Patent Grant
7097737
The present invention is directed to products made from unbleached pulps, the process used to make such products and the processes used to make cellulose solutions from unbleached pulp for spinning into lyocell products.
Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. Cellulose is especially abundant in tree trunks from which it is extracted, converted into pulp, and thereafter utilized to manufacture a variety of products. Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century, strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process, cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. The cellulose is then reacted with carbon disulfide to form cellulose xanthate, which is then dissolved in a dilute caustic soda solution. After filtration and deaeration, the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammonia copper oxide. This property forms the basis for the production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine N-oxides, in particular the tertiary amine N-oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a cellulose fiber precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Acordis, Ltd. presently manufactures and sells a lyocell fiber called Tencel® fiber.
Currently available lyocell fibers are produced from wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose, and lignin. These highly processed pulps are referred to as high alpha (or high α) pulps, where the term alpha (or α) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose and lignin. The processing required to generate a high alpha, low lignin pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
Furthermore, the wood chips are pretreated with an acid before the pulping stage, since it is not possible to obtain acceptable high alpha pulps for lyocell products otherwise through the Kraft process. A significant amount of material, primarily hemicellulose on the order of 10% or greater, of the original wood substance is solubilized in this acid phase pretreatment. Thus process yields are significantly diminished. Omitting the acid phase pretreatment will result in a high hemicellulose pulp. The disadvantage of conventional high alpha pulps is the reduction of yield by eliminating hemicelluloses from the pulp.
Conventional high alpha pulps are also bleached. Bleaching refers to the removal of lignin in a process subsequent to the pulping process. Removing lignin also reduces the overall yield of the original wood material.
In view of the expense of producing lyocell products from high alpha pulps that have small amounts of hemicellulose and lignin, it would be desirable to have alternatives to high alpha, low lignin pulps for making lyocell products.
Thus there is a need for relatively inexpensive, low alpha, high yield, high hemicellulose, and high lignin pulps that are useful for making lyocell products.
In U.S. Pat. No. 6,210,801, fully incorporated herein by reference in its entirety, high hemicellulose containing pulp is described that is useful for lyocell products. The pulp is made by reducing the viscosity of the cellulose without substantially reducing the hemicellulose content, followed by reducing the copper number.
While the methods described in the '801 patent are effective at reducing the viscosity of cellulose without substantially decreasing the hemicellulose content, a further need existed for a process that did not require a separate copper number reducing step, and that was readily adaptable to pulp mills that have oxygen reactors. In U.S. Pat. No. 6,331,354, fully incorporated herein by reference in its entirety, a high hemicellulose, low viscosity pulp is described that is useful for making lyocell products that does not require an additional copper number reducing step. The pulp is made from an alkaline pulp by treating the alkaline pulp with an oxidizing agent in a medium to high consistency oxygen reactor to reduce the viscosity of the cellulose, without substantially reducing the hemicellulose or increasing the copper number of the pulp.
Further efforts to reduce the cost of making lyocell products are described in U.S. application Ser. No. 09/842,274, fully incorporated herein by reference in its entirety. In the '274 application, the pulps are made from sawdust and other low length fiber wood. These pulps are high in hemicellulose and low in viscosity, and are composed of short fibers suitable for producing lyocell products.
However, until now, all of the prior art dissolving pulps for producing lyocell products are low in lignin content. It would be advantageous to develop a high lignin pulp that is useful for making lyocell products as an alternative to the highly refined low yield high alpha pulps.
In accordance with the present invention, lyocell products can be made with unbleached pulps resulting in products with high amounts of hemicellulose and high amounts of lignin as compared to conventional lyocell products. The lyocell products of the present invention are advantageously less expensive to produce but retain the desirable strength of conventional lyocell products.
One embodiment of the invention provides a high hemicellulose, high lignin lyocell fiber. The fiber has a dry tenacity of at least 46 cN/tex at a gauge length of about 10 mm and a dry tenacity of at least 30 cN/tex at a gauge length of about 30 mm. The hemicellulose content of the fiber is at least 7% and the lignin content of the fiber is at least 2% measured as Klason lignin.
In another embodiment of the invention, a lyocell fiber has an average diameter of from 9 to 16 microns; however, other embodiments of fibers have an average diameter of from 12 to 14 microns.
