Method and apparatus for thermally processing polyester pellets and a corresponding pellet preparation

Abstract

Method and apparatus for thermally processing polyester pellets, e.g., polyethylene terephthalate pellets, in order to achieve a partial crystallization, whereby the polyester melt is fed to an underwater pelletizer and pelletized, the pellets obtained are fed to a water/solids separating device and the dried pellets are fed at a pellet temperature of greater than 100° C. to an agitation device that the pellets leave at a pellet temperature of over 100° C.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No. 10/962,614, filed Oct. 13, 2004, which claims priority under 35 U.S.C. §119 of German Patent Application No. 103 49 016.7-43, filed Oct. 17, 2003, and German Patent Application No. 10 2004 021 595.2, filed May 3, 2004, the disclosures of which are expressly incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for thermally processing polyester pellets in order to achieve crystallization and to the polyester pellet preparation produced thereby.

2. Discussion of Background Information

Polyethylene terephthalate, hereinafter also referred to herein as PET for short, is a polyester with repeating ester groups.

PET can be present in different structures, namely in amorphous or in crystalline or partially crystalline form. Amorphous PET is mostly transparent, and crystalline PET is opaque or white. As with all thermoplastics that can be present in amorphous or crystalline form, a 100% degree of crystallization is also not possible with PET. Only a portion of the structure of the PET is able to orient itself, i.e., to crystallize, and PET includes crystalline and amorphous regions which alternate. Therefore, PET is always referred to with respect to partial crystallinity. An approximate 50% degree of crystallinity can be achieved with PET in order to prevent the pellets or granules from sticking to one another. This means that in this state about half of the molecule chains have oriented themselves to one another, and thus have laid themselves parallel next to one another or have wound themselves in a circular manner. The interactions (van der Waals forces) between the molecular chains therefore inevitably become greater in the partially crystalline regions. The chains thus mutually attract one another and thus gaps between the molecules become smaller.

As a thermoplastic, PET can be molded at temperatures of 250 degree. C. The molecular chains then become so mobile that the plastic melts and a viscous mass results that can be made into virtually any desired shape. When it cools, the molecular chains refreeze, and the plastic solidifies in the desired shape—a simple principle that can be repeated multiple times. This method is also used, e.g., in the production of PET bottles. So-called preforms are produced in a first step. As a precursor of the PET bottles, these preforms already have a finished screw thread. In order to obtain proper bottles, they are again softened at 100° C., stretched with compressed air and blown to produce a bottle (stretch blow process).

The production of crystallized PET in pellet form hitherto comprised extensive and complicated fluidized bed methods that required large investments and high operating costs, such as DE 198 48 245 A and its family member WO 00/23497, which are incorporated by reference herein in their entireties.

PET pellets have to be crystallized at a temperature below that at which the material becomes sticky in order to prevent the pellets coalescing into a solid mass that can barely be processed. Although the melting temperature of the crystallized polyester is not reached until 240 to 250° C., it can already become sticky before crystallization at temperatures above approx. 70° C.

In so far as continuous methods for producing dry PET pellets are known, in general they require very large installations, since long crystallization times are necessary.

Thus, for example, U.S. Pat. No. 5,532,335, which is incorporated by reference herein in its entirety, is directed to a method for thermally processing polyester pellets in which the pellets are introduced into a processing vessel and a liquid medium is also introduced into this processing vessel, whereby the pellets and the liquid medium are mixed together. Pressurized water or so-called superheated water is used hereby as a liquid medium in the proposed process. The boiling temperature can easily be controlled by changing the pressure in the reactor vessel. In an exemplary embodiment polyester pellets are processed at 120°-182° C. The water is introduced at 160° C., kept in the liquid state and the pellets are added as long as the pressure in the reactor unit is kept at 7 kg/cm² or higher. It is evident that such a method is extraordinarily expensive and therefore can barely be conducted economically.

Known methods that work with an aerodynamic processing also have the serious disadvantage that they use a large amount of inert gases. The energy and processing costs are also too high here for a practical large-scale application.

In order to sufficiently crystallize the material in the prior art it was therefore always necessary to add sufficient external energy or heat to the crystallization process. These cited problems have hitherto hampered PET recycling.

SUMMARY OF THE INVENTION

The present invention relates to a method for crystallizing PET pellets that is possible without the addition of external energy or heat and that does not require long dwell times. The present invention further relates to the crystallized PET pellet preparation and to that preparation produced according to the method of the invention.

