U.S. patent application number 15/778778 was filed with the patent office on 2018-11-29 for high molecular weight polyamides and copolyamides with uniform rv and low gel content.
The applicant listed for this patent is Ascend Performance Materials Operations LLC. Invention is credited to James E. Polk, Chris E. Schwier, Ashish Sen, Craig A. Trask, Cihan Uzunpinar, Chie-Hsiung Wang, J. Marty Zabcik.
Application Number | 20180340042 15/778778 |
Document ID | / |
Family ID | 58798006 |
Filed Date | 2018-11-29 |
United States Patent
Application |
20180340042 |
Kind Code |
A1 |
Polk; James E. ; et
al. |
November 29, 2018 |
High Molecular Weight Polyamides and CoPolyamides with Uniform RV
and Low Gel Content
Abstract
A customizable polyamide polymer, in particular Nylon 66, Nylon
6, and copolyamides, having a high molecular weight, excellent
color, and low gel content is disclosed. In particular, disclosed
is a polymer having a relative viscosity greater than 50 as
measured in a 90% strength formic acid solution; consistent
viscosity with a standard deviation of less than 1; a gel content
no greater than 50 ppm as measured by insolubles larger than 10
micron; an optical defect content of less than 2,000 parts per
million (ppm) as measured by optical control system (OCS). The
polymer can be made into monofilaments or a multifilament yarn.
Also disclosed is a process of producing the polymer using in-line
vacuum finishing technology in the absence of steam or other gases
in the second, or post condensation, step of the polymer
process.
Inventors: |
Polk; James E.; (Milton,
FL) ; Schwier; Chris E.; (Boston, MA) ; Sen;
Ashish; (Pensacola, FL) ; Trask; Craig A.;
(Pensacola, FL) ; Uzunpinar; Cihan; (Chattanooga,
TN) ; Wang; Chie-Hsiung; (Gulf Breeze, FL) ;
Zabcik; J. Marty; (Pensacola, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ascend Performance Materials Operations LLC |
Houston |
TX |
US |
|
|
Family ID: |
58798006 |
Appl. No.: |
15/778778 |
Filed: |
November 29, 2016 |
PCT Filed: |
November 29, 2016 |
PCT NO: |
PCT/US16/63916 |
371 Date: |
May 24, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62261392 |
Dec 1, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29C 48/267 20190201;
B29C 2791/006 20130101; D01F 6/60 20130101; B29B 9/06 20130101;
B29B 2013/005 20130101; B29C 48/00 20190201; B29C 48/144 20190201;
B29C 48/04 20190201; B29C 48/40 20190201; C08G 69/04 20130101; B29C
48/022 20190201; B29K 2077/00 20130101; C08G 69/08 20130101; C08G
69/28 20130101; C08G 69/36 20130101; C08G 69/06 20130101; B29B
13/00 20130101; B29B 9/12 20130101; C08G 69/30 20130101; B29B
13/022 20130101; B29B 13/06 20130101 |
International
Class: |
C08G 69/30 20060101
C08G069/30; B29B 9/12 20060101 B29B009/12; B29C 47/00 20060101
B29C047/00; B29C 47/08 20060101 B29C047/08; B29C 47/40 20060101
B29C047/40; D01F 6/60 20060101 D01F006/60; C08G 69/36 20060101
C08G069/36 |
Claims
1. A high molecular weight polyamide polymer, wherein the polyamide
polymer is characterized by a precision Relative Viscosity greater
than 50 as measured in a 90% strength formic acid solution, wherein
the precision Relative Viscosity has an RV Standard Deviation of
less than or equal to 1.25.
2. The polyamide polymer of claim 1, wherein the polymer is Nylon
6,6 polymer.
3. The polyamide polymer of claim 1, wherein the polymer is Nylon 6
polymer.
4. The polyamide polymer of claim 1, wherein the polymer is a
random copolymer of Nylon 6,6 and Nylon 6
5. The polyamide polymer of claim 1, wherein the Relative Viscosity
is greater than 70 as measured in a 90% strength formic acid
solution.
6. (canceled)
7. The polyamide polymer of claim 1, wherein the Relative Viscosity
is in the range of from 50 to 200 as measured in a 90% strength
formic acid solution.
8. The polyamide polymer of claim 1, wherein the Relative Viscosity
is in the range of from 75 to 100 as measured in a 90% strength
formic acid solution.
9. (canceled)
10. The polyamide polymer of claim 1, wherein the precision
Relative Viscosity has an RV Standard Deviation of less than
1.0.
11-15. (canceled)
16. The polyamide polymer of claim 1, wherein the polyamide polymer
is characterized by: a Gel Content Parameter of less than 50 ppm as
determined by parts per million insolubles larger than 10 microns
in 90% formic acid at 25.degree. C.; and an Average Optical Defect
Level of less than 2,000 parts per million (ppm) as measured by
optical scanning of pellets.