In another embodiment of the invention, a method for making a lyocell product is described. The method includes modifying a pulp having at least 7% hemicellulose and at least 2% Klason lignin by acid hydrolysis to lower the average degree of polymerization of the cellulose in the pulp, wherein greater than 95% of cellulose has an average D.P. from greater than about 400 to about 1100. The method also includes dissolving the unbleached pulp in a solvent to provide a cellulose solution, and spinning the cellulose solution into a lyocell product. The method uses meltblowing, centrifugal spinning, spun-bonding, or dry-jet wet spinning techniques. Fibers, films, and self-bonded nonwoven webs from unbleached pulps can be produced according to the methods of the invention.
In another embodiment of the invention, a pulp is provided. The pulp has at least 7% hemicellulose and at least 2% Klason lignin. The pulp also has cellulose, wherein greater than 95% of the cellulose has an average D.P. of from greater than about 400 to about 1100.
In another embodiment of the invention, a meltblowing method for making a lyocell product is provided. The method includes extruding a cellulose solution of unbleached pulp through a plurality of apertures to produce continuous cellulose filaments. The method includes stretching the filaments with air.
According to the present invention, an unbleached pulp, having been modified to reduce its average degree of polymerization of cellulose, results in substantial increases in yield. The high amounts of hemicellulose and lignin contribute to this greater improvement in overall yield. This advantage, along with numerous related advantages, makes the lyocell product of this invention more desirable in comparison with previous technology.
Referring to
Conventional dissolving pulps for lyocell products are processed through a sequence of bleaching towers to reduce the lignin content. However, in the present invention, the bleaching stages are omitted. In step 102, the unbleached pulp is modified to reduce its viscosity. Reducing the viscosity of the pulp increases its ability to dissolve. The viscosity of the unbleached pulp is reduced so that the average D.P. of 95% of the cellulose is modified to greater than 400 to about 1100. Unbleached pulps are made into lyocell products by first dissolving the unbleached pulp in an amine oxide and then spinning the solution into filaments followed by regeneration of the filaments. One method for modifying the viscosity of unbleached pulp is by acid hydrolysis. Any acid may be utilized, such as hydrochloric acid or sulfuric acid. The acid may be utilized in the form of a liquid, or may be formed from a gas, such as by dissolving hydrogen chloride gas in an aqueous medium. Another method is by swelling the cellulose in an alkaline solution followed by alkali removal and treatment with a cellulolytic enzyme, preferably an endogluconase enzyme. Alternatively, steam explosion can be utilized. Further, any combination of methods for viscosity reduction can be utilized, such as steam explosion combined with acid hydrolysis. An advantage of utilizing acid hydrolysis to reduce viscosity is that transition metal contaminants in the pulp are removed by the acid treatment. If an acid treatment step is not utilized, then an alternative method of removing transition metals from the pulp can be utilized, such as treatment of the pulp with a chelating agent. Other equally suitable methods for reducing the viscosity of unbleached pulps are described in the aforementioned U.S. Pat. Nos. 6,210,801 and 6,331,354, fully incorporated herein by reference in their entirety. For example, the pulp can be contacted with any oxidant containing a peroxide group such as hydrogen peroxide, oxygen, chlorine dioxide and ozone, as described in U.S. Pat. No. 6,331,354.
Unbleached pulps having their viscosity modified can be spun into lyocell products. Modified unbleached pulps are high in hemicellulose and high in lignin and can readily dissolve to provide cellulose solutions that can be spun into lyocell products.
Methods for measuring pulp viscosity are well known in the art, such as TAPPI T230. Methods used for measuring hemicellulose include, for example, a sugar content assay based on TAPPI standard T29 hm-85. Methods for expressing lignin content are also well known. The amount of lignin can be expressed as Kappa number, Klason lignin, or K (permanganate) number.
Following the pulping process, block 100, or the viscosity reduction process, block 102, the brownstock pulp can be made into a form which is suitable to be transported or stored, or otherwise transformed into a more convenient form for handling. A pulp may be provided in a roll, sheet, or bale, for example.
Referring still to
One embodiment of a method for making lyocell fibers from dope derived from modified unbleached pulp having high hemicellulose and high lignin involves extruding the dope through a die to form a plurality of filaments, followed by washing the filaments to remove the solvent, and then drying the lyocell filaments.
