Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. Cellulose food casings generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use.
Cellulose food casings generally are used in one of two forms. In one form the casing consists of a tubular film of pure regenerated and non-reinforced cellulose having a wall thickness ranging from about 0.025 mm to about 0.076 mm and made in tube diameters of about 14.5 mm to 203.2 mm. The second form is a reinforced casing wherein the tubular wall of the casing consists of a regenerated cellulose bonded to a paper web. Such reinforced casings are commonly called "fibrous" casings to distinguish them from the nonreinforced cellulose casings. Fibrous casings have a wall thickness in the range of 0.050 mm to 0.102 mm thick and are made in diameters of about 40.6 mm to 193 mm or greater. This invention relates to manufacture of the non-reinforced type of cellulose casing hereinafter referred to simply as "cellulose casing".
The cellulose for making this casing is most commonly produced by the so called "viscose process" wherein viscose, a soluble cellulose derivative, is extruded as a tubular film through an annular die into coagulating and regenerating baths to produce a tube of regenerated cellulose. This tube is subsequently washed, plasticized with glycerine or other polyol, and dried. Drying usually is accomplished while the tube is inflated with air at a pressure sufficient both to maintain a constant tube diameter and to orient the film.
The viscose process for making cellulose is well known in the art. Briefly, in the viscose process a natural cellulose such as wood pulp or cotton linters first is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged and treated with carbon disulfide to form cellulose xanthate which is a cellulose derivative. The cellulose xanthate is dissolved in a weak caustic solution. The resulting solution or "viscose" is ripened, filtered, deaerated and extruded. The pulp source and time of aging the alkali cellulose are selected depending upon whether the viscose will be used to make fibrous casing or nonreinforced cellulose casing. For extrusion of a nonreinforced cellulose casing, the selection is such that a relatively more viscous solution is used.
The viscose is extruded as a tube through an annular die and about a self centering mandrel into coagulation and regenerating baths containing salts and sulfuric acid. In the acidic baths the cellulose xanthate, e.g. viscose, is converted back to cellulose. In this respect, the acid bath decomposes the cellulose xanthate with the result that a pure form of cellulose is coagulated and regenerated. Initially, the coagulated and regenerated cellulose is in a gel state. In this gel state the cellulose tube first is run through a series of rinse water dip tanks to remove by-products formed during regeneration. The gel tube then is treated with a glycerine humectant and dried to about 10% moisture based on total casing weight. As noted above, the gel tube is inflated during the drying process to a pressure sufficient to provide a degree of orientation to the dried cellulose tube.
During regeneration of the cellulose from the xanthate solution, sulfur products are liberated and gases such as hydrogen sulfide, carbon disulfide and carbon dioxide are released through both the inner and outer surfaces of the gel tube. It should be appreciated that the gases produced as by-products during regeneration are noxious and toxic so their containment and recovery imposes a considerable burden on the manufacturing process. Moreover, gases generated at the internal surface of the extruded tube can accumulate within the tubular casing and consequently present special problems. The tubular casing while in its gel state is expansible and the pressure build up of gases accumulating within the gel casing causes undesirable diameter variations. To prevent this, the gel casing is punctured periodically to vent the accumulated gases. This puncturing process, involving procedures to puncture, vent, and then seal the punctured gel tube, results in an undesirable interruption of the manufacturing process. Also, gases which evolve within the casing wall may become entrapped causing bubbles which weaken the casing and detract from its stuffability and performance.
Also, the casing in its gel state to some extent retains low residual levels of the sulfur compounds produced during regeneration. While care is taken to remove all residual sulfur compounds by washing the gel tube prior to drying, the dried casing may still contain trace amounts of these compounds.
Despite the problems inherent with the viscose process as described above, it nevertheless remains today as the most commonly used process for the production of cellulose casing for the food processing industry.
An alternate cellulose production method involves forming a cellulose solution by means of a simple dissolution rather than requiring prior derivatization to form a soluble substance (as in the viscose process). A cellulose dissolution process is described in U.S. Pat. No. 2,179,181. This patent discloses the dissolution of natural cellulose by a tertiary amine N-oxide to produce solutions of relatively low solids content, for example 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine N-oxide. The cellulose in the resulting solution is nonderivatized prior to dissolution. U.S. Pat. No. 3,447,939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the tertiary amine N-oxide solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament.
More recent patents such as U.S. Pat. Nos. 4,145,532 and 4,426,288 improve upon the teachings of the '939 Patent. U.S. Pat. No. 4,145,532 discloses a process for making a solution of cellulose in a tertiary amine oxide such as NMMO which contains 10-35% by weight of cellulose. This higher solids content, achieved in part by including an amount of water (from 1.4% to about 29% by weight) in the tertiary amine oxide solvent, provides a solution adapted for shaping into a cellulosic article such as by extrusion or spinning. In U.S. Pat. No. 4,426,288 the NMMO-cellulose solution contains an additive which reduces decomposition of the cellulose polymer chain so that molding or spinning substances are obtained with only slight discoloration and which will yield molded shapes distinguished by improved strengths upon precipitation in a nonsolvent such as water.