17. The polyamide polymer of claim 16, wherein the polymer is Nylon
6,6 polymer.
18. The polyamide polymer of claim 17, wherein further the Nylon
6,6 polymer exhibits a Gel Content Parameter of less than 40 ppm as
determined by parts per million insoluble larger than 10 microns in
90% formic acid at 25.degree. C.
19-25. (canceled)
26. A method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and low gel content comprising:
(a) providing a first polyamide polymer melt comprising a first
polyamide polymer with a first Relative Viscosity; (b) feeding the
first polyamide polymer melt to a twin screw extruder; (c)
melt-processing the first polyamide polymer melt under vacuum in
the twin screw extruder in the absence of added steam to remove
steam and other volatiles therefrom, thereby increasing the
molecular weight of the polymer melt to provide a second polyamide
polymer melt comprising a second polyamide polymer with a second
Relative Viscosity, said second polyamide polymer being
characterized by either: (i) a precision Relative Viscosity greater
than 50 as measured in a 90% strength formic acid solution with an
RV Standard Deviation of less than or equal to 1.25; or (ii) a Gel
Content Parameter of less than 50 ppm as determined by parts per
million insoluble larger than 10 microns in a 90% formic acid
solution at 25.degree. C.; (d) optionally feeding the second
polymer melt to a residence time dwell vessel and melt-processing
the second polymer melt in the residence time dwell vessel to
provide a third polyamide polymer melt comprising a third polyamide
polymer with a third Relative Viscosity higher than the second
Relative Viscosity of the second polyamide polymer, said third
polyamide polymer being characterized by either: (i) a precision
Relative Viscosity greater than 50 as measured in a 90% strength
formic acid solution with an RV Standard Deviation of less than or
equal to 1.25; or (ii) a Gel Content Parameter of less than 50 ppm
as determined by parts per million insoluble larger than 10 microns
in a 90% formic acid solution at 25.degree. C. and an Average
Optical Defect level of less than 2000 ppm as measured by optical
scanning at 50 micron resolution; and (e) recovering a product
polyamide polymer characterized by either: (i) a precision Relative
Viscosity greater than 50 as measured in a 90% strength formic acid
solution with an RV Standard Deviation of less than or equal to
1.25; or (ii) a Gel Content Parameter of less than 50 ppm as
determined by parts per million insoluble larger than 10 microns in
a 90% formic acid solution at 25.degree. C. and an Average Optical
Defect level of less than 2000 ppm as measured by optical scanning
at 50 micron resolution.
27. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 26, wherein the polyamide polymer melt is melt-processed
in the twin screw extruder at a temperature in the range of from
280.degree. C. to 350.degree. C.
28. (canceled)
29. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 26, wherein the polyamide polymer melt is melt-processed
in the twin screw extruder under vacuum in the range of 600 mm Hg
vacuum to 725 mm Hg vacuum.
30. (canceled)
31. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 26, wherein the polyamide polymer melt is melt-processed
in the twin screw extruder for a residence time in the extruder of
less than 60 seconds.
32. (canceled)
33. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 26, wherein the polyamide polymer melt is melt-processed
in the twin screw extruder for a residence time in the extruder of
less than 20 seconds.
34. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 26, wherein the polyamide polymer melt is melt-processed
in the twin screw extruder for a residence time in the extruder of
from 10 seconds to 60 seconds.
35. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 26, comprising feeding the second polymer melt to a
residence time dwell vessel and melt-processing the second polymer
melt in the residence time dwell vessel to provide the third
polyamide polymer melt comprising a third polyamide polymer with a
third Relative Viscosity higher than the second Relative Viscosity
of the second polyamide polymer, said third polyamide polymer being
characterized by either: (i) a precision Relative Viscosity greater
than 50 as measured in a 90% strength formic acid solution with an
RV Standard Deviation of less than or equal to 1.25; or (ii) a Gel
Content Parameter of less than 50 ppm as determined by parts per
million insoluble larger than 10 microns in a 90% formic acid
solution at 25.degree. C. and an Average Optical Defect level of
less than 2000 ppm as measured by optical scanning at 50 micron
resolution.
36. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 35, wherein the polyamide polymer melt is melt-processed
in the residence time dwell vessel at a temperature in the range of
from 280.degree. C. to 350.degree. C.
37-38. (canceled)
39. The method of making a high molecular weight polyamide polymer
with a precision Relative Viscosity and a low gel content according
to claim 35, wherein the polyamide polymer melt is melt-processed
in the residence time dwell vessel for a residence time in the
residence time dwell vessel of at least 1 minute.
40. (canceled)
Description
CLAIM FOR PRIORITY
[0001] This patent application is a national phase application of
PCT/US2016/063916 FILED 29 Nov. 2016 which was based on U.S.
