This is a continuation-in-part (CIP) of application Ser. No. 07/496,585, filed on Mar. 21, 1990, now abandoned, which is a division of application Ser. No. 07/261,000, filed Oct. 21, 1988, now U.S. Pat. No. 4,999,149. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the production of high tensile strength, solvent-spun cellulose fiber which is stable in alkaline solutions. More particularly, the invention relates to a process in which cellulose is solubilized in a zinc chloride solution then extruded into a coagulation medium in which the cellulose fibers form. The fibers are removed from the coagulation medium, treated to remove the coagulation medium, stretched, and recrystallized in water, thus forming the high tensile strength, solvent-spun cellulose fibers of the present invention. 
     BACKGROUND OF THE INVENTION 
     Cellulose, the most abundant polymer on earth, is a straight-chain polymer of anhydroglucose with beta 1-4 linkages. Cellulose fiber in its natural form comprises such materials as cotton and hemp, while solvent-spun fiber comprises products such as rayon. 
     The process most frequently used to produce solvent-spun fiber is known as the viscose process. In the viscose process cellulose xanthate, a cellulose derivative, is solubilized then spun into a coagulation bath where the fiber forms. The resulting fiber is a product known as viscose. 
     A second solvent-spun fiber product, cellulose acetate, is produced when a cellulose derivative is solubilized in an organic solvent, then spun into water or alcohol where it coagulates to form fiber. The fiber may be regenerated from its derivative form to true (nonderivatized) cellulose using an alkaline solution, but such a regeneration step is rarely performed. 
     In addition to the production of viscose and cellulose acetate, a third embodiment of the solvent-spun process involves the dissolution of true cellulose in a solvent which is then spun into a coagulation bath in which fiber formation occurs. 
     Due to the high processing costs and the generally inferior properties of the fiber products formed when nonderivatized cellulose is employed in the solvent-spun process, derivatized cellulose such as that used in the viscose process is generally employed when producing solvent-spun fibers. 
     Approximately 20 years ago it became apparent that the production of solvent-spun fiber by methods such as the viscose process was becoming disadvantageous due to the high capital costs and environmental considerations associated with their use. For this reason, modified or alternative methods for producing solvent-spun fiber were sought. 
     Several cellulose solvents were tested for use in a modified or alternative solvent-spun process. A few achieved favorable results in solubilizing the cellulose, but were ultimately deemed to be impractical for other reasons. 
     Specifically, solvents comprising solutions of SO 2  /NH 3 , and SO 2  /(CH 3 ) 2  HN were tested and found to form good cellulose solutions (i.e., solutions having reasonable viscosities and practical degrees of polymerization). Unfortunately, it was impractical to regenerate cellulose fiber, and to recover the solvent from the coagulation medium. 
     Similarly, a 85% H 3  PO 4  solution was tested for use as a cellulose solvent and was found to dissolve cellulose well, however, the resulting solution contained gels and fibers which made filtration very difficult. Additionally, when phosphoric acid was tested, the cellulose went into solution well, but the phosphoric acid could not be washed from the resulting fiber. D. M. MacDonald, The Spinning of Unconventional Cellulose Solutions in Turbak et al., &#34;Cellulose Solvent Systems&#34; ACS Sym. Seri. 58 (1977). 
     Solutions of 52.5% Ca(CNS) 2 , DMF/N 2  O 4 , and DMSO/para-formaldehyde were also tested. These too proved unsuccessful for use, for while the solutions were found to be acceptable cellulose solvents, they either formed weak fibers or were difficult to recover from the coagulation medium once fiber formation occurred. Hudson, S. M., Cuculo, L. A., J. Macromolecular Science Rev., Macromolecular Chemistry (1980) C18(1) p. 64. 
     In addition to the solvents listed above, MacDonald (supra) also reported testing a 64% ZnCl 2  solution. As with the previous solvents, the results were unacceptable. In this case, the solubilized cellulose could not be spun and the coagulated fibers were noncohesive. 
