CA2381724A1

CA2381724A1 - Extruder screw

Info

Publication number: CA2381724A1
Application number: CA002381724A
Authority: CA
Inventors: Vaclav George Zboril; Eric Clavelle
Original assignee: Nova Chemicals Corp
Current assignee: Nova Chemicals Corp
Priority date: 2002-04-12
Filing date: 2002-04-12
Publication date: 2003-10-12

Abstract

The capacity of an extruder fed with a high viscosity liquid having a volatile component may be limited by the ability of the high viscosity liquid to completely fill the flights of the screw in the barrel o f the extruder. The liquid may incompletely fill the initial sections of the screw and may be subject to a partial or complete vapour lock. This problem may be reduced by using a screw having in the section adjacent the feed port an increasing flight volume from the back to the front of the feed port. It may also be reduced in combination or independently through the use of guide vanes or baffles placed in an appropriate position of the extruder feed port.

Description

FIELD OF THE INVENTION
The present invention relates to the extrusion of viscous material.
More particularly the present invention relation to the extrusion of viscous material containing a volatile component. The present invention relates to feeding a viscous liquid or melt containing a volatile fraction to an extruder.
In particular the viscous material may be a polymer melt containing a 1o volatile component such as a solvent.
BACKGROUND OF THE INVENTION
There is a significant amount of prior art relating to the extrusion of polymers. Generally, the polymers are fed to the extruder in the form of solid pellets. Under the shear and temperatures in the extruder the solid polymer melts and becomes a viscous material. Howeveir, there is no art, applicants are aware of, relating to feeding a viscous material containing a volatile component to an extruder.
When a viscous liquid such as a polymer melt is fed to an extruder if the viscous material contains a volatile component having a low vapour pressure under some conditions such as a high viscosity ~e.g, typically greater than 1000 Pa.s) the melt does not flow easily into the feed port and the pressure within the feed port may be reduced below the vapour pressure of the volatile component. As a result the volatile component tends to vapourize and the feed of viscous material is reduced or in some cases stops. In short a partial or complete vapour lock may occur.
The present invention seeks to overcome this problem.
1TT Specs19239-Cda.doc ai SUMMARY OF THE INVENTION
The present invention provides in an extruder for use with a viscous liquid feed material the improvement comprising using one or more screws having an increasing flight volume from the back of the feed port to the front of the feed port.
The present invention also provides in process comprising passing 1o a polymer melt containing a volatile component through and extruder and extruding and pelletizing said polymer the improvement comprising using adjacent the feed port one or more screws as described above.
DETAILED DESCRIPTION
Figure 1 is a schematic drawing of an extruder using one embodiment of the present invention, Figure 2 is a schematic drawing of the cross section of one embodiment of the invention in which there is a guide in the feed port of the extruder.
As used in this specification the back of the feed port is the portion of the feed port furthest from the exit from the extruder.
As used in this specification the front of the feed port is the portion of the feed port closest to the exit from the extruder.
3o Without wishing to be bound by theory it is believed that by increasing the flight volume from the back to the front of the feed port more viscous material is able to flow into the extruder and there is a lower potential that the pressure at the back of the feed port will drop below the vapour pressure of the volatile component in the viscous feed and cause a partial or complete vapour lock.
1TT Specs\9239-Cda.doc One embodiment of the present invention will be described in association with figure 1. Figure 1 is a schematic drawing of the feed port area of an extruder 1. The extruder comprises a drive means not shown which powers a shaft 2. The shaft is within the body or barrel of the extruder 3. The feed port 4 has a front 6 and a back 5. There is a flight 7, which is attached to the screw and forms a spiral or screw around the 1o shaft. In the embodiment shown in the drawing the pitch of the flights is uniform. (e.g. the spacing between parallel sections of the flight is constant). In the embodiment shown the flight is constructed so that it has a constant outer diameter. However in the embodiment shown the root mean diameter of the shaft is tapered from the back 5 to the front 6 of the feed port 4. As a result there is an increasing volume from the back to the front of the feed port. The fill volume at the back of the feed port is smaller so that as viscous material is moved forward to the front of the feed port and then into the extruder the flights being filled by the polymer uniformly along the length in the feed port.