Provisional Application Ser. No. 62/261,392 filed 1 Dec. 2015, both
entitled High Molecular Weight Polyamides and CoPolyamides with Low
Gel Content and Low Impurities. The priorities of the foregoing
applications are hereby claimed and their disclosures incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a customizable high
molecular weight polyamides, including Nylon 66, Nylon 6, and
copolyamides, and others having uniform RV and low gel content. The
resultant polymer is suitable for various applications of fiber and
film formation, in particular fiber spinning.
BACKGROUND
[0003] Traditionally, continuous polyamide production, in
particular Nylon 6 (also referred to as N6 (poly caproamide) and
Nylon 6,6, N66 or hexamethylene adapamide) polymerization processes
involve an expansive and intensive multi-vessel infrastructure in
order to achieve the final desired relative viscosity (RV). This is
due to the necessity of boiling off large amounts of solution water
along with the long hold up times required for the polymerization
kinetics with currently practiced catalyzed and/or non-catalyzed
systems.
[0004] An objective of the present invention is to be able to
create customizable nylon 6 or nylon 66 polymer or copolymers
having uniform RV and low gel content. A separate objective is to
simplify the continuous N6 and N66 polymerization process by
reducing the polymer residence time and thus polymer
degradation.
[0005] The resulting polymer is useful for making N6 or N66 polymer
for the injection molding, film, and fiber industries.
[0006] The present invention addresses the need for a low residence
time process for the continuous production of polyamides, in
particular nylon polymer and products neat and compounded through
the utilization of extruder and vacuum technology to simultaneously
increase molecular weight (MW) and compound additives from a direct
fed polymer melt stream.
DESCRIPTION OF THE RELATED ART
[0007] Numerous references describe polyamides and copolyamides,
fibers and films formed from the materials and procedures for
producing the polymers and articles. Following is a brief summary
of related art.
[0008] U.S. Pat. No. 6,235,390, to Glenn Alan Schwinn et al.,
discloses polyamide filament with formic acid relative viscosity of
at least 140 and tenacity in the range of 4.5 to 7 gpd for use in
papermaking machine felts and other staple fiber applications.
Although the patent discloses improved relative viscosity, the
range of tenacity of the monofilament obtained indicates lower
strength of the polyamide filament.
[0009] U.S. Pat. No. 8,211,340, to Swu-Chen Shen et al., discloses
a process to produce a squared-analogous cross-section polyamide
filament for uncoated airbag fabrics using melt extrusion
technique. The polyamide filament obtained has a reported tenacity
in the range of 7.5 to 9.5 g/denier and elongation of breakage of
18 to 30%.
[0010] U.S. Pat. No. 7,381,788 to Tsujii Yasuhito et al discloses a
method for continuous production of polyamide polymer having a
relative viscosity with low standard deviation. This is also
elaborated in Table 1 of the patent.
[0011] The patents to Yuo, U.S. Pat. No. 5,298,598, U.S. Pat. No.
5,298,597, U.S. Pat. No. 5,298,594, U.S. Pat. No. 5,290,747, U.S.
Pat. No. 5,264,541, U.S. Pat. No. 5,264,406, U.S. Pat. No.
5,260,246, each disclose a reactive extrusion process that includes
polyamides and an alkali metal hypophosphite compound.
[0012] U.S. Pat. No. 6,900,267, to Royer, discloses a reactive
extrusion process combining at least one polymer, oligomer, or
combination thereof, and a carbon dioxide containing fluid in an
extruder.
[0013] U.S. Pat. No. 5,651,927, to Auda, discloses an extruder
whereby multiple sequential chemical reactions are carried out
within multiple reaction zones.
[0014] U.S. Pat. No. 5,169,582, to Illing, discloses a method for
making caprolactam by feeding the mass to an extruder provided at
an increased temperature and vacuum to attain the desired degree of
polymerization.
[0015] U.S. Pat. No. 5,102,594, to Burlet, discloses a process for
making thermoplastic polymers using vented extruders.
[0016] U.S. Pat. No. 4,902,455, to Wobbe, discloses a method for
degassing thermoplastic melts over a wide range of viscosities
using a degassing extruder including a plurality of sequential
degassing sections.
[0017] U.S. Pat. No. 3,657,195, to Doerfel, discloses a process for
making high molecular weight nylon 6,6 by continuous further
condensation of low molecular weight nylon 6,6 in a self-cleaning
screw extruder reactor. The extruder includes at least one
degassing orifice at elevated temperature and pressure.
[0018] U.S. Pat. No. 4,760,129, to Haering, discloses a process for
preparing highly viscous polyhexamethyleneadipamide (nylon 6,6)
using an extruder and injection of steam or gas having a residence
time of 1-4 minutes.
