Abstract:
Process makes a raw cotton linter matted sheet by dispersing a loose mass of raw cotton linters in water, removing a portion of the water from the dispersed loose mass, and pressing the wet loose mass of raw cotton linters into a resulting matted sheet. The loose mass of raw cotton linters can be precleaned before dispersing into water. The final sheeted product can be cut into water. The final sheeted product can be cut into rectangle pieces or wound on a roll or dried or calendered. This sheet is used as a starting material for making cellulose ethers.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates to a process for making raw cotton linters into sheet form and more particularly to a process for making raw cotton linters into sheet form that is substantially free of any extraneous chemicals. This invention also relates to the sheeted raw cotton linters composition prepared from the above process.  
       BACKGROUND FOR THE INVENTION  
       [0002]     Prior to the present invention, manufacturers of cellulose derivatives have traditionally used highly purified cellulose furnishes derived from cotton linters or wood to make cellulose ethers. These materials are fabricated into rolls of paper sheets for ease of handling. The extensive purification of raw cotton linters to make purified cotton linters added cost and, in certain instances, created environmental problems.  
         [0003]     Both cellulose purification and paper manufacturing of raw cotton linters to form purified cotton linters typically require drying a wet mass of cellulose. Paper making of the cotton linters provides a number of benefits in both the purification of the cellulose and several subsequent cellulose derivative manufacturing processes. Unfortunately extensive purification of crude cellulose adds cost and can pose environmental problems. In U.S. patent application Ser. No. 10/822,926, filed Apr. 13, 2004, entitled “Improved Raw Cut Cotton Linters, Method of Making, and Uses Thereof”, to P. Gillette et al, high performance cellulose ethers were prepared using raw cut cotton linters.  
         [0004]     Although several previous inventors have disclosed the use of cotton linters for the production of paper, these inventions differ in significant respects from the present invention. U.S. Pat. No. 3,235,443, filed Feb. 15, 1966, E. G. Greeman and P. T. Kitze, entitled “Process for Forming Transparentized Paper Containing Cotton Linters Fibers and Paper Thereof” disclose a transparent paper that includes cotton linters in a complex formulation impregnated with a polymeric film-forming acrylic resin. Such compositions have different applications (e.g. tracing papers) and would not be suitable as starting materials for cellulose derivative manufacture. U.S. Pat. No. 5,976,320, filed Nov. 2, 1999, G. Lund and R. Tauber, entitled “Method for Producing Paper Pulp from Fibers of Annual Plants”, and European Patent 0 824 160 A1, filed Jul. 22, 1997, G. Lund and R. Tauber, entitled “Verfahren zur Herstellung von Papiermasse”, propose a pulping process that includes bleached, shortened cotton linters fiber. The present invention involves the production of a sheet of raw linters that has not been bleached. Bleaching typically reduces molecular weight, which is not desired. U.S. Pat. No. 6,174,412, filed Jan. 16, 2001, T. Paterson-Brown, B. T. Painter, S. T. Zuanic, and T. A. White, entitled “Cotton Linters Tissue Products and Method for Producing Same”; European Patent 1 058 751 B1, filed Feb. 3, 1999, T. Paterson-Brown, B. T. Painter, S. T. Zuanic, and T. A. White, entitled “Cotton Linters Tissue Products and Method for Producing Same”; and T. Paterson-Brown, B. T. Painter, S. T. Zuanic, and PCT WO 99/45204, filed Mar. 2, 1999, T. A. White, entitle “Cotton Linters Tissue Products and Method for Producing Same” describe a tissue product produced using raw linters and a cationic starch derivative. This product is substantially thinner than the sheets described by the present invention and includes an undesired binder as well. U.S. 2003/0070262 A1, filed Apr. 17, 2003, J. O. B. Andersen, entitled “Dry Production of a Non-Woven Fibre Web”, proposes a dry process for producing a non-woven fiber web from cotton linters pulp.  
