GB2361402A - Process for producing shaped cellulose bodies - Google Patents

Abstract

A process for the production of a shaped cellulose body, comprises the steps of:<BR> ```- providing a solution of non-chemically derivatised cellulose, wherein said solution comprises at least one solvent for dissolving said cellulose and at least one processing aid for assisting the extrusion of said solution through an orifice; and<BR> ```- extruding said cellulose solution through said office. The solvent is preferably N-methylmorpholine N-oxide. The processing aid is preferably a second solvent that is miscible with N-methylmorpholine N-oxide, and may be selected from dimethyl sulfoxide, tetramethyl ammonium chloride or N-methylol caprolactam.

Description

2361402 PROCESS FOR PRODUCING SHAPED CELLULOSE BODIES The present

invention relates to a process for the production of shaped cellulose bodies. In particular this invention relates to improvements in the processability of N-methylmorpholine N-oxide (NMMO) solutions of cellulose used to produce food casings for sausages.

Cellulose food casings are well known in the art for producing stuffed or wrapped foods such as sausages and the like. Cellulose casings are currently most commonly produced by the well known viscose process. For a brief discussion of this process see for example the introductory portion of US Patent, No. 5,277,857. Briefly, the viscose process involves making a solution of the sodium xanthate salt of cellulose and extruding this chemically derivatised cellulose solution into a coagulating and regenerating bath to form regenerated cellulose as a continuous tube. This process however presents some problems, for example carbon disulphide which is used to derivatise the cellulose is highly inflammable and toxic. Furthermore the viscose process uses sodium hydroxide, sulphuric acid and its salts, large amounts of water, and produces toxic gaseous sulfur by-products such as H2S' Clearly these chemicals present contamination and disposal problems for the manufacturer. Furthermore industrial waste disposal and degree of water usage may be strictly regulated.

2 An alternative process which is gaining useful recognition for forming cellulose articles such as sausage casings is the N-methylmorpholine N-oxide solution process.

This process does not chemically derivatise the cellulose, but instead involves cellulose dissolution in the NMMO solvent to f orm a solution known as a dope.

Briefly, the NMMO cellulose dope is made by mixing cellulose pulp with NMMO which contains about 50% water and then applying reduced pressure and heat so that the mixture boils at about 700C. Water is removed from the mixture by condensing the generated steam. At about 12% water content, in relation to the amount of NMMO, the NMMO forms a mono-hydrate and the cellulose then fully dissolves on increasing the temperature to 950C to form the dope.

Stabilisers are added to reduce degradation and discolouration of the end produced cellulose body. Further reduction of pressure removes any air bubbles from the dope which, at about 1000C, becomes a visco-elastic melt having high viscosity compared to viscose and a pronounced elastic behaviour. This whole procedure takes approximately 3 hours using laboratory scale equipment and is believed to take less than one hour using industrial scale equipment.

The dope is solid at room temperature (about 250C) and can be ground into pellets ready for feeding into for example a screw extruder. The solid dope is chemically stable to 3 deterioration, and pellets of this dope can be stored for a considerable time without substantially affecting subsequent dope extrusion performance and also the properties of cellulose casing made therefrom.

Sausage casings may be formed by extruding melted dope through an annular die orifice and passing the tubular cellulose solution extrudate into a precipitation bath containing water or a solution of NMMO in water. A positive pressure applied to the inside of the freshly extruded tube causes the tube to remain open, that is inflated and the extruded tube is longitudinally stretched by rotating nip rolls in the bath. This extrusion method is known as blown extrusion and results in oriented biaxially stretched tubes. Furthermore provision is made for the precipitation liquid to fill the inside of the tube. The precipitation liquid is a non-solvent for cellulose and a solvent for NMMO. The dissolved cellulose thus precipitates to form a solid cellulose semi-gel tube.

This cellulose tube is processed further to remove any remaining NMMO, and treated with a plasticising agent prior to drying when the sausage casing is formed.

Although the NMMO cellulose solution process has been described for the manufacture of sausage casings, see for example US 5,277,857, certain limitations exist with current technology. One such limitation is related to the 4 extrusion rate of NMMO cellulose dope. This rateis currently too low to satisfactorily meet the present market demand for sausage casings manufactured by the NMMO technology. It is thus desirable to improve the processability of the NMMO cellulose dope so that higher extrusion rates can be achieved without compromising casing characteristics such as uniformity and strength. Therefore it is an object of the present invention to obviate or mitigate these aforementioned problems.

The present invention provides a process for the production of a shaped cellulose body, comprising the steps of:

providing a solution of non-chemically derivatised cellulose, wherein said solution comprises at least one solvent for dissolving said cellulose and at least one processing aid for assisting the extrusion of said solution through an orifice; and extruding said cellulose solution through said orifice.

In particular the shaped cellulose body is a film, typically a seamless tubular film.

It is to be understood that suitable solvents for cellulose are those which possess a large dipole moment.