The cellulose solution derived from the modified, unbleached pulp dope is forced through extrusion orifices in a process called spinning, block 210, to produce cellulose filaments that are then regenerated with a non-solvent, block 212. Finally, the lyocell filaments or fibers can be washed and dried, block 214. Filaments may also be aggregated into a nonwoven web before regeneration.
In meltblowing, the cellulose solution is forced from extrusion orifices into a turbulent airstream. Meltblowing produces continuous cellulose filaments and causes stretching of the filaments with the air. Where the fibers are produced by centrifugal spinning, the cellulose solution is fed to a rotating head and expelled through small orifices into air. The cellulose filaments are drawn or stretched by the resistance of the air caused by the rapidly rotating head. In meltblowing and centrifugal spinning, the latent filaments are regenerated in a bath or with sprayers. In other embodiments, conventional processes for forming lyocell fibers can be used which continuously mechanically pull the extruded filaments linearly downward into a regenerating bath through an air gap. The latter process is sometimes referred to as a “dry-jet wet” spinning process.
In the dry-jet wet spinning process, cellulose solution is extruded from a multiplicity of fine apertured spinnerets into an air gap. The filaments of cellulose dope are continuously mechanically drawn. They are then led into a non-solvent, usually water, to regenerate the cellulose. Examples of this process are described in McCorsley in U.S. Pat. Nos. 4,142,913; 4,144,080; 4,211,574; 4,246,221; and others. These patents are expressly incorporated herein by reference in their entirety.
More specifically, in meltblowing, a supply of dope is pumped through an extrusion head having a multiplicity of orifices. Compressed air or any other suitable gas is supplied to the extrusion head. Latent cellulose filaments are extruded from the orifices. These thin strands of cellulose solution are picked up by the high velocity gas stream exiting from the orifices and are stretched or elongated by the gas. The latent filament strands can be regenerated by passing between spray pipes that carry water or any other suitable regenerating liquid. Alternatively, the strands can pass into a regenerating bath. The regenerated filaments are then picked up by a rotating pickup roll where they accumulate until a sufficient amount of fiber has accumulated.
In centrifugal spinning, the cellulose solution is directed into a hollow cylinder or drum with a base and a multiplicity of small apertures in the sidewalls. As the cylinder rotates, cellulose solution is forced out horizontally through the apertures as thin cellulose filaments or strands. As these strands meet resistance from the surrounding air, they are drawn or stretched. The cellulose strands will fall by gravity or are gently forced downward by an air flow into a non-solvent where they coagulate into individual oriented fibers.
Processes for meltblowing cellulose solutions are described in U.S. Pat. Nos. 6,235,392 and 6,306,334, incorporated herein by reference in their entirety. The centrifugal spinning method is described in U.S. Pat. No. 6,235,392.
In meltblowing, centrifugal spinning, and spun-bonding, as the cellulose filaments encounter resistance from the air, they will be drawn or stretched by contact with the air. The cellulose filaments will also undergo a reduction in diameter. The amount of stretching will depend on readily controllable factors such as orifice size, dope viscosity, cellulose concentration in the dope, air speed, nozzle diameter, spinning temperature, winder speed, bath temperature, etc. Meltblowing and centrifugal spinning can be used to produce nonwoven webs, as described in U.S. Pat. No. 6,235,392.
One particularly useful embodiment of an apparatus for making a lyocell nonwoven web made from an aggregate of lyocell fibers is shown in U.S. Pat. No. 6,235,392. However, in the present invention, the cellulose material is provided from a modified unbleached pulp having high hemicellulose and high lignin. Referring to
As can be seen from
Microfibers can be produced by the meltblowing spinning process; however, self-bonded nonwoven webs using the meltblown process is also of great commercial interest. Self-bonded meltblown nonwoven webs made from modified unbleached pulp are produced. Referring now to
Lyocell fibers produced by meltblowing, centrifugally spinning, or spun-bonding modified, unbleached cellulose pulps possess a natural crimp quite unlike that imparted by a conventional stuffer box. Crimp imparted by a stuffer box is relatively regular, has a relatively low amplitude, usually less than a 1 fiber diameter, and a short peak-to-peak period, normally not more than 2 or 3 fiber diameters. The fibers made according to the present invention have an irregular amplitude greater than 1 fiber diameter, usually much greater, and an irregular period exceeding about 5 fiber diameters, a characteristic of fibers having a curly or wavy appearance. The fiber diameter along the length of each individual fiber can vary, as well as the average fiber diameter between fibers. A quantifiable measurement of this variability is termed coefficient of variability. The fibers of the present invention have a range of diameters and tend to be somewhat curly giving them a natural crimp. This natural crimp is quite unlike the regular sinuous configuration obtained in a stuffer box. Both amplitude and period are irregular and are at least several fiber diameters in height and length. Most of the fibers are somewhat flattened and some show a significant amount of twist. The methods are useful in producing microfibers. In one embodiment, average fiber diameter varies from about 6 to 42 microns. In one embodiment, average fiber diameter is about 9 to 16 microns. In one other embodiment, average fiber diameter is about 12–14 microns.
