U.S. patent application number 11/888229 was filed with the patent office on 2008-03-20 for method and apparatus for thermally processing polyester pellets and a corresponding pellet preparation.
Invention is credited to Theodor Bruckmann.
Application Number | 20080071061 11/888229 |
Document ID | / |
Family ID | 34524035 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080071061 |
Kind Code |
A1 |
Bruckmann; Theodor |
March 20, 2008 |
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.degree. C. to an
agitation device that the pellets leave at a pellet temperature of
over 100.degree. C.
Inventors: |
Bruckmann; Theodor; (Xanten,
DE) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG & WOESSNER, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Family ID: |
34524035 |
Appl. No.: |
11/888229 |
Filed: |
July 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10962614 |
Oct 13, 2004 |
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11888229 |
Jul 31, 2007 |
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Current U.S.
Class: |
528/480 ;
264/142; 425/308 |
Current CPC
Class: |
B29K 2067/00 20130101;
C08J 2367/02 20130101; C08J 3/12 20130101; B29B 9/16 20130101; C08G
63/88 20130101; B29B 9/065 20130101; B29K 2995/0041 20130101 |
Class at
Publication: |
528/480 ;
264/142; 425/308 |
International
Class: |
C08G 63/02 20060101
C08G063/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2003 |
DE |
103 49 016.7-43 |
May 3, 2004 |
DE |
10 2004 021 595.2 |
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.degree. C. to no
greater than 160.degree. C., feeding the dried pellets at a pellet
temperature of greater than 100.degree. 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.degree. 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.degree. 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.degree. 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.degree. C. by underwater granulation;
cooling the surface temperature of the pellets with water to about
110.degree. 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.degree. 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.degree. C. to approximately 110.degree.
C. and then allowed to heat to a surface temperature of 140 to
150.degree. 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.degree. to 150.degree. C. during the
partial crystallization step.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 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. .sctn.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
[0002] 1. Field of the Invention
[0003] The invention relates to a method for thermally processing
polyester pellets in order to achieve crystallization and to the
polyester pellet preparation produced thereby.
[0004] 2. Discussion of Background Information
[0005] Polyethylene terephthalate, hereinafter also referred to
herein as PET for short, is a polyester with repeating ester
groups.
[0006] 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.
[0007] 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.degree. C.,
stretched with compressed air and blown to produce a bottle
(stretch blow process).
[0008] 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.
[0009] 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.degree. C., it can already become
sticky before crystallization at temperatures above approx.
70.degree. C.
[0010] 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.
[0011] 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.degree.-182.degree. C. The water is introduced at 160.degree.
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/cm.sup.2 or
higher. It is evident that such a method is extraordinarily
expensive and therefore can barely be conducted economically.
[0012] 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.
[0013] 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
[0014] 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.
[0015] 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.degree. C. to an agitation
device, and removing pellets from the agitation device at a pellet
temperature of over 80.degree. C.
[0016] 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.
[0017] 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.
[0018] The method can further include flowing a fluid around the
pellets during agitation of the pellets in the agitation
device.
[0019] 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.
[0020] The pellets can be fed to the agitation device at a pellet
surface temperature of over 110.degree. C.
[0021] The pellets can be conveyed from the underwater pelletizer
to the water/solids separating device with hot process water.
[0022] The process water can have a temperature of 98.degree.
C.
[0023] The thermal processing leading to partial crystallization
can utilize specific heat present in the pellets.
[0024] The polyester pellets can comprise polyethylene
terephthalate pellets.
[0025] The conveyor device can comprise a conveyor channel.
[0026] The conveyor device can comprise a vibrating conveyor
device.
[0027] The vibrating conveyor device can comprise a conveyor
channel.
[0028] 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.
[0029] The conveyor device can be surrounded at least in part by a
housing.
[0030] The water/solids separating device can comprise a
centrifuge.
[0031] 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.
[0032] 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.degree. 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.degree. C., since it is ensured that the
extrusion temperature of the PET is maintained for as long as
possible.
[0033] 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.
[0034] 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.degree. 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.
[0035] Another object of the invention is to propose a device with
which the effective agitation of the pellets is possible.
[0036] 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.
[0037] 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.
[0038] 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
[0039] 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
[0040] 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.
[0041] 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.
[0042] Related to the examples described, a hot plastic with a
temperature of from, for example 130 to 180.degree. 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.degree. 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.
[0043] 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.degree.-150.degree. 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.
[0044] 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.degree. C. The pellets leave the water/solids separating device
3 at a temperature of over 110.degree. 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.degree. C. and can be fed via a
so-called pellet diverter valve 8 to an after treatment device 9 or
a silo 10.
[0045] These pellets are crystallized at least to 40%, and can be
handled.
[0046] 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.
[0047] Moreover, the surface temperature of the pellets can be
estimated as being around 20.degree. C. to 30.degree. 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.degree. C. to
30.degree. C. to the measured surface temperature.
[0048] 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.
* * * * *