WO1996014199A1

WO1996014199A1 - Extrusion method

Info

Publication number: WO1996014199A1
Application number: PCT/NZ1995/000110
Authority: WO
Inventors: Roderick Munro Mcdonald
Original assignee: The Horticulture And Food Research Institute Of New Zealand Limited
Priority date: 1994-11-02
Filing date: 1995-10-27
Publication date: 1996-05-17
Also published as: AU3818395A

Abstract

A method for extruding material having a sharp melting point characterised by the steps of: a) extruding the material in a molten state past the first cooling means capable of cooling the molten material to just above the melting point of the material, and b) extruding the now cooled molten material past the second cooling means capable of freezing the molten material so that the material remains substantially intact when removed from the extrusion process.

Description

EXTRUSION METHOD

TECHNICAL FIELD

This invention relates to an extrusion method.

In particular, the present invention relates to an extrusion method most suitable for compositions having a sharp melting point.

Reference throughout this specification shall be made to a device for producing controlled-release medicaments for animal disease control, and particularly to the manufacture of zinc-oxide-containing boluses for the control of facial eczema.

However, it should be appreciated that discussion relating to these boluses is made purely by way of example, and the present invention has applications far wider than this.

BACKGROUND ART

The ruminant animal disease, facial eczema, which is due to the ingestion of a fungal toxin from pasture can be prevented by administration of zinc, which protects the liver from damage. Administration is often done by regular (usually daily) forcible injection of a zinc compound into the animal's stomach through the mouth, which is known as drenching. It is also done by adding soluble zinc salts to the farm water troughs.

Drenching is only economic when the animals are being handled frequently for other purposes, for example for milking. This does not apply to animals which are farmed for reasons other than milking such as meat production. Provision of zinc salts in the water supply is unsatisfactory because of the high variation in water intake which occurs between animals and between days. As a consequence a significant number of animals in a herd are likely to be either unprotected or exposed to toxic doses of zinc.

It has been shown that an efficient and reliable method of administering zinc to animals is by means of a slow-release device. Zinc, for example in the form of zinc oxide, can be released from the device at a substantially constant rate into the rumen, and this appears to give a high level of protection. The rate of release can be such that a single device may be effective for more than three weeks, which makes forcible administration by mouth practical even for animals which are not normally handled frequently.

A suitable slow-release device has been made using a mixture of zinc oxide and compounds which are slowly water-soluble. These are thoroughly mixed and formed into the shape of a rod, then coated with a layer of water-insoluble material except at one or both ends. The whole construction is referred to as a bolus.

The effect of this coating is to restrict dissolution of the rod to the end or ends of the rod, so that the surface area available for dissolution, and hence the rate of dissolution, is substantially constant throughout the life of the device. The coating is designed so that it crumbles away as the rod within erodes, so that the eroding face is not shielded by the coating from abrasion by stomach contents.

While boluses are an effective means by which zinc can be delivered, manufacture of the boluses has a number of requirements as described below. Requirement 1

In order that the bolus should reliably last the required time, it must not fracture while in the stomach. This requires that the bolus should be strong, and also that the length-to-diameter ratio should not be too great.

Requirement 2

The daily quantity of zinc required by the animal for facial eczema protection is high compared with the quantities typically required for trace elements, and to achieve a reasonable term of protection the quantity of zinc oxide must be large, for example 40 grams for a sheep. The diameter of bolus which can be inserted into a sheep by mouth without undue distress to the animal is limited to less than 20 mm.

Requirement 3

As in a ruminant animal, slowly-eroding zinc oxide boluses are prone to fail due to the formation of insoluble deposits on the boluses, it is preferred these deposits do not form.

Requirement 4

It has been found that boluses may be regurgitated, and hence protection may be lost, if they are not of high density, preferably greater than 3000 kg/m³. The alternative to high density is the addition of a retention device such as unfolding plastic extensions or "wings", but this alternative can increase the bolus cost substantially or possibly damage the animal internally. Requirement 5

Requirements 1 and 2 cannot be met by dosing the animal with a number of short boluses in a single dosing operation, because the slow erosion rate required for each bolus would make them prone to failure from deposit formation. Therefore long boluses are required.

Requirement 6

To meet Requirement 3 it has been found necessary to formulate the boluses so that the erosion rate is reasonably high, and only one or two boluses are used in a dose. To further meet Requirements 1, 2 and 4 it is also necessary to keep the bolus volume to a minimum. This means that the ratio of the weight of zinc oxide to the weight of other ingredients or components of the device must be high, and the formulation should be physically compressed to a high degree.