     In view of these results and until the present invention, the use of ZnCl 2  as a cellulose solvent has only been successfully utilized in a limited number of processes. 
     In one such process, the carbon fiber process, cellulose is solubilized in a ZnCl 2  /HCl solution then extruded into a methanol bath wherein the cellulose coagulates to form fibers. These fibers are usually weak, and while they can generally be handled with tweezers, they are not usually strong enough to permit spinning. 
     Following coagulation, the ZnCl 2  /HCl is removed from the cellulose by prolonged soaking in the methanol bath. The fibers are then carbonized. 
     In addition to the carbon fiber process, the production of &#34;vulcanized fiber&#34; or nonwoven mats also involves the use of a ZnCl 2  cellulose solvent. See, eg. Young and Miller, Formation and Properties of Blended Nonwovens Produced by Cellulose-Cellulose Bonding in Gould et al. &#34;Blended Nonwovens,&#34; ACS Symp. Ser. 10 (1975). 
     In producing vulcanized fiber, cellulose is swollen and softened into a gel using a concentrated ZnCl 2  solution. The gel is then pressed into sheets which are leached with water in order to extract the ZnCl 2  from the cellulose. This results in the formation of a tough, rigid, nonwoven plastic sheet. 
     Prior to the present invention, the use of ZnCl 2  as a cellulose solvent has had limited application as described above. Moreover, its use in a solvent-spun process has until now proven impractical. 
     The present invention is advantageous, therefore, for it teaches the use of ZnCl 2  as a cellulose solvent in a solvent-spun process, as well as teaching the production of a high tensile strength, solvent-spun cellulose fiber resulting therefrom. The use of ZnCl 2  in the present invention is further beneficial, for ZnCl 2  is nontoxic, less corrosive than previously employed solvents and is easily recoverable for reuse. Further advantages of the present invention are set-forth below. 
     Absent the teachings of the present invention, it is generally preferred that the cellulose starting material employed in solvent-spun processes have a high degree of polymerization (hereinafter &#34;DP&#34;), i.e. a DP preferably above 600, but at least above 300 (hereinafter &#34;high DP cellulose&#34;), when strong fiber products are desired. Unfortunately, this usually means that the cellulose starting material must be obtained from conventional pulping processes. While the DP of such pulp is normally high, the conventional processes are relatively expensive to operate and their pulp products are generally costly. 
     Unlike the solvent-spun processes previously discussed, the solvent-spun process of the present invention enables one to use both high DP cellulose and cellulose with a DP ranging from about 100 to 300 (hereinafter &#34;low DP cellulose&#34;). In fact, the cellulose starting material of the present invention may range in DP from about 100 to 3000. Therefore, pulp obtained from a non-conventional, acid pulping process (a potentially cheap and efficient process yielding dissolving grade cellulose having a DP of 400 or less) which is generally unsuitable for use in existing solvent-spun fiber processes, may be used herein according to the teachings of the present invention. For purposes of the present invention, &#34;dissolving grade&#34; cellulose comprises substantially lignin-free cellulose. 
     In addition to the above, the ability to use low DP cellulose in the process of the present invention is further advantageous because low DP cellulose is theoretically both cheap and abundant, potentially derivable from both municipal and agricultural wastes such as used paper, corn stalk, and sugar cane bagasse. 
     As a further advantage, the process of the present invention may be employed (with only slight modifications described infra) to form cellulose fibers and films suitable for use in food and pharmaceutical applications. In such instances, food-grade cellulose and food-grade ZnCl 2  are taught for use herein. 
     In addition to the DP of the cellulose starting material being determinative of the strength of the cellulose fibers of the existing solvent-spun processes, it has also been observed that fiber strength is dependent upon the arrangement of the cellulose crystals within the fiber as well. 
     To clarify, cellulose may exist in amorphous or crystalline form. In fact, both amorphous and crystalline regions form within the cellulose fiber upon coagulation. The ratio and orientation of these regions vary, but in the existing solvent-spun processes both are determined during the coagulation step. 
