Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2000-12-26
2001-10-09
Soohoo, Tony G. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S091000, C366S092000, C366S285000, C366S338000
Reexamination Certificate
active
06299342
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a mixing device and more particular to a device for compounding, i.e. mixing and homogenisation of polymers by elongational deformation.
TECHNICAL BACKGROUND
When producing a polymer composition the ingredients thereof, such as different polymers, fillers, additives, etc, should be mixed intimately in order to obtain as homogenous a composition as possible. This intimate mixing is done by compounding the ingredients in a compounding machine, continuous or batchwise; the former type can be exemplified by an extruder which may be of the single screw of twin screw type. When the composition comprises two or more different polymers these should be so thoroughly mixed with each other that, ideally they form a completely homogenous polymer blend. To achieve this the polymers are mixed with or without external heating so that they are melted and converted into liquids and the liquid polymers are mixed at sufficiently high shear rates.
Although the conventional way of compounding polymer compositions in many instances leads to acceptable results, problems are encountered when compounding multi-modal polymer compositions, and more particularly multi-modal polymer compositions comprising a low molecular weight ethylene polymer and a high molecular weight ethylene polymer. Thus, when compounding polymer compositions, e.g. for pipes, so called “white spots” occur in the compounded material. These white spots have a size of about 10-50 &mgr;m and consist of high molecular weight polymer particles that have not been adequately compounded in the composition. In addition to being disfiguring the white spots may adversely influence the strength of the composition. Further, when compounding polymer compositions, e.g. for the production of film, gel particles with a size of about 0.01-1 mm often occur. These gel particles appear as disfiguring inhomogeneities in the finished film and consist of high molecular weight polymer particles that have not been adequately compounded in the composition.
The above described white spots and gel particles are a serious problem in the polymer industry and a solution to the problem would mean the removal of an obstacle to use otherwise superior multi-modal polymer compositions.
One has tried to solve the problem with gel particles and white spots by applying large amounts of deformation energy to the polymer composition. Normally, this is achieved by compounding the polymer composition at high shear rates and at high temperatures. However, such excessive energy input frequently results in degradation of the polymer especially when the polymer composition comprises polymer materials with large differences in viscosity. For polymer compositions comprising polymers with large differences in viscosity it has been demonstrated that mixing by applying elongational deformation is more effective and favourable than mixing by pure shear deformation.
Mixing by applying elongational deformation as in the present invention is also referred to as “dispersive mixing” as opposed to conventional “distributive mixing”. While distributive mixing aims at achieving a homogenous concentration, e.g. by splitting up the stream as many times as possible, dispersive mixing aims at breaking down one phase by applying high shear or tension forces in order to obtain blending of the components of the stream. The blend thus obtained is free from visual inhomogenities (gels), but does not have to satisfy the requirement of homogenous concentration. Dispersive mixing is primarily used when the ratio between the viscosity of the two phases which are to be mixed is higher than 10:1.
As an example of a device for mixing of viscous liquids, such as polymer compositions by elongational deformation may be mentioned U.S. Pat. No. 5,451,106 to Xuan et al., the contents of which is hereby incorporated by reference. This patent discloses a mixer which has a vertical, cylindrical casing which encloses a lower die member and an adjustable, upper die member. Opposed, annular and concentric die member protrusions define alternating concentric slits and chambers. An inlet opening in the side wall of the casing allows liquid polymer to be fed into the device, through the slits and chambers between the dies where the polymer is subjected to elongational deformation, and finally the polymer exits through a central bottom outlet.
The device according to U.S. Pat. No. 5,451,106 has a number of disadvantages. Thus, arranging the inlet on the side of the casing and the outlet in the bottom of the casing results in an uneven material distribution in the device which gives problem with wear due to pressure differences in the mixing chamber. The inlet and outlet arrangement also makes it difficult to connect the mixing device to existing compounding equipment in a simple manner. Further, the geometry of the circular dies with concentric protrusions confines the number of concentric protrusions and thus the elongational deformation if the outer diameter of the device is to be kept within acceptable limits.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a mixing device which mitigates or eliminates the disadvantages of prior art devices of the type mentioned above. It is thus an object of the present invention to provide a device which gives an even material distribution and which is easy to connect to or integrate with existing compounding equipment. A further object of the present invention is to provide a device which may include a large number of alternating slits and chambers for enhanced elongational deformation without the device having an excessive outer diameter.
The above objects are achieved by providing a polymer mixing device with a casing and a core defining between them a conically tapered mixing space which is divided by circumferential protrusions into slits alternating with chambers, said protrusions being arranged on the core and/or the inner wall of the casing. Thus, the present invention provides a device for mixing and homogenisation of polymers by elongational deformation having
a casing with an inlet and an outlet;
said casing having inner walls defining an enclosure which communicates with the inlet and the outlet;
a core arranged in the enclosure and defining a mixing space between the core and the enclosure wall;
said mixing space being divided into several slits and chambers by protrusions;
characterised in that the enclosure and the core are concentrically arranged around a common axis and are conically tapered towards the outlet;
and that to achieve elongational deformation the protrusions are circumferentially arranged as continous annular beads on at least one of the core and the enclosure wall with a slit gap in the range from about 0.5 mm to about 10 mm and a width of the slit gap in the range of 0 to about 5 mm.
These and other advantages and characterising features of the present invention will appear from the following specification and the appended claims.
The core or casing of the device may be static or arranged to rotate or oscillate. Thus, the core which is arranged in the enclosure of the casing may be static, i.e. non-rotatable, or rotatable by means of a motor. At present it is preferred that the core (and the casing) is static, inter alia because of the simpler structure it entails.
In order to achieve a proper elongational deformation of the polymer during the processing thereof in the mixing device according to the invention the height of the slits, i.e. the slit gap should have suitable value. This value lies within the range from about 0.5 mm to about 10 mm, more preferably from about 0.5 mm to about 5 mm, and most preferably from about 1 mm to about 3 mm. Within the indicated, preferred general range, the optimal slit gap value depends on several parameters such as the type of polymer to be processed, the polymer flow, and the overall geometry of the apparatus. To adapt the mixing device to a particular type of polymer and polymer flow the slit gap is preferably made adjustable so that a s
Eggen Svein Staal
Nygaard Rune
Borealis Technology Oy
Merchant & Gould P.C.
Soohoo Tony G.
LandOfFree
Mixing device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Mixing device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mixing device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2607727