The present invention relates to a method for thermally processing polyester pellets in order to achieve partial crystallization, comprising feeding polyester melt to an underwater pelletizer and pelletizing the polyester melt in the underwater pelletizer to obtain pellets, feeding the pellets to a water/solids separating device to dry the pellets, feeding the dried pellets at a pellet temperature of greater than 100° C. to an agitation device, and removing pellets from the agitation device at a pellet temperature of over 80° C.

The present invention also relates to an apparatus for carrying out a method for the thermal processing of polyester pellets in order to achieve a partial crystallization of the pellets, the apparatus comprising a melt pump; a screen changer; an underwater pelletizer; a water/solids separating device; and a conveyor device for transporting pellets, the conveyor device being arranged downstream of said underwater pelletizer and said water/solids separating device, said conveyor device being constructed and arranged to agitate pellets and crystallize pellets during transport through specific heat of the pellets.

The present invention also relates to polyester pellets, preferably PET pellets that are crystallized at least to 40% by the specific heat present in the pellets. That specific heat results from the pellet formation. The pellets have an outermost layer spheroylitic structure which is equal or smaller than that at the center. This characteristic can be distinguished by polarization-contrast optical microscopy and by the change in visible pellet character from translucency to opacity. In particular, the polyester pellets have a degree of crystallization at their centers which is at least as great as the degree of crystallization of their outermost layers. The pellets also have an acetaldehyde content between 0.5 and 100 ppm.

The method can further include flowing a fluid around the pellets during agitation of the pellets in the agitation device.

The pellets can be in a form of a pellet layer, and further comprising flowing a fluid around the pellet layer during agitation of the pellets in the agitation device.

The pellets can be fed to the agitation device at a pellet surface temperature of over 110° C.

The pellets can be conveyed from the underwater pelletizer to the water/solids separating device with hot process water.

The process water can have a temperature of 98° C.

The thermal processing leading to partial crystallization can utilize specific heat present in the pellets.

The polyester pellets can comprise polyethylene terephthalate pellets.

The conveyor device can comprise a conveyor channel.

The conveyor device can comprise a vibrating conveyor device.

The vibrating conveyor device can comprise a conveyor channel.

The conveyor device can a plurality of spaced apart dams distributed over the length of the conveyor device or conveyor channel, each of said plurality of spaced apart dams causing a damming up of material.

The conveyor device can be surrounded at least in part by a housing.

The water/solids separating device can comprise a centrifuge.

In other words, it is proposed that the PET starting material is extruded in an extruder at a suitable temperature. Subsequently impurities are filtered out using, e.g., screen changer technology. The polymer melt is fed to an “underwater hot strike off pelletizing system,” referred to below as “underwater pelletization,” and processed into pellets that due to the underwater pelletization have a ball shape or a lenticular shape and have a high core temperature.

These PET pellets are conveyed via a conveyor line at high speed to a water/solids separation device, whereby hot water, preferably up to 98° C., is used as a flow medium. One important aspect for the effectiveness of the method according to the invention is relatively short conveyor paths between the pelletization chamber and the water/solids separating device. The PET pellets leave the water/solids separating device at a core temperature of 130-180° C., since it is ensured that the extrusion temperature of the PET is maintained for as long as possible.

The pellets having this temperature are then subjected to an agitation whereby the crystallization begins. This crystallization according to the method according to the invention is determined by the specific heat and it is thus achieved that the product, i.e., the pellets, do not agglomerate and no longer stick to one another. This effect is also increased in that the product to be crystallized has a ball shape or a lenticular shape, and thus manages with the smallest possible contact surfaces to one another.

The dwell time of the ball-shaped pellets in the agitation phase as the pellets pass through an agitation apparatus, such as a conveying device, is, e.g., 3 to 8 minutes and after this phase has been completed, up to 40% and more of the PET pellets are crystallized and have a temperature of greater than 100° C. The transport of the hot PET pellets into a storage silo or subsequent processing station is possible, since the pellets no longer stick together.

Another object of the invention is to propose a device with which the effective agitation of the pellets is possible.

Preferably, a so-called crystallization channel is provided as the agitation device for the pellets. This crystallization channel is constructed in a similar manner to a pellet conveyor channel, but, seen in the conveyor direction, is divided into successive chambers that are separated from one another by dams. The crystallization channel has vibration motors so that the pellets located therein are permanently agitated and thus can give off their intrinsic energy to other pellets. A rotation of the PET pellets takes place in the individual chambers and a sticking of the pellets is no longer possible.

With the method according to the invention and the apparatus according to the invention a gentle, economic and rapid crystallization of PET pellets is achieved.

Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed description which follows, in reference to the sole FIGURE of drawings by way of non-limiting example of exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.