     By way of example, when cellulose is coagulated in a typical solvent-spun process, some molecules randomly orient themselves in a crystalline matrix, the degree of crystallization being determined, generally, by the presence and amount of water in the coagulation medium. Neither the ratio nor the orientation of the crystalline regions can be controlled in such a process, because the crystallization occurs simultaneously with the coagulation of the fiber. 
     In view this, it was theorized by the present inventor that by separating the steps of coagulation and crystallization in a solvent-spun process, the ratio of crystalline and amorphous regions in the fiber could be controlled. Moreover, by applying tension to the fiber after coagulation but before crystallization, the fiber could be stretched thereby orienting the amorphous and crystalline regions therein. This would result in a solvent-spun cellulose fiber having high tensile strength. 
     Based on this theory, and in view of the fact that ZnCl 2  was known to be a good cellulose solvent, a process for producing high tensile strength, solvent-spun cellulose fiber was developed. 
     Additionally, a second embodiment of the present process was found to produce a low tensile strength cellulose fiber particularly suitable for food and pharmaceutical applications when food-grade starting materials and reagents were employed. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, high tensile strength, alkaline stable, solvent-spun cellulose fiber may be produced from dissolving grade cellulose having a DP in the range of from about 100 to 3000. 
     The present invention generally comprises a process in which cellulose is mixed, heated and solubilized in solvent comprising a solution of ZnCl 2 . The cellulose/ZnCl 2  solution is then extruded into a coagulation medium comprising one or more organic solvents wherein the cellulose coagulates to form fibers. The resulting fibers are then treated under tension to stretch and orient the fibers, as well as to remove any solvent or coagulation medium therefrom. The fibers are then placed in a water bath where recrystallization is fully achieved. The fibers may then be dried or otherwise treated for packaging, shipment or use. 
     More specifically, the process of the present invention comprises the steps of: 
     a) adding a solvent to cellulose to form a mixture, the solvent comprising a solution of zinc chloride (ZnCl 2 ), and the resulting mixture comprising cellulose/ZnCl 2  ; 
     the mixture having a final cellulose concentration in the range of from about 5% to about 45% weight to volume (w/v), and a final ZnCl 2  concentration in the range of from about 55% to about 80% weight to volume (w/w), more preferably from about 62% to about 76%, concentrations at the upper and lower extremes of this range are suggested for producing the strongest cellulose fibers; 
     b) stirring and heating the cellulose/ZnCl 2  mixture at a temperature ranging from about 40° C. to about 120° C, more preferably from about 40° C. to about 100° C., and most preferably at about 65° C., until the cellulose dissolves and the mixture becomes clear, thereby forming a cellulose/ZnCl 2  solution; 
     c) extruding the cellulose/ZnCl 2  solution into a coagulation medium wherein the cellulose in the cellulose/ZnCl 2  solution coagulates to form fiber, the fiber not being fully crystallized; 
     the coagulation medium comprising one or more alcohols or ketones, the alcohols generally being selected from the group consisting of straight or branched chain C 1  to C 4  alcohols such as methyl, ethyl, propyl and isopropyl-alcohol and the ketones generally being selected from the group consisting of C 3  to C 5  ketones such as acetone or methylethylketone (MEK); 
     d) removing the fiber from the coagulation medium and treating the fiber to remove residual solvent and coagulation medium therefrom; 
     e) applying tension to the fiber sufficient to stretch the fiber and orient the molecules therein; 
     the above treatment may comprise evaporation or other conventional means known to those skilled in the art; 
     f) submerging the fiber in a bath containing water to fully crystallize the fiber; 
     g) removing the fiber from the water bath for drying and/or further treatment. 