         [0005]     To appreciate the utility of the present invention it is informative to review cellulose ether manufacturing processes. The “steep press” cellulose ether production process (L. Brandt,  Cellulose Ethers  in Ullmann&#39;s Encyclopedia of Industrial Chemistry (5 th  Edition), Vol. A5, (VCH Publishers, 1986), p. 466) involves initially subjecting purified cellulose to a highly concentrated aqueous caustic solution. After an appropriate period of time, the swollen mass is compressed in a manner that removes excess caustic solution where upon it is shredded. The resulting alkali cellulose is then mixed and reacted with appropriate reagents to form the desired cellulose derivative of interest. Use of stacked sheets of cellulose facilitates distribution of the caustic solution. A common practice involves shredding the sheet prior to adding derivatizing reagents. Due to the inherent difficulty in attaining uniform alkalization as well as uniformly mixing the derivatizing reagents within the shredded alkali cellulose, the production of high quality derivatives is difficult by this process. Nevertheless this processing method provides a simple, relatively inexpensive means for producing cellulose ether derivatives.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is directed to a process for making a raw cotton linters matted sheet comprising a) dispersing a loose mass of raw cotton linters in water, b) removing a portion of the water from the dispersed loose mass, and c) pressing the wet loose mass of raw cotton linters into a resulting matted sheet.  
         [0007]     The present invention is also related to the raw cotton linters composition that is prepared by the above-mentioned process. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0008]     It has been unexpectedly found that raw cotton linters fabricated into a sheet serves as a raw material suitable for the production of high performance cellulose derivatives. The resulting derivatives are especially well suited for demanding nonregulated applications such as paint, construction, and oil field. Raw linters can be processed using conventional papermaking processes.  
         [0009]     In this invention raw cotton linters are converted to sheet form using a conventional paper machine to provide a raw material especially well suited for the production of high performance cellulose derivatives. For purposes of this patent application “sheet” refers to matted, non woven web that is held together by pressure (that is commonly referred to a paper) that has been cut into rectangular pieces as well as that which has been wound into rolls. The use of raw cotton linters in this physical form permits cellulose derivative manufacturers to utilize existing manufacturing assets to produce high performance products at substantially reduced cost. The cost reduction stems both from the elimination of many purification steps heretofore practiced, as well as other unexpected benefits found for a raw linters-based sheet. It has been found that the resulting derivatives are especially well suited for demanding applications in nonregulated industries such as paint, construction, and oil field.  
         [0010]     In accordance with the present invention, an example of a process for producing the matted sheets of raw cotton linters includes a step wherein bales of raw linters are broken apart and dispersed in a low solids water slurry to form a pulp. The resulting pulp can be formed into a sheet using a variety of papermaking processes. It is preferred that the sheet be made without any chemicals, fillers, or other additives traditionally employed by the papermaking industry. In addition, it is preferred that the raw cotton linters fibers not or minimally be subjected to processes such as disk refiners that are designed to fibrillate the fiber&#39;s surface. Fibrillated materials occupy a greater volume which decreases the amount of material that can be loaded into a cellulose reactor. Both dry and wet cleaning processes may be employed to remove impurities prior to the formation of the paper sheet. Examples of such impurities include, but are not limited to, inorganic materials as well as cottonseed hull fragments.  
         [0011]     The optimum dimensional, basis weight, density, and mechanical property characteristics of the raw cotton linters sheets are determined by the desired specific cellulose derivative process characteristics. Basis weight refers to the weight per unit area of a representative sample of the raw cotton linters matted sheet. It is preferred that the sheet be dried to a moisture content of 2 to 20% water. It is most preferred that sheet be dried to a moisture content in the range of 4-12%. The drying should not be done in a manner that causes hornification of the cellulose. Hornification refers to excessive drying of cellulose which renders the material less reactive in subsequent derivatization reactions. It is generally attributed to the formation of strong hydrogen bonds between cellulose anhydroglucose units that are difficult to disrupt.  
         [0012]     In the production of high quality cellulose derivatives, it is preferred that the raw cotton linters sheet undergo shredding and/or cutting prior to chemical derivitization. In the most preferred embodiment of the invention the raw cotton linters sheet is fed to at least one cutter whereupon it is cut in a manner that the fibers become individually separated from each other prior to being used to make cellulose ethers.  