In a conventional extruder having a constant root mean diameter, a constant pitch and a constant flight diameter the volume of the feed section of the extruder is constant. This works well for solid resin pellets in 3o that the pellets fall into and substantially fill volume of the open flights at the back end of the extruder. Any unfilled volume is filled as the free space between the flights traverses the feed port. However if the feed is a thick viscous liquid (e.g. having a viscosity greater than 500 Pa.s; typically from 1000 Pass to 30,000 Pa.s) the flow pattern of the liquid into the open volume between the flights may not be able to completely fill the space 1TT Specs\9239-Cda.doc t between the flights in the barrel at the back of the feed part. As a result the limiting rate for the extruder capacity is not the mechanical limitations of the equipment but rather the inability to completely fill the flights in the barrel of the extruder. As noted above if the viscous material contains a volatile component, such as a solvent there may be a partial or complete vapour lock. There are several potential approaches to the problem. The 1o viscosity of the feed could be reduced by heating the viscous material.
(One would not want to add a solvent or diluent as this raises issues of devolatilizing the viscous material). The pressure on the viscous material could be increased for example by a ram feed. Both of these proposals are expensive in terms of energy and equipment.
A simpler solution is to have a relatively larger flight volume at the front of the feed port and a relatively lower flight volume at the back of the feed port. There is a small volume at the back of the feed port to permit sufficient viscous material to flow into the empty first flight and to subsequently fill the down stream flights in the extruder barrel as their volume increases. Thus the polymer flow is not restricted to the back of the feed port as it is distributed along the whole length of the feed port.
One method of achieving the result has been disclosed above.
3o There are several other methods of achieving this result. 'The root mean diameter of the screw could remain constant and the flight diameter would remain constant and the pitch of the flight could be increased (e.g. there would be mare flights per unit of distance at the feed port), An alternate method of achieving the results would be to maintain a constant pitch and root mean diameter of the screw and to alter the outer diameter of the ITT Specs19239-Cda.doc flights. The diameter would increase from the back of the feed port to the front of the feed port. Combinations of the foregoing approaches could also be used. For example the root mean diameter of the screw and the pitch of the flights could increase from the front to the back of the feed port while maintaining a constant outer diameter of the flights. Root means diameter of the screw could be kept constant and the pitch and the outer diameter of the flights could be increased from the front to the back of the feed port. The root mean diameter of the screw could decrease from the back to the front of the feed port while simultaneously the outer diameter of the flights increases from the back to the front of the feed port. This would allow the meshing without interference of a twin screw extruder.
The present invention may be applied to a single screw extruder or to a twin screw extruder. The twin screw extruder may be a co- or counter rotating twin screw extruder.
In a further embodiment of the present invention one or more baffles or guide vanes may be installed in the feed port to improve the flow pattern in such a way that the flights of the screw are more easily filled.
That is the baffles or guide vanes direct the flow to the section of the feed port drawing feed into the extruder. For a single screw extruder the a 3 o single baffle or guide vane may direct the flow to the side of the feed port drawing feed into the extruder (e.g. where the rotation of the flights is down into the extruder) for a twin screw extruder the section of the feed port where feed is drawn down into the extruder may be on the edges of the feed port in which case the baffle would direct the flow to the edge of the feed port and away from the central region (e.g. a peeked baffle over 1TT Specs19239-Cda.doc the center or mid line region of the feed port) or in the center in which case two baffles would direct the flow from the edge of the feed port to the center. One embodiment of such an invention will be described in association with figure 2. Figure 2 is a schematic drawing of an end view of the feed port area of an extruder 1. The direction of rotation of the extruder screw 2 is indicated. The feed port 4 is above extruder screw.