Requirement 7

To maintain a substantially constant release rate the slightly-water- soluble materials must be uniformly distributed throughout the volume of the rod, and intimately mixed with the zinc oxide powder. This cannot be adequately achieved by simple mixing of the solid ingredients. It has been found by the applicant that it is necessary to melt the slightly-water- soluble materials and mix them with the zinc oxide to form a paste. Any further processing must then be done without allowing the paste to solidify, or the hard mass so formed is difficult to handle in an economical manner. Requirement 8

The low unit value of the animals which must be treated means that the production process must be low in cost.

Requirement 9

The applicant's solution to Requirements 6, 7 and 8 is to use a heated extrusion device to act simultaneously as ingredient mixer and high- pressure pump. The rod shape required for the core of the bolus can then be formed continuously by extrusion through an orifice. This is an economical production method, especially if the boluses are to be made with two eroding faces, because the water-insoluble coating can be applied continuously on the extrusion line, and the boluses formed by simply cutting the rod to length.

When the boluses are to be made with a single eroding face the process is less simple, because the extruded rod has to be cut and rounded on one end before coating. Nevertheless the extrusion process is still attractive because of the simplicity of materials handling, low labour costs and homogeneity of product.

The use of extrusion allows a high pressure to be applied in the manufacture of the bolus so as to achieve the high density required as outlined previously.

This method has a problem, however, in that the formulations found to be suitable for making a satisfactory bolus do not have the physical properties considered to be necessary for conventional extrusion methods - for example, as used in the plastics industry. The desired formulation in this example has a large percentage of zinc oxide and a small percentage of water soluble compounds.

In the plastics industry, plastics material usually has what is known as a wide melting point range. Although there may be an average melting point for the plastics material, the actual melting range is quite wide on account of the differing lengths of polymer chains in the plastics material. This means that the plastics material is not too sensitive to temperature and can be extruded with an even consistency over a temperature range of a few degrees.

In addition to the effect of a wide melting point range, an even consistency of plastics material is also achieved on account of the relatively high melt strength of the plastics material. That is, although part of the extrusion of plastics material may be closer to solidifying than the rest of the material, the more solid material does not shear from the more molten material. Instead, the whole of the plastics material within the extruder die tends to move together at the same rate.

With formulations derived by the applicant for zinc boluses, the molten material is pumped along a die and cooled by heat conduction through the walls of the die like most plastics materials. However, at some point the material in contact with the die will solidify. Because the formulations have a sharp melting point and a low "melt strength", the material a short distance from the wall of the die has low shear strength. The consequence is that a layer freezes to the wall of the die and stops moving, while the inner part of the extruded material continues to flow freely. Further cooling of the inner part is inhibited by the insulating properties of the frozen outer layer. Thus, at the end of the die there emerges initially a thin stream of molten material, which may eventually stop flowing.

The result of all of the above may be a jammed die, or a pattern of alternating molten and frozen sections in the extruded rod. Such a rod will have poor strength and uniformity. Further, because of the low tensile strength of the solid formulation, this rod cannot be readily pulled from the extrusion die, unlike most plastics extrusions.

It can be seen that conventional methods of plastics material extrusion and other types of extrusion cannot be readily applied to this particular problem.

It is an object of the present invention to address the above problems or at least to provide the public with a useful choice.

Further aspects and advantages of the present invention will become apparent from the following description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided a method for extruding material having a sharp melting point characterised by the steps of:

a) extruding the material in a molten state past a first cooling means capable of cooling the molten material to just above the melting point of the material, and b) extruding the now cooled molten material past a second cooling means capable of freezing the molten material so that the material remains substantially intact when removed from the extrusion process.

According to a further embodiment of the present invention, there is provided an animal treatment remedy produced by the aforementioned method.

Throughout this specification use of the present invention will be described in relation to material as being formulations including zinc oxide, however it should be appreciated that the present invention has applications for a wide variety of other formulations in which the molten formulation has a relatively sharp melting point and a low melt strength.

It can be seen in the present invention that it overcomes a number of the problems associated with the prior art.

Cooling the material to just above its melting point prior to freezing the material addresses the two problems of sharp melting point and low melt strength. This is particularly so if in preferred embodiments the whole of the material is cooled to the same temperature, or at least has a minimal temperature gradient.

With the whole of the material being just above melting point, the material can be rapidly frozen without having substantial temperature gradients forming across the material. That is, applying a second cooling means to freeze the material will not cause the outer skin of the material to freeze leaving a highly molten interior. Further, if a temperature gradient does form, the shear or melt strength of the material is greater than at higher temperatures. This means that the resultant sluggish consistency of the material tends to carry the whole of the material through the extruder rather than having the middle part of the material breaking away from the outer skin.