This requirement is described in US Patent, No. 4,255,300 wherein a comprehensive discussion of solvents suitable for dissolving cellulose can be found. Preferable solvents for dissolving cellulose are amine oxides, particularly cyclic amine oxides. Cyclic amine oxide solvents are described in relation to their ability to dissolve a host of compounds in US Patent, No. 3,447,939. The most desirable solvent for dissolving cellulose is N-methylmorpholine N-oxide (NMMO). This solvent is soluble in water which allows it to be washed or leached from a precipitated cellulose tube following extrusion into a water bath.

Processing aids are generally known in the art of extrusion technology and in the present case these aids may be added to the cellulose dope to alter the processability thereof during extrusion. It is to be understood that processability refers to the behaviour of a material during physical processing. For example, such aids may internally lubricate the extruder and die surfaces so that the cellulose dope passes through the extruder and flows from the die orifice more easily. Other aids may alter the viscosity of the dope so that a freer flow is achieved.

Some aids may individually alter the processability in several ways. Processing aids may thus increase extrudate output f rom the die at a given screw speed. Combinations of several processing aids may also be used. The present invention may in particular be applied to the production of sausage casings. These casings are formed by extruding the 6 cellulose solution through an annular die orifice to form a continuous seamless length of tube, commonly by the blown extrusion method.

The processing aid may be incorporated into the NMMO -cellulose dope by any suitable method. For example the aid may be added during preparation of the NMMO-cellulose dope prior to the extrusion. Alternatively the aid may be added to the NMMO-cellulose dope while the dope is resident within the extruder prior to passing through the die thereof. This is typically achieved by adding the aid, held in a hopper, through an entry port into the extruder shaft which contains the extruder screw and melted or partially melted NMMO-cellulose dope.

It should be understood that a less viscous cellulose solution should not be too fluid, that is, runny otherwise for example tube formation may be difficult to achieve; collapse or disintegration of the tubular wall, either during the blown extrusion process or following formation of the cellulose tube might occur. Also casing tubes made from less viscous solutions may have unacceptably thin walls, and hence have a low burst resistance. Such casings would not easily withstand the rigours of shirring and stuffing and therefore be unsuitable for use as sausage casings.

A suitable processing aid which may lower the viscosity of a cellulose solution is a suitable solvent, 7 that is a second solvent (for cellulose) which is different from the existing, or first, solvent. Such a second solvent should preferably be non-toxic and dissolve in the precipitation liquid for example water to ensure its effective removal from the cellulose tube during a washing treatment with the precipitation liquid. Further processing may be employed however to remove the second solvent from the precipitated cellulose tube. Such a second solvent should preferably, but not necessarily, be miscible with the first solvent, for example the amine oxide solvent, particularly the NMMO solvent.

Advantageously the second solvent may also allow larger amounts of cellulose to dissolve to give more concentrated cellulose solutions than are obtainable with the first is solvent alone. More concentrated cellulose solutions allow an increase in production capacity by allowing more cellulose casing to be produced per unit volume of dope.

More concentrated cellulose solutions tend to be more viscous than more dilute cellulose solutions at a given temperature. More viscous solutions are difficult to process, and may reduce the extruder output capacity.

Addition of a processing aid or second solvent may therefore overcome these problems by increasing the ease of processing of the cellulose solution. Use of a second solvent may be particularly advantageous because more concentrated solutions and better processable solutions may 8 be obtained.

A further advantage associated with using a second solvent is that the degree of cellulose de-polymerisation may be reduced compared to when a second solvent is not used. Depolymerisation is a process whereby the polymer chains of the cellulose break to give shorter chains. This may occur for a variety of reasons and its prevention will tend to maintain the cellulose molecular integrity. Shaped cellulose bodies made from dissolved cellulose which has experienced minimal de-polymerisation can match the properties, for example tensile strength, of the cellulose before dissolution more closely than callulose which has experienced greater amounts of de-polmerisation. use of a second solvent has the particular benefit of increasing the strength and stiffness of the resultant extruded cellulose casing, that is, the elastic modulus is increased. This is important because a stronger casing will better withstand the rigours of shearing and stuffing operations and general handling.

Preferred second solvents are dimethyl sulfoxide, tetra methyl ammonium chloride and N-methylol caprolactam.

The preferred second solvent is N-methylol caprolactam. A suitable method of incorporating the second solvent into the NMMO-cellulose solution is to add it during the dope preparation stage. Another suitable method is to incorporate the second solvent in a relatively large amount 9 into the NMMO-cellulose dope which may then be used as a master additive. This additive can be added to the NMMOcellulose dope while it is resident in an extruder via an empty port so that intimate admixture of the second solvent with the dope occurs prior to expulsion through a die orifice. The resultant bicomponent solvent may then be used to dissolve cellulose to form a dope solution having reduced viscosity compared to dope lacking the second solvent for a given concentration of cellulose. The extrusion speed may be increased when using such a lower viscosity dope so that speeds in excess of 5 metres per minute (m.min-') of extrudate, that is of tubular cellulose sausage casing can be attained. Speeds greater than 20m. min-' are desirable, preferably greater than 50m.min-1. To meet current market demands for sausage casing production, speeds of between 80 and 100m.min-1 are most suitable. Such extrusion speeds have previously been unobtainable with NMMO-cellulose solutions without compromising physical properties of the extruded cellulose tube. This represents a significant step forwards in bringing NMMO-cellulose solution extrusion technology to production levels which are competitive with existing food casing production technologies.