1o An example of a baffle or flow guiding device 8 is also displayed. The guide is shaped, preferably wedge shaped, and positioned to aid the flow in entering the flights of the screw andlor the pocket 9 that may or may not be present in the barrel. (i.e. the side or section of the feed port where the screw draws the material down into the extruder). This is the section of flight which draws feed into the extruder.
The present invention may be practiced with various viscous materials. The invention is particularly useful with polymer melts. The polymer melt may contain a volatile component such as unreacted monomer or oligomers or it may contain residual so solvent or diluent.
One polymer which may be used in accordance with the present invention is polyethylene. The polyethylene may comprise 80 to 100, preferably 80 to 95, weight % of ethylene and from 20 to 0, preferably 20 3o to 5, weight % of one or more copolymerizable monomers.
Some copolymerizable co-monomers include a C4-10, preferably C4_~
alpha olefins including butene, hexene and octene. However the copolymerizable co-monomer could be one or more of propylene (e.g.
ethylene - propylene rubbers (EP) and a non-conjugated diene such as 1,4-hexadiene, dicyclopentadiene, and ethylidene norbornene. (EPDM) or \'1T Specs\9239-Cda.doc 7 the copolmerizable monomer could be a functional monomer such as a C3_ $ ethylenically unsaturated acid, imide or amide such as acrylic or methacrylic acid or the corresponding imides.
Typically the alpha olefin polymer may have a melt index as determined by ASTM D 1238 (190°C I 2.16 kg) of up to 200 dg/min.
ASTM means the American Standard Test Method and the conditions of 1p the test are at 190°C and under a load of 2.16 kg. While the melt index may be fractional the lowest melt index would be that useful for extrudable polymers. Typical ranges would include melt indexes from 0.1 to 150, most typically from 0.1 to 120 dglmin.
The homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins may have densities in the range of, for example, about 0.900-0.970 g/cm3 and especially 0.910-0.965 g/cm3; the polymers of higher density, e.g. about 0.960 and above, being homopalymers. Such polymers may have a melt index, as measured by the method of ASTM D-1238, condition E; in the range of, for example, 0.1-200 dglmin, typically from about 0.1 to 150 dglmin., and especially in the range of about 0.1 to 120 dglmin. The polymers may be manufactured with narrow or broad molecular weight distribution. For example, the polymers may have a 3 o stress exponent, a measure of the molecular weight distribution, in the range of about 1.1-2.5 and especially in the range of about 1.3-2Ø Stress exponent is determined by measuring the throughput of a melt indexer at two stresses (2160 g and 6480 g loading) using the procedures of the ASTM melt index test method, and the following formula:
1TT Specs\9239-Cda.doc n Stress Exponent = 1/(0.477) x (Log. wt extruded with 6480 g weight) / wt, extruded with 2160 g wt).
Stress exponent values of less than about 1.40 indicate narrow molecular weight distribution while values above about 1.70 indicate broad molecular weight distribution.
There are several methods for preparing such. polymers. The polymers may be prepared in the gas phase using for example BP's INNOVENE process or Dow's UNIPOL process. The polymers may be prepared in a slurry process such as that developed by Phillips. The polymer may be prepared by a solution process such as that used by NOVA Chemicals Corporation. In a solution process the polymer is prepared in a solvent at temperatures at which the resulting polymer is soluble in a solvent. The solvent may be an inert C5_~o hydrocarbon which may be unsubstituted or substituted by a C~.4 alkyl radical such as for example, heptane, hexane, heptane, octane, cyclohexane, methylcyclohexane, and hydrogenated naphtha.
After the solution of polymer leaves the reactor a significant portion of the solvent is separated from the polymer. However there is still some residual solvent entrained within the polymer. The polymer is maintained 3o in the form of a melt and may be fed to an extruder to be pelletized. The polymer melt comprises one or more components (e.g. solvent) having a vapour pressure higher than the pressure in a comparable extruder having a conventional screw design. If such a melt were fed to a conventional extruder there may be difficulty feeding sufficient polymer melt to the feed CfT Specs\9239-Cda.doc section (e.g. the open flights at the beginning of the extruder screw) to permit the extruder to operate at full capacity.