Reference has been made to the first cooling means and second cooling means. It shall be appreciated however that these are terms which are not intended to be physically limiting on the apparatus used in accordance to the present invention. For example, the first cooling means and the second cooling means may form part of a single cooling unit having differing temperatures at different locations in the cooling process. Alternatively, there may be multiple cooling units provided which are used to thermally manipulate the material in accordance with the present invention.

In typical extrusion processes, the extrusion die is relatively short in length, being only a few centimetres. However, the applicant has found that at typical extrusion speeds this short distance does not allow the whole of the material in the die to reach substantially the same temperature.

According to one aspect of the present invention there is provided a method wherein there is a first passage associated with the first cooling means, and a second passage associated with the second cooling means, characterised in that the first passage is longer than the second passage.

Therefore, in preferred embodiments of the present invention, there is provided an extrusion passage which is considerably longer than conventional passages, being in the order of one to two metres in length. It is envisaged that the majority of the length of this extrusion passage would have the first cooling means applied to it. As the second cooling means is concerned with more rapid cooling/freezing of the material, it is envisaged that the length of the passage associated with it will be considerably less than that associated with the first cooling means.

In a preferred embodiment of the present invention there is provided a method of producing a substantially rod-shaped extrusion with high density, uniformity and homogeneity from materials with a sharp melting point and high melt strength, characterised by the steps of: a) mixing the ingredients at a temperature below the melting point of any of the ingredients, and b) feeding the coarsely mixed ingredients to an extruder which has a number of zones along its axial length, the temperature of each zone being maintained substantially independently, and c) ensuring that the first zone to which the ingredients are transported is at a temperature below the melting point of any of the ingredients, and d) ensuring that a subsequent zone is at a temperature higher than the melting point of some of the ingredients, and e) ensuring that some recirculation of the mixture takes place within the machine, in order to achieve thorough mixing and a high degree of homogeneity within the mix, and f) allowing the resulting paste to pass through a long passage or die which has walls maintained at a temperature a little above the freezing point of the paste, and g) ensuring that the passage is long enough that the mixture reaches a high viscosity over most of the cross-section of the passage, and h) allowing the resulting paste to pass through a second passage or die which has walls maintained at a temperature a little below the freezing point of the paste, and i) supplying sufficient pressure that the adhesion between solidifying paste and the die walls is broken, and that the mixture is extruded out of the die with a substantially uniform velocity over the whole cross-section of the die, and j) collecting the extruded rod at the exit of the die, and k) optionally shaping the end of the rod, and

1) perhaps coating the rod, and m ) cutting the rod to length, and n) collecting the cut rods for further processing or packing.

Throughout this specification the shape of the extruded object is described as a rod, but other shapes may be produced instead. For example an extrusion in the form of a fluted column may have advantages for improving the durability of the coating, or an extrusion with an oval cross section may ease the administration of the boluses.

The handling of the raw materials can be a simple operation, specifically placing bulk quantities of the raw materials into a simple mixer, mixing for a short while, and feeding the mix at a controlled rate into the extruder. All these operations could be carried out in a single machine, thereby minimising handling labour and dust production. Melting, mixing to a homogeneous mass and coating of solid particles with molten material, and generation of pressure to perform the extrusion may all take place within a single machine with very few moving parts, which typically will be a screw extruder (although other extruders could be used). Production of the rod is preferably a continuous process and it is possible to maintain a high degree of uniformity of composition, density and strength throughout the length of the rod. Very little labour or supervision is likely to be required.

The preferred extrusion device may be a single-screw or twin-screw extruder, in which materials are conveyed along a passage under the action of a rotating screw or screws. In the first section of such an extruder, the clearance between screws and passage walls, the shape of the screws and the viscosity of the mixture should be such that little of the conveyed material can escape the conveying effect, and very high pressures can be generated. These are necessary for the final rod- forming stage.

In the second section of the extruder the mixture can be melted and as a result of the reduced viscosity, or by a change in the clearance or shape of the screws a substantial flow of material against the conveying direction takes place, thereby achieving a high degree of homogeneity in the mixture.

In some embodiments of the present invention there may be provided an additional feed device to assist in the movement of material during the extrusion process. This feed device may come in a variety of forms and in some embodiments may be the provision of a vacuum. However, in preferred embodiments the feed device is in the form of pump (say an hydraulic motor or gear pump) which is situated at the end of the extruder before the first cooling means. Such an additional feed means assists in ensuring that there is steady flow of the material through the extrusion process. The applicants have found that the pressure applied by the first feed device or extruder and the subsequent pressure applied by the second feed device, say gear pump, affects the quality of the product produced. For example, if the extruder does not apply any additional pressure to the material but just mixes it, and the gear pump applies pressure of between 3 and 8 MPa, a high quality structurally sound product is produced.