         [0013]     In order to overcome the problems of the prior art, a number of manufacturers employ nonreactive liquid diluents (U.S. Pat. No. 2,517,577, filed Aug. 8, 1950, E. D. Klug and J. S. Tinsley, entitled “Preparation of Carboxyalkyl Ethers of Cellulose”) to form a cellulose slurry. This approach enables better mixing of reactants. From an economic perspective, it is desirable (and the objective of the manufacturer) to fill a reactor with as much cellulose raw starting material as possible without sacrificing product quality. This objective implies minimizing nonreactive diluent usage to a level that still permits good mixing. To increase reactor cellulose loading manufacturers often cut cellulose fibers. Fiber length reduction increases bulk density, thereby permitting greater reactor loading. In the production of high quality cellulose derivatives, it is preferred that the sheet undergo shredding and/or cutting prior to chemical derivatization. In the most preferred embodiment of the invention, the sheet is feed to at least one cutter whereupon it is cut in a manner that the fibers become individually separated from each other and reduced in length.  
         [0014]     There are a variety of methods to reduce fiber length. A preferred method makes use of rotary cutters fitted with appropriately-sized mesh screens. Examples of such cutters include CS cutting granulators from Netzsch-Condux Mahitechnik, Herbold SMF grinder, as well as Rotoplex granulators from Hosokawa-Alpine. For this process, it is important to be able to feed the cutter at a uniform rate. Cellulose furnishes in the form of paper rolls provide a simple means of material handling and feeding. It has been found that cutting of raw linters-based sheets requires substantially less energy and results in lower cutter temperatures than commercially available purified cellulose sheets. This permits cutters to be operated at higher throughputs than previously.  
         [0015]     According to the present invention, raw linters can be processed using conventional papermaking processes. An example of such a process includes a step wherein bales of raw linters are broken apart and dispersed in a low solids water slurry to form a pulp. Typical concentrations of fibers are from 0.01 to 4%. The optimum concentration depends upon the specific papermaking process and fiber characteristics. For example (J. E. Williamson,  Wet - laid Systems in Nonwovens—Theory, Process, Performance, and Testing , p. 142”), conventional papermaking machines generally are produced at 0.3 to 0.7%, whereas wet-laid nonwovens are in the range of 0.01 to 0.05% solids. The resulting pulp can be formed into a sheet using a variety of papermaking processes. It is preferred that the sheet be made without any chemicals, fillers, or additives traditionally employed by the papermaking industry to enhance paper properties for other applications or reduce costs.  
         [0016]     A wide range of equipment is available (G. A. Smook,  Handbook for Pulp and Paper Technologists  (2 nd  Edition), (Angus Wilde Publications, Vancouver: 1992), p. 195 ff) to alter fiber properties from the standpoint of both papermaking and end-use paper properties. Conical refiners (e.g. Jordan, Claflin), disk refiners, and Valley or Hollander beaters are commonly employed to this end. This equipment impacts fiber morphology in a variety of ways including fiber length reduction and fibrillation. The relative extent to which these effects occur depends upon the specific design and operation of the refiner/beater. From a cellulose ether manufacturing perspective, reduction of fiber length and less fibrillation are generally considered desirable, since these requirements lead to higher bulk density that translates into higher reactor loading. It may, however, be necessary to balance these requirements in order to attain good papermaking machine operation and paper characteristics.  
         [0017]     Both dry and wet cleaning processes may be employed to remove impurities prior to the formation of the paper sheet. Examples of such impurities include, but are not limited to, inorganic materials as well as cottonseed hull fragments. Mechanical dry cleaning methods may be employed prior to the formation of the pulp slurry. An example of appropriate dry cleaning equipment is a Continental Eagle IMPCO LC-410D Linters Cleaner. Wet cleaning methods include screen-based processes, both pressurized and nonpressurized, which are particularly useful for removing larger contaminants. Examples of such a process are the Voith Minisorter screen and vibrating flat screens. In addition, centrifugal cleaners (G. A. Smook,  Handbook for Pulp and Paper Technologists  (2 nd  Edition), (Angus Wilde Publications, Vancouver: 1992), p. 113 ff) (e.g., liquid cyclones, hydrocylcones, vortex cleaners, and centricleaners) are especially well suited for cleaning. Although Sarimsakov et al (A. Sarimsakov, T. S. Saypiev, G. V. Nikonovich, N. D. Burkhanova, S. M. Yugai, and S. Sh. Rashdova,  Synthesis and Properties of Na - CMC from Cotton Cellulose Produced by Different Methods , Cell. Chem. &amp; Tech., 36(5), 423(2002)) describe a CMC sample made from “cotton linters with centrycleaner screening”, it does not appear that the furnish was made into a sheet or cut. Depending upon the specific characteristics of the cotton linters, it may be desirable to use one or more of such devices in series to selectively remove small impurities (e.g., inorganics or hull pepper) or large impurities (e.g., seed hull fragments) prior to sheet formation. Since reject orifice plugging of the cleaning devices can present problems, the order in which these operations are performed can impact process efficiency. For example it may be preferable to remove larger diameter contaminants prior to removing small diameter species.  
