Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2001-04-24
2003-08-12
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S357000, C428S364000
Reexamination Certificate
active
06605350
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to sawdust pulps useful for making lyocell fibers, to methods of making such pulps useful for making lyocell fibers, and to lyocell fibers made from the compositions of the present invention. In particular, the present invention is directed to sawdust pulps having a high hemicellulose content, a low copper number, short fiber length and includes cellulose having a low average degree of polymerization.
BACKGROUND OF THE INVENTION
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. This is reacted with carbon disulfide to form cellulose xanthate which is then dissolved in 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 so-called 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 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 that were 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 underivatized 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-xides 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 fiber composed of cellulose 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 high quality wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose. These highly processed pulps are referred to as dissolving grade or high alpha (or high &agr;) pulps, where the term alpha (or &agr;) 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. The processing required to generate a high alpha pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom. Typically, the cellulose for these high alpha pulps comes from softwood, which generally has longer fibers than hardwoods. Softwoods must be made into wood chips to make them suitable to be pulped in the digesters. The digesters are normally equipped with a system for recycle of the black liquor. Recycle provides a desirable homogenous mixture throughout the digester that leads to uniform pulping conditions. In order to move the black liquor, digesters are equipped with screens to keep wood chips from the pump inlet.
Since conventional Kraft processes stabilize residual hemicelluloses against further alkaline attack, it is not possible to obtain acceptable quality dissolving pulps, i.e., high alpha pulps, through subsequent treatment of Kraft pulp in the bleaching stages. Thus, in order to prepare dissolving type pulps by the Kraft process, it is necessary to give the raw material an acidic pretreatment before the alkaline pulping stage. 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 and thus process yields drop. Under the prehydrolysis conditions, the cellulose is largely resistant to attack, but the residual hemicelluloses are degraded to a much shorter chain length and can therefore be removed to a large extent in the subsequent Kraft cook by a variety of hemicellulose hydrolysis reactions or by dissolution.
A relatively low copper number, reflective of the relative carbonyl content of the cellulose, is a desirable property of a pulp that is to be used to make lyocell fibers because it is generally believed that a high copper number causes cellulose and solvent degradation, before, during, and/or after dissolution in an amine oxide solvent. The degraded solvent can either be disposed of or regenerated; however, due to its cost it is generally undesirable to dispose of the solvent. Regeneration of the solvent suffers from the drawback that the regeneration process involves dangerous, potentially explosive conditions.
A low transition metal content is a desirable property of a pulp that is to be used to make lyocell fibers because, for example, transition metals accelerate the undesirable degradation of cellulose and NMMO in the lyocell process.
In view of the expense of producing commercial dissolving grade pulps, it would be desirable to have alternatives to conventional high alpha dissolving grade pulps as a lyocell raw material. In addition, pulp manufacturers would like to minimize the capital investment necessary to produce such types of pulps by utilizing existing capital plants.
In order to control lyocell fiber properties, lyocell manufacturers utilize dopes that comprise a blend of different pulps having different ranges of average degree of polymerization values. In view of this, there is also a need for pulp manufacturers to produce pulps having an average degree of polymerization within a broad band to eliminate the need for blending.
Thus, there is a need for relatively inexpensive, low alpha (e.g., high yield) pulps that can be used to make lyocell fibers, for a process of making the foregoing low alpha pulps using capital equipment that is currently available to pulp manufacturers, and for lyocell fibers from the foregoing low alpha pulp. Preferably, the desired low alpha pulps will have a desirably low copper number, a desirably low lignin content and a desirably low transition metal content but broad molecular weight distribution.
In a prior a
Luo Mengkui
Persinger, Jr. W. Harvey
Sealey, II James E.
Wester Brian
Christensen O'Connor Johnson & Kindness PLLC
Gray J. M.
Kelly Cynthia H.
Weyerhaeuser Company
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