In some embodiments the extruder may apply all of the pressure and the gear pump may be at the same pressure as the extruder. This still produces a structurally sound product but as not as high a quality as that previously discussed.

In the first part of the extrusion die the wall temperature can be accurately controlled by the first cooling means at a temperature which is just above the temperature at which the mixture will freeze and adhere to the walls of the die. The temperature may be maintained by, for example, circulating a liquid such as water in a jacket around the die. Typically the water temperature would be less than 2 degrees Celsius above the melting temperature of the mixture. This part of the die may be very long, for example two metres long, so that the temperature of the inner parts of the extruded rod closely approaches the temperature of the outer parts.

In the second part of the extrusion die the temperature is not required to be as accurately controlled as with the first part of the extrusion die. This is because all that is required is a temperature sufficiently below the melting point of the material to freeze the material to form a solid rod. It is envisaged however that the second cooling means will be positioned close to the outlet of the extrusion die so that the solid rod need only to be pushed a short distance along the extrusion die before emerging from same.

It is envisaged that in most embodiments the pressure of the extruder (and perhaps an additional pump) on the molten material behind the solid rod will act to push it sufficiently to exit the die. At this consistency, the shear strength is high and the outer surface of the rod is more likely to break away from the surface of the die to flow with the rest of the material making up the rod.

In addition to the force from the extruder (and optional pump) pushing the rod, there may also be provided a pulling force external to the extruder. This is possible if the rod is in substantially solid form and unlikely to break when gently pulled.

In some embodiments of the present invention there may be provided automatic chafing or rounding of one end of the rod as it exits the apparatus. Rounding of the end can make the rod more palatable to the animals if the rod is to be introduced as a bolus.

Either before or after the chafing, there may be provided an automatic means by which the rod is cut. For example, it is envisaged that there will be provided continual extrusion of the rod. The length of the rod may be measured and then the rod may be clamped prior to being cut to a required length. The rod may be cut by a variety of means, but in one embodiment a disc saw may be used.

In some embodiments, the clamp and saw may be on a carriage which enables the rod to be carried to where the end can be rounded.

The carriage may also allow the rod to be subsequently dispensed. For example, in one embodiment the clamp may release the rod onto a weighing device. The device can determine whether the rod is of the required density. The weighing device may have a selection means which can dump the rod (perhaps for recycling) if it does not meet the required density or ensure the rod passes to the next stage of the process.

The next stage of the process may be storage of the rods or perhaps dipping of the rods in molten wax or other coating material.

The applicant has found that sometimes it is necessary to rest the rods for two to three days before dipping, otherwise there is a tendency for the wax coating to crack. The reason for this is unknown, but it is suspected that the boluses gradually leach compressed air contained within their matrix. If this is the case, then the applicant proposes in some embodiments of the present invention to subject the boluses to a vacuum system to extract the air before dipping.

The vacuum system may be applied to the whole of the apparatus or their may be provided a specialised chamber.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from the ensuing description which is given by way of example only and with reference to the accompanying drawing in which:

Figure 1 illustrates a method of extrusion in accordance with one embodiment of the present invention, and

Figure 2 is a side view of one embodiment of the present invention, and Figure 3 is an exploded view of the components which make up one embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Figure 1 illustrates an extruder generally indicated by arrow 1 for use in one embodiment of the present invention.

Discussion of the method of extrusion shall now be made to a formulation having a melting point of 54°C. It should be appreciated however that this melting point is given by way of example only and the present invention can apply to formulations having other melting points.

In this embodiment, the extruder 1 has a number of parts. The first part 2 is the reservoir of the extruder 1 in which the well mixed formulation 3 has a temperature in the order of 65 °C.

The extruder screw (not shown) pushes the formulation 3 along a long extruder passage 4. The extruder passage 4 is approximately 1 metre in length. The first 80 cm or so of the passage 4 is surrounded by a first cooling means 5 in the form of a cooling jacket. The temperature of the cooling jacket 5 is thermostatically controlled to 56°C. The length of the passage 4 and the rate of the extruder screw ensure that the material 3 leaves the cooling means 5 at approximately 56°C with only a slight temperature gradient if any.

The material 3 then passes by a second cooling means 6 also in the form of a cooling jacket. In this embodiment the cooling jacket 6 has a temperature of approximately 40°C. This causes the material 3 to snap freeze without causing the outer parts to stop moving while the inner parts flow on. The solid rod is then pushed out through the extruder die 7 to exit the extruder 1.