         [0018]     In addition to the distinct solid impurities already mentioned, the concentration of other chemical species present on or in the cotton linters fibers may be significantly reduced by appropriate processing. For example wax and oil residues are typically found on the surface of raw cotton linters fibers. Washing or extraction with appropriate organic solvents (e.g., alcohols, ether) can be utilized to assist in the removal of these materials. In the case of aqueous extraction, elevated temperatures, preferably 70 C to 95° C., and more preferably above 85 C can be employed. This operation may be conveniently performed during the pulping of the cotton linters. Other synergistic additives such as soaps may also be employed to promote the extraction and removal impurities. The use of organic solvents can present additional difficulties in their complete removal, disposal, and/or purification for re-use. For such processes pulping followed by de-watering may be desirable to remove excess waxes and oils.  
         [0019]     A variety of processes may be used to form the sheet including, but not limited to, Fourdrinier, cylinder, wet (J. B. Calkin (ed),  Modern Pulp and Paper Making , (Reinhold Publishers, New York: 1957), p. 16) machines, belt washers, and disk savealls. Examples of inclined wire Fourdrinier machines include Tampella Sandy Hill Deltaformer®, Dörries Hydraformer®, and the Neue Bruderhas No-Wo Former®.  
         [0020]     To assist in removing water from the sheet, a variety of pressing methods may be employed. Examples of appropriate processes include two roll presses, shoe (extended-nip) presses, and platen presses. A series of nips, typically three, in which the pressure progressively increases at each successive nip represents a most preferred embodiment.  
         [0021]     It is preferred that the sheet be dried to a moisture content of 2 to 20% and most preferably in the range of 4-12%. The drying should be done in a manner that does not cause hornification of the cellulose since this can result in a cellulose that will not be uniformly swollen by alkali. A variety of processes may be used to dry to sheet including heated cylinder “dryer cans”, through-air dryer processes, and pulp dryers (e.g., Flakt pulp dryer).  
         [0022]     Calendering of the dry or partially-dry sheet may be used to improve sheet density, thickness uniformity, and mechanical properties.  
         [0023]     The optimum dimensional, base weight, density, and mechanical property characteristics of the sheet are determined by the specific cellulose derivative process characteristics. For processes employing cutters, typical cellulose cutters used for industrial manufacturing processes range in width from 0.25 to 2.5 m, most preferably in the range of 0.50 to 1.25 m wide. The sheet fed to such cutters should be approximately the same width, but no wider than, the width of the knives of the rotary cutter. The basis weight of the sheet is preferably in the range of 250 to 900 g/m 2  and most preferably in the range of 400 to 700 g/m 2 . Sheet thickness is preferably in the range of 0.04 to 0.30 cm, most preferably in the range of 0.08 to 0.20 cm, and should vary by less than most preferably ±5% within the sheet. This combination of base weight and thickness implies a range of bulk density of the matted sheet. High bulk density is generally desirable, since it results in lower shipping costs and less frequent roll changes. Regarding mechanical properties of the sheet, the primary concern for materials in roll form is that they be able to be unwound without breaking or tearing. Since materials are unwound at relatively slow, uniform rates into cutters, this does not generally present a problem since the material must have a minimum tensile strength to be consistently wound during paper manufacturing.  
       EXAMPLE  
       [0024]     Second cut raw linters were initially pulped in a Tornado pulper for 30 minutes. The pulp was subsequently transferred to a blend chest whereupon it was diluted to 1.0% consistency after which it was transferred to a machine chest. Paper was then produced using a 1.1 m Deltaformer paper machine at a headbox consistency of 0.08% without the use of any additional papermaking chemicals or fillers. The sheet was dried on the machine to a final moisture content of 3.5%. The resulting raw linters sheet had a basis weight of 700 g/m 2  with a caliper of 2.0 mm.  
         [0025]     While this invention has been described with respect to specific embodiments, it should be understood that these embodiments are not intended to be limiting and that many variations and modifications are possible without departing from the scope and spirit of this invention.