The invention provides a sufficient heat energy to the pellets with their formation so that the pellets have a sufficient heat capacity to allow post-formation “after-curing.” The “after-curing” causes the above described partial crystallization of the pellets. This crystallization is made possible by the intrinsic heat energy present in the pellet cores so that this procedure can be called “direct crystallization.” For example, following extrusion of the polyester melt and pellet formation from the melt, the pellet surfaces can be cooled down in order to produce dimensional pellet stability. However, if the pellets are not cooled down intensively, then enough heat energy exists in the pellet cores is sufficient to cause the surface temperature of the pellets to slowly rise and accomplish the desired thermal treatment causing crystallization.

Related to the examples described, a hot plastic with a temperature of from, for example 130 to 180° C. can be molded into pellets, e.g., PET plastic, by means of the above described underwater granulation. While the pellets spend a dwell time in the water, their surface temperature cools down to a value of approximately 110° C. The core temperature of these pellets, however, remains significantly higher. If the pellets are now separated from the water, such as by a centrifugal dryer, the further comparatively intensive cooling of the pellets is interrupted because the pellets release their heat energy to surrounding air slower than they would to surrounding water.

After the dryer, the pellets are agitated for a period of time as discussed above. During an agitation dwell time of several seconds, preferably even several minutes, a slow heating up of the pellet surfaces takes place through the distribution of the higher pellet core heat energy to the pellet surfaces. The surfaces reach, for example, 140°-150° C. whereby the desired effect of direct crystallization occurs. This crystallization is indicated visually by the color change of the pellets, which change from a first glassy or translucent condition into an opaque white coloring.

In the drawing, 1 is used to label a melt pump 14 and a screen changer 12 to which a polyester is fed according to the arrow F.sub.1. At the outlet of the screen changer 12, an underwater pelletizer 2 is provided through which pellets with a ball shape or a lenticular shape are produced. These pellets are guided through a conveyor device to a water/solids separating device 3, e.g., a centrifuge, whereby the conveying is carried out by process water that preferably has a temperature of over 80° C. The pellets leave the water/solids separating device 3 at a temperature of over 110° C. and are fed to a conveyor channel 4 to which supply air can be fed at 5 that leaves the conveyor device 4 at 6 and ensures a removal of the moisture. The conveyor device 4 is embodied as a conveyor channel with dams 7 aligned crosswise with respect to the conveyor direction and vibration motor 11, and the pellets leave the conveyor device 4 at a pellet temperature of over 100° C. and can be fed via a so-called pellet diverter valve 8 to an after treatment device 9 or a silo 10.

These pellets are crystallized at least to 40%, and can be handled.

Surface temperature of pellets according to the present invention can be measured by contactless infrared devices, such as those that are readily available in the market, e.g., model “Raynger MX” of the brand “RAYTEC”, which model is pistol-shaped and with which a surface temperature can be measured over a distance of several feet.

Moreover, the surface temperature of the pellets can be estimated as being around 20° C. to 30° C. lower than the core temperature of the pellets, so that by measuring the surface temperature of the pellets, a rough idea of the core temperature can be deducted. Accordingly, measurement of the surface temperature can provide an estimation of the core temperature of the pellets by adding about 20° C. to 30° C. to the measured surface temperature.

It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.

Claims

1. A method for thermally processing polyester pellets in order to achieve partial crystallization, comprising feeding polyester melt to an underwater pelletizer followed by pelletizing the polyester melt in the underwater pelletizer to obtain pellets having a ball shape or lenticular shape, feeding the pellets to a water/solids separating device to dry the pellets so that the pellets have a pellet surface temperature of no less than 100° C. to no greater than 160° C., feeding the dried pellets at a pellet temperature of greater than 100° C. to an agitation device, transporting and agitating the pellets on the agitation device to obtain partial crystallization utilizing specific heat present in the pellets, and removing pellets from the agitation device at a pellet temperature of over 80° C.

2. The method according to claim 1, further comprising flowing a fluid around the pellets during agitation of the pellets in the agitation device.

3. The method according to claim 1, wherein the pellets are in a form of a pellet layer, and further comprising flowing a fluid around the pellet layer during agitation of the pellets in the agitation device.

4. The method according to claim 1, wherein the pellets are fed to the agitation device at a pellet surface temperature of over 110° C.

5. The method according to claim 4, further comprising flowing a fluid around the pellets during agitation of the pellets in the agitation device.

6. The method according to claim 4, wherein the pellets are in a form of a pellet layer, and further comprising flowing a fluid around the pellet layer during agitation of the pellets in the agitation device.