Embodiments of the present invention will now be described by way of non-limiting example, with reference to Figure 1 which shows a schematic representation of an NMMO- cellulose processing apparatus for providing tubular cellulose casings.

The apparatus comprises an extruder 10 which is typically a screw extruder, a die 25, water supply and water return means designated 50 and 55 respectively, and pressure control means 45.

The NMMO-cellulose dope is contained in a feed hopper which is situated on the extruder screw shaft 5. The shaft 5 and die 25 are heated to about 1000 C. The dope is typically used as solid pellets or granules and passes through an entry port into the extruder shaft, where it then melts. The melted dope is simultaneously mixed and transported along the extruder shaft 5 by the action of the turning screw. The melted dope travels along the shaft 5 in the direction indicated by arrow 7 and passes through a filter screen (not shown) to a gear metering pump section 20. The processing aid such as a second solvent may be added to the dope during its preparation stage so that modified dope may be added to the hopper 15 and extruded directly. However further methods of adding the processing aid may be used such as adding a processing aid down stream of the hopper 15 at a general position along the shaft 5 indicated by arrow 17. The dope and processing aid are together resident in the extruder shaft 5 for a time sufficient for their intimate admixture prior to extrusion through die 25. An annular die is used to give seamless 11 tubular f ilm. The die is positioned above a bath 40 containing a precipitation liquid which is usually water at a temperature of 100C - 200C.

The freshly extruded dope is allowed to pass into the water bath 40 whereupon the cellulose precipitates to give a solid semi-gel tube 35. Water is provided to the inside of tube 35 by a conduit and feed pump 50 so that the cellulose also precipitates at the interior of the tube.

During precipitation of the cellulose some NMMO and processing aid will dissolve in the water to give a dilute aqueous solution of NMMO. This solution is withdrawn from the tube interior via a conduit and pump 55, and replaced with fresh water via the conduit and pump 50. This water replenishment ensures effective cellulose precipitation in is the interior of the extruded tube. The portion, 30, of extrudate positioned between the die and precipitation bath is pressurised internally with a pressure of between 0.2 - 2mbar above atmospheric pressure supplied via pressure control means 45. This extrusion process in combination with nip rolls 4 which pull the tube 35 longitudinally through the bath 40 gives an oriented biaxially stretched tube. The water/NMMO/processing aid precipitation bath mixture is further processed to recover the dissolved NMMO and processing aid which may then be used to prepare further quantities of cellulose -NMMO-process ing aid dope.

It is possible to achieve NMMO recoveries of up to 99.5% 12 which represents excellent conservation of material.

Further embodiments of the invention may be envisaged without departing from the inventive concept. For example a solid aid rather than a liquid solvent may be used to improve the NMMO-cellulose dope processability.

Claims (15)

13 CLAIMS

1 A process for the production of a shaped cellulose body, comprising the steps of:

providing a solution of non-chemically derivatised cellulose, wherein said solution comprises at least one solvent for dissolving said cellulose and at least one processing aid for assisting the extrusion of said solution through an orifice; and extruding said cellulose solution through said orifice.

2. The process according to claim 1 wherein the shaped cellulose body is a film.

3. The process according to claim 2 wherein said film is a seamless tubular film.

4. The process according to any preceding claim wherein said solvent is an amine oxide.

5. The process according to claim 4 wherein said amine oxide is a cyclic amine oxide.

6. The process according to claim 5 wherein said cyclic amine oxide is N-methylmorpholine N-oxide.

7. The process according to any preceding claim wherein 14 said processing aid is a second solvent for cellulose different from the said at least one solvent.

8. The process according to claim 7 wherein said second solvent is miscible with the said at least one solvent,

9. The process according to claim 8 wherein said second solvent is miscible with the said amine oxide solvent.

10. The process according to claim 9 wherein said second solvent is miscible with N-methylmorpholine N-oxide.

11. The process according to claim 10 wherein said second solvent is chosen from dimethyl sulfoxide, tetra methyl ammonium chloride and N-methylol caprolactam.

12. The process according to claim 11 wherein said second solvent is N-methylol caprolactam.

13. The process according to any previous claim wherein said extrusion is conducted at a speed of greater than m.min-' of extrudate,

14. The process according to claim 13 wherein said speed is greater than 20m.min-1.

15. The process according to claim 14 wherein said speed is greater than 50m.min-1.

1G. The process according to claim 15 wherein said speed is between 80m.min-1 and 100m.min-1.