     Several optional manipulations may be performed at step a), for instance: 
     i) pre-wetting the surface of the cellulose with water (optionally containing one or more of the chlorides as listed below) prior to its dissolution in the ZnCl 2  solution, thereby facilitating the rapid formation of a homogenous cellulose/ZnCl 2  solution low in solid particles and having a chemical and physical structure suitable for extrusion; 
     ii) adding chlorides of magnesium, calcium, lithium or aluminum directly to the cellulose, or to the cellulose/ZnCl 2  mixture, thereby aiding in the mixing, dissolution and the extrusion of the cellulose/ZnCl 2  by lowering its viscosity. 
     As disclosed in sub-step i) above, pre-wetting the cellulose reduces the number of solid particles normally encountered when a solution of zinc chloride is added directly to dry cellulose. The presence of solid particles is generally disadvantageous in solvent-spun processes, for the solid particles tend to clog the narrow openings of the spinnerettes which are frequently employed in these processes. Furthermore, occasionally the DP of the cellulose in the cellulose/ZnCl 2  solution decreases prior to extrusion. Pre-wetting the cellulose prior to its mixture with the ZnCl 2  solvent appears to alleviate this problem as well. 
     In regard to coagulation step c), it should be noted that the coagulation medium serves to remove the ZnCl 2  and other salts present in the extruded cellulose by dissolving and diluting them in the medium. This results in a decreased concentration of ZnCl 2  in the extruded cellulose/ZnCl 2  solution so as to enable the cellulose to coagulate to form fibers. 
     It is also important to note that in keeping with the inventor&#39;s goal to separate the steps of coagulation and crystallization in the present process, the water content in the coagulation medium should be kept to a minimum in order to prevent premature recrystallization of the fiber prior to stretching and orientation. 
     Likewise, it should also be noted that when extruding the cellulose/ZnCl 2  solution into the coagulation medium, the solution may be extruded directly or indirectly into the medium. Direct extrusion entails the introduction of the cellulose/ZnCl 2  solution into the coagulation medium through a nozzle or spinnerette immediately at or below the surface of the medium, while indirect extrusion may be achieved, for example, by extrusion of the solution into air then into the coagulation medium. 
     While indirect extrusion methods may be employed in the process of the present invention, such methods are not preferred for they tend to form fibers of non-uniform dimension and strength. 
     The above description provides the basic steps in the formation of the high tensile strength cellulose fiber of the present invention. It will be apparent to those skilled in the art that modifications and variations can be made in this process without departing from the scope or spirit of the invention. For example, the fibers may be finished according to common practice, and/or various modifying agents may be added to the solvent, the coagulation medium or the crystallization bath. 
     Additionally, by slight modification to the present process, a cellulose fiber or film suitable for use in food and pharmaceutical applications may be produced. 
     For example, a food-grade cellulose fiber may be formed which can be woven into a netting or binding for use as a packaging on food products such as meats. Similarly, when the cellulose/ZnCl 2  solution of the present invention is extruded as a film, it may be used as an edible film or casing on sausages and the like. 
     To form products such as those listed above, food-grade reagents must be employed, namely, food-grade cellulose and a food-grade solvent. The present invention is important, therefore, for it teaches the use of ZnCl 2  as a cellulose solvent in a solvent-spun process, and it teaches a solvent-spun process in which the zinc chloride may be successfully and practically used to form a fiber suitable for the uses envisioned and disclosed herein. 
     The use of ZnCl 2  is advantageous in the present process for it is nontoxic and is available in food-grade quality. Furthermore, while the use of food-grade quality reagents in most processes is prohibitively expensive, ZnCl 2  is recoverable from the coagulation medium of the present process for reuse, thus significantly lowering its cost per use. 
     In the food applications listed above, the need for a fiber having a high tensile strength is not always great, therefore, after dissolution and extrusion the fibers (or film) may be removed from the coagulation medium and transferred directly into the crystallization bath. Alternatively, the fibers or film may also be coagulated and recrystallized simultaneously. This would result in fibers with tensile strengths of about 1.6 to 1.9 g/den (see Example 5 below). 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are set forth below: 
     EXAMPLE I 
     Cellulose (Avicel PH 101) was pre-wet by mixing with water. A 76% zinc chloride solution (w/v) was added to the pre-moistened cellulose and the mixture was stirred immediately and continuously at 60° C. for about 15 minutes, by which time the cellulose had dissolved. 