7. The method according to claim 1, wherein the pellets are conveyed from the underwater pelletizer to the water/solids separating device with hot process water.

8. The method according to claim 7, wherein the process water has a temperature of 98° C.

9. The method according to claim 1, wherein thermal processing leading to partial crystallization utilizes specific heat present in the pellets.

10. The method according to claim 1, wherein the polyester pellets comprise polyethylene terephthalate pellets.

11. An apparatus for carrying out a method for the thermal processing of polyester pellets in order to achieve a partial crystallization of the pellets, said apparatus comprising: a melt pump; a screen changer; an underwater pelletizer; a water/solids separating device; and a conveyor device for transporting pellets, said conveyor device being arranged downstream of said underwater pelletizer and said water/solids separating device, said conveyor device being constructed and arranged to agitate pellets and crystallize pellets during transport through specific heat of the pellets.

12. The apparatus according to claim 11, wherein the conveyor device comprises a conveyor channel.

13. The apparatus according to claim 11, wherein the conveyor device comprises a vibrating conveyor device.

14. The apparatus according to claim 13, wherein the vibrating conveyor device comprises a conveyor channel.

15. The apparatus according to claim 11, wherein the conveyor device comprises a plurality of spaced apart dams distributed over the length of the conveyor device, each of said plurality of spaced apart dams causing a damning up of material.

16. The apparatus according to claim 12, wherein the conveyor device comprises a plurality of spaced apart dams distributed over the length of the conveyor channel, each of said plurality of spaced apart dams causing a damming up of material.

17. The apparatus according to claim 13, wherein the vibrating conveyor device comprises a plurality of spaced apart dams distributed over the length of the vibrating conveyor device, each of said plurality of spaced apart dams causing a damming up of material.

18. The apparatus according to claim 14, wherein the vibrating conveyor device comprises a plurality of spaced apart dams distributed over the length of the vibrating conveyor device, each of said plurality of spaced apart dams causing a damming up of material.

19. The apparatus according to claim 11, wherein the conveyor device is surrounded at least in part by a housing.

20. The apparatus according to claim 11, wherein the water/solids separating device comprises a centrifuge.

21. The apparatus according to claim 11, wherein the water/solids separating device comprises a centrifuge.

22. A polyester pellet crystallized at least to 40% by specific heat present in the pellet from its formation.

23. A PET pellet crystallized by specific heat present in the pellet from it formation, wherein said pellet is obtained by a process comprising: forming hot PET into pellets with a core temperature of 130-180° C. by underwater granulation; cooling the surface temperature of the pellets with water to about 110° C., while the core temperature in the pellets is higher; separating the pellets from the water; and allowing the surface temperature of the pellets to rise to 140-150° C. to effect crystallization of the pellets.

24. A polyester pellet crystallized at least to 40% by specific heat present in the pellets from in formation, which pellet has an outermost layer spheroylitic structure of a particle which is equal or smaller than at the center of the particle as distinguishable by polarization-contrast optical microscopy.

25. The polyester pellet according to claim 24, wherein the pellet has a degree of crystallization at the center of a particle which is at least as great as the degree of crystallization of the outermost layer thereof.

26. The polyester pellet according to claim 24, which has an acetaldehyde content between 0.5 and 100 ppm.

27. The polyester pellet according to claim 26 which has an acetaldehyde content between 0.5 and 70 ppm.

28. The polyester pellet according to claim 27 which has an acetaldehyde content between 0.5 and 60 ppm.

29. The polyester pellet according to claim 24 which has a heat of fusion of less than 50 kJ per kg.

30. A method of producing a PET pellet, which comprises using specific heat present in the pellet from its formation to produce a pellet with at least 40% crystallization and an acetaldehyde content between 0.5 and 100 ppm.

31. The polyester pellet according to claim 25, which has a heat of fusion greater than 50 kJ per kg.

32. A PET polyester pellet produced by direct crystallization and having an opaque white coloring.

33. A PET polyester pellet according to claim 32 wherein the crystallization is at least 40%.

34. A PET polyester pellet according to claim 32 wherein during direct crystallization, the pellet surface has been cooled from a maximum temperature of 160° C. to approximately 110° C. and then allowed to heat to a surface temperature of 140 to 150° C. by transfer of heat energy from the pellet core.

35. A PET polyester pellet according to claim 32 that has undergone a change in color from a first translucent condition to opaque, white coloring during its direct crystallization.

36. The method according to claim 1 wherein the pellet surface temperature rises to 140° to 150° C. during the partial crystallization step.