Using NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose, as in the viscose process. Consequently it eliminates the disadvantages attendant to the viscose process such as the problems associated with the generation of toxic and noxious gases and sulfur compounds.
However, while nonderivatized cellulose resulting from the process of dissolving cellulose in NMMO eliminates certain problems associated with the viscose process, to applicants' knowledge, NMMO-cellulose solutions heretofore have not been used in the manufacture of cellulose food casings. This perhaps is due in part to the fact that the nonderivatized cellulose solution is thermoplastic with a melting point of about 65.degree. C. so it is normally solid at the temperature heretofore used in the extrusion of viscose (e.g. cellulose xanthate) for producing cellulose food casings.
Cellulose dissolution generally occurs by four methods: it behaves as an electron donor base, or an electron acceptor acid, or complexes with another reagent, or forms a derivative in which the cellulose is covalently bonded through alcohol groups with various reagents to form new molecules. The latter includes sodium cellulose xanthate, ie. cellulose being an alcohol can react to make esters such as xanthate derivative which is soluble in aqueous, nonaqueous or strongly polar organic solvents. The solubilization is mainly due to the disrupting of the hydrogen bonding by the derivative bonds. The salient feature of this step is that the derivatizing groups can be easily removed by hydroxylic materials such as, for example, aqueous acid to yield pure cellulose. In this type of dissolution the cellulose is truly regenerated whereas in the first three mentioned types of dissolution the cellulose is mainly precipitated or coagulated, ie. reorganized into a shape. Notwithstanding these differences, the resulting cellulose article is chemically identical irrespective of whether it is reprecipitated from solutions or chemically regenerated.
It is speculated that another reason why nonderivatized cellulose has not been commercially used in manufacture of food casings is that the solution at 65.degree. C. has a viscosity significantly higher than the viscosity of the derivatized cellulose heretofore used in the production of cellulose food casings. In particular, nonderivatized cellulose in solution may have a molecular weight of about 80,000 to 150,000 and a viscosity in the range of about 1,000,000 to 3,500,000 centipoises. The high molecular weight and viscosity is because the dissolution of the cellulose does not affect the degree of polymerization. Viscose for casing manufacture (wherein the degree of polymerization is affected by the derivatization process) has a molecular weight in the range of about 95,000 to 115,000 for nonfibrous casing and a viscosity of 5,000 to 30,000 centipoises.
From a cellulose article manufacturing process standpoint these differences are important because after dissolution the process steps (including cellulose recovery) are dependent on whether cellulose has entered into a covalent bond with the solubilizing reagent, i.e., has been derivatized. This is so in the case of the well-known and commercially practiced viscose process. When a cellulose derivative is processed into the shaped article, the derivative such as viscose is first partially coagulated in the extrusion bath and then subsequently hydrolyzed back to cellulose, i.e., cellulose is regenerated. During this hydrolysis and while the derivative is still in a "plastic" state, the reforming cellulose crystallites can be stretched and oriented to give desirable commercial properties such as high tensile strength or burst strength. However, a disadvantage of this general approach is that since a cellulose derivative has been hydrolyzed, additional byproducts are formed. This significantly complicates cellulose recovery.
By contrast in the nonderivative cellulose dissolution methods such as NMMO/H.sub.2 O, orienting the cellulose molecules during the reorganization of the cellulose article is more difficult because there is no covalent bond to break. So reorganization is essentially a physical dilution or decomplexation. However recovery is less complex and, at least in the cellulose/NMMO/H.sub.2 O system, commercially feasible.
The prior art such as McCorsley III U.S. Pat. No. 4,246,221 and East German Patent No. DD 218 121 has taught that such nonderivatized cellulose containing mixtures with NMMO and water may be forced through a nozzle and longitudinally guided through a 12 inch long air gap into a precipitating bath to form very small diameter solid fibers. More recently the nonderivatized cellulose fiber spinning prior art teaches that such long air path lengths should be avoided. As for example stated in Jurkovic et al U.S. Pat. No. 5,252,284, a long air gap leads to sticking of the fibers, uncertainties in spinning and fiber breakage at high degrees of drawing. According to Jurkovic et al, by using selected orifice diameters and nozzle channel lengths, the air gap is desirably reduced to at most 35 mm (1.4 inches).
It will be appreciated by those skilled in the art that manufacture of individual solid cellulose fibers by extrusion through orifices of 2-4 mils diameter is nonanalogous to manufacture of cellulose food casings which are extruded as a hollow tube of at least about 700 mils inside diameter with wall thickness typically on the order of 40 mils.