     When pre-wetting the cellulose, the amount of water added to each sample was controlled so that upon the addition of the 76% ZnCl 2  solution thereto, the final ZnCl 2  concentration in the resulting cellulose/ZnCl 2  mixtures ranged from 66% to 74.6% as depicted below in Table 1. The final cellulose concentration in each sample was 10% (w/v). 
     
                       TABLE 1______________________________________FI-  76%              CELLU- FINAL    TENSILEBER  ZnCl.sub.2        H.sub.2 O                 LOSE   CONC.    STRENGTHNo.  (ml)    (ml)     (g)    ZnCl.sub.2 (%)                                 (g/den)______________________________________1    2       0.23     0.223  74.4     4.12    2       0.44     0.244  72.7     3.33    2       0.68     0.268  70.8     3.04    2       0.98     0.298  68.6     2.85    2       1.35     0.335  66.0     4.5______________________________________ 
    
     Without cooling, the cellulose/ZnCl 2  solutions were extruded by syringe through a 22 gauge hypodermic needle into acetone which served as the coagulation media. After about 15 minutes, cellulose fibers about three feet in length were removed from the media. The ends of the fibers were fixed to a table and the fibers were then allowed to air dry. The drying caused the fibers to shrink. Due to their attachment to the table, this shrinkage exerted tension on the fibers causing them to stretch. 
     After the fibers were completely dried and stretched, they were submerged in a water, i.e. crystallization bath for about ten minutes. The fibers were then removed and dried in an oven at 60° C.. 
     The resulting fibers had tensile strengths as depicted in Table 1. 
     EXAMPLE II 
     A cellulose/ZnCl 2  mixture containing 15% cellulose (w/v) was prepared according to the procedure described in Example I. The final zinc chloride concentration in the mixture was 67% (w/w). ) After heating and dissolution, the cellulose/ZnCl 2  solution was extruded into six baths individually containing the following coagulation media: acetone, ethyl alcohol, acetone:water (2:1), acetone:water (5:1), ethyl alcohol:water (5:1), and acetone:ethyl alcohol (1:1). The resulting fibers were then treated as described in Example I. The tensile strengths of the 6 resulting fibers are listed in Table 2. 
     
                       TABLE 2______________________________________                TENSILE STRENGTHCOAGULATION BATH SOLVENT                (g/den)______________________________________Acetone              5.1Ethyl Alcohol        5.5Acetone:water (2:1)  2.4Acetone:water (5:1)  3.8Alcohol:water (5:1)  2.7Acetone:Alcohol* (1:1)                3.0______________________________________ *For purposes herein, &#34;alcohol&#34; comprises ethyl alcohol. 
    
     EXAMPLE III 
     A cellulose fiber was prepared from a cellulose/ZnCl 2  mixture as described in Example II using ethyl alcohol as the coagulation medium. The procedure set-forth in Example I was followed. The resulting fiber had the following characteristics: tensile strength-- 5.7 g/den and fiber elongation--13%. Solubility of the fiber in an alkaline solution was undetectable and the zinc content in the fiber was less than 0.4% by weight. 
     EXAMPLE IV 
     A cellulose fiber was formed using alpha-cellulose (DP--400, Sigma Chemical Co.) as the cellulose starting material and zinc chloride solution as the cellulose solvent. The final ZnCl 2  concentration in the cellulose/ZnCl 2  mixture was 67% (w/w). The cellulose concentration was 10% (w/v). 
     The procedure of Example I was followed, and acetone or ethyl alcohol were used as the coagulation media. The characteristics of the resulting fibers were compared and the results achieved are as follows: 
     1) coagulation medium--acetone, tensile strength--6.2 g/den, % elongation--15%; 
     2) coagulation medium--ethyl alcohol, tensile strength--3.6 g/den, % elongation--not calculated. 