In some embodiments of the present invention there may be provided an additional feed means in the form of gear pump situated between the extruder 1 and the first cooling means 5.

Figure 2 is a more detailed diagram of one embodiment of the present and Figure 3 is exploded view of the components which make up the embodiment illustrated in Figure 2.

The ingredients for the rod are placed into feed auger from where they travel to a mixing auger.

Preferably, no pressure is applied to the material within the mixing auger and the only pressure applied is when a material passes through the gear pump before entering the extrusion dye.

At the end of the extrusion dye there are a number of "post-production" devices to place the rod in its final form.

As the rod extends from the dye, the length is determined by a sensor. The rod is then clamped in the appropriate position so that the rod can be cut by a circular saw.

The clamped and cut rod can then be moved to a shaping tool which rounds the end of the rod.

The rod is then transferred by an arm to a weigh station. If the rod is outside a desired weight range, then a reject actuator disposes of the rod which may be recycled.

If the rod is within the required weight range it is then loaded into a carousel stack.

The carousel stack may only be used in some embodiments of the present invention and it is envisaged that this could be useful for allowing the rods to rest prior to dipping the rods in wax.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the appended claims.

Claims

WHAT WE CLAIM IS:

1. A method for extruding material having a sharp melting point characterised by the steps of: a) extruding the material in a molten state past the first cooling means capable of cooling the molten material to just above the melting point of the material, and b) extruding the now cooled molten material past the second cooling means capable of freezing the molten material so that the material remains substantially intact when removed from the extrusion process.

2. A method as claimed in claim 1 wherein the material includes zinc oxide.

3. A method as claimed in either claim 1 or claim 2 wherein the two cooling means are incorporated into a single cooling unit.

4. A method as claimed in either claim 1 or claim 2 wherein multiple cooling units provide the cooling means.

5. A method as claimed in any one of claims 1 to 4 wherein there is a first passage associated with the first cooling means, and a second passage associated with the second cooling means, characterised in that the first passage is longer than the second passage.

6. A method as claimed in any one of claims 1 to 5 wherein the material is extruded by a screw extruder

7. A method of producing a substantially rod-shaped extrusion, characterised by the steps of: a) mixing the ingredients at a temperature below the melting point of any of the ingredients, and b) feeding the coarsely mixed ingredients to an extruder which has a number of zones along its axial length, the temperature of each zone being maintained substantially independently, and c) ensuring that the first zone to which the ingredients are transported is at a temperature below the melting point of any of the ingredients, and d) ensuring that a subsequent zone is at a temperature higher than the melting point of some of the ingredients, and e) ensuring that some recirculation of the mixture takes place within the machine, in order to achieve thorough mixing and a high degree of homogeneity within the mix, and f) allowing the resulting paste to pass through a long passage or die which has walls maintained at a temperature a little above the freezing point of the paste, and g) ensuring that the passage is long enough that the mixture reaches a high viscosity over most of the cross-section of the passage, and h) allowing the resulting paste to pass through a second passage or die which has walls maintained at a temperature a little below the freezing point of the paste, and i) supplying sufficient pressure that the adhesion between solidifying paste and the die walls is broken, and that the mixture is extruded out of the die with a substantially uniform velocity over the whole cross-section of the die, and j) collecting the extruded rod at the exit of the die, and k) optionally shaping the end of the rod, and

1) perhaps coating the rod, and m) cutting the rod to length, and n) collecting the cut rods for further processing or packing.

8. A method as claimed in any one of claims 1 to 7 wherein there is provided an additional feed device that assists in the movement of the material through the extrusion process.

9. A method as claimed in claim 8 wherein the additional feed device is a pump interposed between the extruder and the first cooling means.

10. A method as claimed in any one of claims 1 to 9 wherein the first or second cooling means includes a water jacket.

11. An animal treatment remedy produced by a method as claimed in any one of claims 1 to 10.

12. An animal treatment remedy as claimed in claim 11 which includes zinc oxide.

13. Apparatus for extruding material including an extruder, a first cooling means capable of cooling molten material to just above the melting point of the material, and second cooling means capable of freezing the molten material so that the material remains substantially intact when removed from the apparatus.

14. Apparatus as claimed in claim 13 which includes an additional feed device.

15. A method substantially as herein described with reference to and as illustrated by the accompanying drawings.

16. An animal treatment remedy substantially as herein described with reference to and as illustrated by the accompanying drawings.

17. Apparatus substantially as herein described with reference to and as illustrated by the accompanying drawings.