U.S. patent application Ser. No. 07/822,506 filed Jan. 17, 1992 in the names of Paul E. DuCharme et al and issued on Jan. 11, 1994 as U.S. Pat. No. 5,277,857 discloses a method of and apparatus for manufacturing cellulose food casing from a solution comprising nonderivatized cellulose, NMMO and water, and the specification of the DuCharme et al application is incorporated herein by reference.
According to the DuCharme et al invention, it was unexpectedly discovered that nonderivatized cellulose solutions are suitable for use in making both cellulose and fibrous food casings. Nonderivatized cellulose in a molten state can be extruded as a tubular film into a nonsolvent liquid such as a water bath. For purposes of this specification "nonderivatized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces such as hydrogen bonding. "Nonsolvent" means a liquid which is not a cellulose solvent. In the water bath, the nonderivatized cellulose precipitates and the resulting gel tube can be treated with water, a polyhydric alcohol such as glycerine, or other water soluble softening agent such as a polyalkylene oxide or a polyalkylene glycol prior to drying.
More specifically, in the manufacturing method of the DuCharme et al application the following steps are employed:
(a) providing a solution comprising nonderivatized cellulose in an amine oxide solvent; PA0 (b) downwardly extruding the solutions from an annular orifice to form a seamless tube of at least 14.5 mm diameter; PA0 (c) passing the extruded seamless tube of solution downwardly from the orifice first through an air gap and then into a bath of nonsolvent liquid; PA0 (d) introducing a nonsolvent liquid into the interior of said extruded seamless tube at a location below the annular orifice and above the surface of the bath of nonsolvent liquid; PA0 (e) downwardly flowing the nonsolvent liquid concurrently with the inner surface of said downwardly moving extruded seamless tube of solution and into said bath as the tube moves through said air gap, and contacting the inner surface of said extruded seamless tube with nonsolvent liquid in the course of said concurrent flows to precipitate nonderivatized cellulose at said inner surface from said solution; PA0 (f) maintaining said extruded seamless tube of solution in said bath with its inner and outer surfaces in direct contact with said nonsolvent liquid thereby further precipitating said nonderivatized cellulose from said solution and forming a nonderivatized cellulose tube; and PA0 (g) removing said nonderivatized cellulose tube from said bath and contacting same with a water soluble softener. PA0 1) Cut six samples 2 inches long machine direction (MD) .times.1 inch long transverse direction (TD), and identify as MD. PA0 2) Cut six samples 1 inch long MD .times.greater than 1 inch long TD, and identify as TD. PA0 3) Measure thickness of samples with a micrometer having a range up to 0.1 inch and accuracy of 0.0001 inch, basing measurement on minimum thickness (the weakest point). PA0 4) Soak samples in room temperature water for 20 minutes. PA0 5) Measure flat width of wet samples as well as their thickness in the same manner as 3). PA0 6) Set the Testing Machine crosshead speed and the chart speed at 20 inches/minute. PA0 7) Set the gauge length at 1 inch and zero the pen. PA0 8) Calibrate the Testing Machine to a full scale load of 20 lbs. PA0 9) Clamp the specimen squarely between the jaws. PA0 10) Run the crosshead down until the specimen ruptures. PA0 11) Calculate the specimen tensile strength in lbs/inch.multidot.mil thickness in accordance with the following formula: ##EQU1## 12) Calculate the arithmetic average of six tensile strength readings for MD and the six readings for TD. These are the values reported hereinafter in Tables A and B.
The nonderivatized cellulose food casings prepared by the teachings of the DuCharme et al invention are somewhat limited in the sense that their tensile strength properties are not equivalent to those of commercially employed viscose-derived cellulose casing. More particularly, based on a flat width of about 2.24 inches and wall thickness of about 0.80 mil, the machine direction (MD) tensile strengths of the NMMO-based nonderivatized cellulose tube prepared according to the teachings of DuCharme et al is about 3.77 lbs/inch.multidot.mil, and the MD tensile strengths of a viscose-derived NOJAX type cellulose food casing manufactured and sold by Viskase Corporation is about 4.18. So from the MD tensile strength standpoint, the two casings are comparable.
However, the transverse direction (TD) tensile strength of the NMMO-based nonderivatized cellulose tube is about 1.60, whereas the NOJAX cellulose tube has TD tensile strength of about 3.15. It will be apparent from the foregoing that the former's TD strength is limiting and further that the NMMO-based nonderivatized tube does not have balanced tensile strengths, i.e. the MD/TD is about 3.76 in contrast to the balanced NOJAX food casing tensile strength where the MD/TD ratio is about 1.33.
An object of this invention is to provide an improved method of forming a seamless cellulose tube (suitable for use as a food casing) from a solution comprising nonderivatized cellulose, tertiary amine N-oxide and water.
Another object is to provide such an improved method from a nonderivatized cellulose-NMMO-water solution yielding a cellulose tube with TD tensile strength of at least about 2.0.
A further object is to provide such an improved method yielding a cellulose tube with both MD and TD tensile strengths of at least about 2.5 and an MD/TD tensile strength ratio below about 2.