Lyocell products, including fibers, films, and nonwovens made from modified unbleached pulp having high hemicellulose and high lignin content are high in tenacity regardless whether they are made from meltblowing, centrifugal spinning, spun-bonding, dry-jet wet or any other method. Modified, unbleached pulps having high hemicellulose and high lignin can also be made into microfibers having an average size of about 0.1 denier.
In one embodiment, a lyocell fiber was made from modified unbleached pulp having high hemicellulose, and high lignin. The lignin content of the fiber was at least 2% Klason lignin. The hemicellulose content of the fiber was at least about 7%. The lyocell fiber further contains cellulose, wherein greater than 95% of the cellulose has an average D.P. of from greater than 400 to 1100. The lyocell fiber has a dry tenacity of at least 46 cN/tex at a gauge length of about 10 mm or a dry tenacity of at least 30 cN/tex at a gauge length of about 30 mm. The average diameter of the fiber can range from about 6 to about 19 microns using a dry-jet wet process to about 2 to about 46 microns using a meltblowing process. The median appears to lie between about 9 to about 16 microns, or between about 12 to about 14 microns.
Using the dry-jet wet spinning process, lyocell fibers produced from modified unbleached Kraft pulp having high hemicellulose and high lignin content were produced having the diameter distribution shown in
Using the meltblowing spinning method, lyocell fibers produced from modified unbleached Kraft pulp having high hemicellulose and high lignin content were produced having the diameter distribution shown in
A scanning electron microscope (SEM) or an optical microscope was used to observe the morphology of the fibers or the nonwoven web.
The mechanical properties of the fibers produced by the dry-jet wet method was measured with an Instron tester. Measurement of fiber bundles of 10 filaments was used to determine the average.
The following examples now merely illustrate the best mode now contemplated for practicing the invention, but should not be construed to limit the invention.
Lyocell fibers were produced using laboratory process equipment from both the Thuringisches Institut Fur Textil Und Kunststoff-Forschung (TITK) and the Weyerhaeuser Technology Center (WTC). Table 1 illustrates some of the process conditions used for the dry-jet wet and meltblowing methods.
It was found that the average diameter of the WTC dry-jet wet lyocell fibers from modified unbleached Kraft is about 12 microns, and the fibers have a denier of about 1.3. The diameter distribution is shown in
An Instron tester was used to measure the mechanical properties of the WTC dry-jet wet lyocell fibers. The properties are shown in Table 2.
The tenacity measurement of the fibers is reasonably reproducible considering that the equipment is normally used for paper testing. The tenacity is dependent on the gauge length.
The dry-jet wet lyocell fibers from the modified unbleached Kraft pulp have a dry tenacity of about 46 cN/tex at a gauge length of 10 mm. If a gauge length of 30 mm is used for testing, the dry tenacity for the lyocell fibers is 30 cN/dtex.
A decrease in tenacity with increasing gauge length is typical. The result, nevertheless, indicates the potential to produce strong lyocell fibers from modified unbleached Kraft pulps. For example, TITK uses 10 mm gauge length for their testing and usually gives a tenacity of about 35 to 45 cN/tex for dry-jet wet lyocell from bleached pulp.
Using the meltblown spinning system from WTC, lyocell fibers from modified unbleached Kraft pulp were obtained. The meltblown lyocell fiber had 3.4% of Klason lignin and the diameter results are summarized in Table 3.
As expected, the diameter of the lyocell fiber decreased with decreasing throughput. The diameter will also be affected by air pressure, temperature, and winder speed, etc. For meltblown lyocell fibers, diameter distribution is usually broader than those from dry-jet wet process.
While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
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