     EXAMPLE V 
     Cellulose (Avicel PH101) was dissolved in a solution of zinc chloride and the mixture was prepared according to the procedure described in Example I. The final ZnCl 2  concentration in the cellulose/ZnCl 2  mixture was 67% (w/w) and the cellulose concentration was 10% (w/v). 
     The mixture was heated and the resulting solution extruded into a bath containing ethyl alcohol as the coagulation medium. Three fibers were formed. One fiber was removed from the bath, air dried and stretched. The second fiber was transferred from the coagulation medium and placed directly into a water bath without drying or stretching, while the third fiber was removed from the coagulation medium, dried and stretched and recrystallized in a water bath as described in Example I. 
     The tensile strengths of the three fibers were 1.6, 1.9 and 5.2 g/den, respectively. 
     From consideration of the specification and examples, and practice of the invention as disclosed herein, other embodiments of the invention will be apparent to those skilled in the art It is intended that the specification and examples be considered as exemplary only, with the scope and spirit of the invention being indicated by the following claims. 
     EXAMPLE VI 
     Cellulose (Avicel, microcrystalline cellulose, DP about 100, 30 g) was mixed with six ml of the calcium chloride solution (This solution was prepared by adding 0.84 g of CaCl 2 .2H 20  in 6.8 ml of water). Zinc chloride solution (48.7 g in 18 ml water) was then added to wet cellulose. The mixture was stirred at 75° C. about 15 min. with a spatula until cellulose dissolved, and a clear solution was obtained. The cellulose solution was transferred into a syringe and was extruded through a #27 gauge hypodermic needle into a coagulation bath containing ethanol. The coagulation fibers were then removed from coagulation bath, and dried in air. Precautions were taken to prevent shrinkage during drying. The fiber was then stretched and then soaked in water for about one hour. The fiber was then dried at 60° C. The tensile strength of the fibers were then tested and the results were: 
     
         ______________________________________Trial       Tensile strength                   elongation______________________________________1.          12.7        21.3%2.          11.7        22.8%3.          14.5        18.1%______________________________________ 
    
     EXAMPLE VII 
     Cellulose (alpha cellulose from Sigma Chemical Company, 15 g) was wetted with 60 ml of water. Zinc chloride solution (487 g in 180 ml water) was added to wet cellulose. The mixture was stirred with an egg beater at 70° C. for about 15 min. until a clear cellulose solution was formed. The cellulose solution was then spun through a spinnerette with 40 holes (80 microns in diameter) into a coagulation bath containing ethanol. The coagulated fiber was picket up by a reel. Some fiber was removed from reel and placed in a beaker containing ethanol to remove excess zinc chloride. They were then dried completely before they were placed into water. After soaking in water, the fibers were dried. The fibers were tested for tensile strength and elongation. The results are shown as follows: 
     
         ______________________________________Trial       Tensile strength                   elongation______________________________________1.          16.45       5.0%2.          15.82       5.6%______________________________________ 
    
     EXAMPLE VII 
     As Example VI, cellulose solution was prepared and extruded through a hypodermic needle into a coagulation bath. Except that the coagulation bath contained cold water (0° C.) to prevent crystallization of cellulose. The coagulated fiber were then removed from water and dried at 60° C. oven. Cautions were taken to prevent shrinkage during drying. The formed fibers were then soaked in water and then dried. The tensile strength was 8 g/den. Elongation was not recorded. 
     Examples VI and VII show a high strength cellulose fiber of from about 11 to nearly 17 g/den can be achieved.
The present invention relates to the production of high tensile strength, solvent-spun cellulose fiber which is stable in alkaline solutions. More particularly, the invention relates to a process in which cellulose is solubilized in a zinc chloride solution then extruded into a coagulation medium in which the cellulose fibers form. The fibers are removed from the coagulation medium, treated to remove the coagulation medium, stretched and recrystallized in water, thus forming the high tensile strength, solvent-spun cellulose fibers.