1TT Specs~9239-Cda.doc ~ 0

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an extruder for use with a viscous liquid feed material the improvement comprising using one or more screws having an increasing flight volume from the back of the feed port to the front of the feed port.

2. The extruder according to claim 1, wherein said viscous liquid material comprises a volatile material.

3. The extruder according to claim 2, wherein said viscous liquid material further comprises a polymer melt

4. The extruder according to claim 3, wherein adjacent said feed port said one or more screws have a constant pitch and flight outer diameter and a shaft tapered over the length of said feed port such that the root mean diameter of the shaft at the front of the feed port is less than the root mean diameter of the shaft at the back of the feed port.

5. The extruder according to claim 4, which is a single screw extruder.

6. The extruder according to claim 5, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

7. The extruder according to claim 4, which is a twin screw extruder.

8. The extruder according to claim 7, in which the screws are counter rotating.

9. The extruder according to claim 7, in which the screws are co-rotating.

10. The extruder according to claim 8 or 9, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

11. The extruder according to claim 3, wherein adjacent said feed port said one or more screws have a constant root mean shaft diameter, a constant flight outer diameter and an increasing pitch from the back of the feed port to the front of the feed port.

12. The extruder according to claim 11, which is a single screw extruder.

13. The extruder according to claim 12, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

14. The extruder according to claim 11, which is a twin screw extruder.

15. The extruder according to claim 13, in which the screws are counter rotating.

16. The extruder according to claim 13, in which the screws are co-rotating.

17. The extruder according to claim 15 or 16, further comprising one or more baffles running at least a portion of the length of the feed port which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

18. The extruder according to claim 3, wherein said one or more screws have in the area adjacent the feed port a shaft tapered such that the root mean diameter of the shaft at the front of the feed port is less or equal to the root mean diameter at the back of the feed port, a constant flight outer diameter and an increasing pitch from the back of the feed port to the front of the feed port.

19. The extruder according to claim 18, which is a single screw extruder.

20. The extruder according to claim 19, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

21. The extruder according to claim 18, which is a twin screw extruder.

22. The extruder according to claim 21, in which the screws are counter rotating.

23. The extruder according to claim 21, in which the screws are co-rotating.

24. The extruder according to claim 22 or 23, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

25. The extruder according to claim 3, wherein adjacent said feed port said one or more screws have a constant root mean diameter, a constant pitch and an increasing outer diameter of the flights from the back to the front of the feed port.

26. The extruder according to claim 25, which is a single screw extruder.

27. The extruder according to claim 26, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

28. The extruder according to claim 25, which is a twin screw extruder.

29. The extruder according to claim 28, in which the screws are counter rotating.

30. The extruder according to claim 28, in which the screws are co-rotating.

31. The extruder according to claim 29 or 30, further comprising one or more baffles running at least a portion of the length of the feed part in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

32. The extruder according to claim 3, wherein adjacent said feed port said one or more screws have a constant root mean diameter, a constant pitch, a decreasing root mean diameter from the back to the front of the feed port and an increasing outer diameter of the flights from the back to the front of the feed port.

33. The extruder according to claim 32, which is a single screw extruder.

34. The extruder according to claim 33, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

35. The extruder according to claim 32, which is a twin screw extruder.

36. The extruder according to claim 35, in which the screws are counter rotating.

37. The extruder according to claim 35, in which the screws are co-rotating.

38. The extruder according to claim 36 or 37, further comprising one or more baffles running at least a portion of the length of the feed port in which one or more baffles directs the viscous liquid to the portion of one the feed port where it is drawn down inside the extruder.

39. In a process comprising passing a polymer melt containing a volatile component through an extruder and extruding and pelletizing said polymer the improvement comprising using adjacent the feed port one or more screws according to claim 1.

40. The process according to claim 39, wherein said polymer comprises 80 to 100 weight % of ethylene and from 20 to 0 weight % of one or more copolymerizable monomers.

41. The process according to claim 40, wherein one or more of said copolymerizable monomers is selected from the group consisting of C4-8 alpha olefins.

42. The process according to claim 41, wherein said polymer melt comprises one or more components having a vapour pressure higher than the pressure in a comparable extruder having a conventional screw design.