[0019] U.S. Pat. No. 5,079,307, to Taylor et al., discloses a high
molecular weight polyamide production from carboxy terminated
polyamide prepolymers using a twin extruder and a catalyst as
polymerization aid.
[0020] U.S. Pat. No. 5,543,495, to Anolick et al., discloses a
process for increasing the molecular weight of polyamides and other
condensation polymers, using a twin extruder under gas, with a
catalyst, an activator, and a residence time of seconds to
minutes.
[0021] U.S. Pat. No. 5,683,808, to Earl Blaine Adams et al.,
discloses a polyamide monofilament having a formic acid relative
viscosity of at least 60, tenacity greater than 10 grams per denier
(gpd), an along end standard deviation of tenacity of less than 0.1
gpd, and a hot air shrinkage at 177.degree. C. of less than 15%.
The polyamide monofilament is extruded by injecting low pressure
steam or heated, which may contaminate the polyamide filament and
further lower the overall tensile strength.
[0022] U.S. Pat. No. 5,707,733, to Max Kurt et al., discloses a
nylon 6,6 monofilament with improved initial modulus, strength,
LASE and wet relaxation as compared to standard polyamide (PA 66)
monofilament. The patent also discloses that the nylon 6,6
monofilament has breaking extension of less than 25%. There
continues to exist a need for a customizable polymer having desired
properties (i.e., high molecular weight, high uniformity of
molecular weight, low gel content) for particular end uses, as well
as a process to produce a polyamide with greater efficiency than
currently known in the art.
SUMMARY OF THE INVENTION
[0023] The present invention is directed to high molecular weight
polyamides, with uniform viscosity substantially free of gels,
wherein the relative viscosity (RV) ranges from about 50 or so to
200. The objective is to produce a customizable uniform polyamide
polymer having minimal to substantially no gel content. The high
molecular weight polymer, primarily nylon 6 and nylon 6,6 polymer
and random copolymers thereof, substantially free of gels,
preferably comprises a viscosity preferably greater than 50 RV, a
uniform viscosity (RV) with a standard deviation of less than 1.0,
a gel content as measured by insolubles larger than 10 micron less
than 50 parts per million (ppm), and an optical defect content as
measured by optical control systems (OCS) scanning technology of
less than 2000 parts per million (ppm). The resulting polymer can
be heat stabilized and formed into fibers (monofilament or
multifilaments).
[0024] The polyamides of the invention have unexpectedly superior
properties in terms of RV uniformity, gel content, optical
appearance and fiber spinning performance as compared to existing
products. Moreover, the inventive polymer when spun into fiber
exhibits an unexpectedly low pack pressure rise, leading to greater
pack life, for example greater than 10 days and preferably greater
than 15 days. Applications for this polymer include converting to a
monofilament or a multifilament fiber (yam) having the following
properties: tenacity greater than 9.0 grams per denier (g/d);
elongation greater than 18%, and broken filaments less than 2 per
20 lbs bobbin.
[0025] Another aspect of the present invention is directed to the
process of melt polymerization of polyamides, with a description of
N66 to a high molecular weight polymer in the presence of an active
phosphorous based polyamidation catalyst in a heated vented vacuum
process extruder, in the absence of added steam or gas.
[0026] While the invention is described relative to polyamides, and
in particular Nylon 66, Nylon 6, and copolyamides thereof, the
invention can be applied to all polyamides ranging from aliphatic
polyamides (traditionally N6 and N66 or other aliphatic nylons) to
polyamides with aromatic components (for example
Paraphenylenediamine and terephthalic acid), to copolymers such as
adipate with 2-methyl pentmethylene diamine and
3,5-diacarboxybenzenesulfonic acid (or sulfoisophthalic acid in the
form of its sodium sulfonate salt).
[0027] It has not heretofore been seen in the art to have a process
to produce a polymer having an adjustable precision RV with such
high level of RV uniformity and low level of gel as described
herein.
[0028] Generally, gel bodies are not visible to the eye in the
polymer without an optical microscope. It is also necessary to have
a method to enhance the contrast between the polymer and the gels,
with use for example of a broad spectrum ultraviolet (UV) or near
ultraviolet (UV) light for florescence excitation. U.S. Pat. No.
4,760,129 (1988, assigned to Werner & Pfleiderer) discloses
production of highly viscous (RV equal or greater than 4) high
molecular weight (Mn=34000, Mw=2.1) nylon 6,6 polymer using added
superheated steam in the post polycondensation reaction. The data
reports gels in terms of present or absent in the resultant
polymer. There are no details provided on the measurement of gel or
impurity content other than an assumed visual observance test. It
is now known that for gels to be visible to the human eye, the gel
content is incredibly high in the polymer. The minimum size an
adult can discern is on the order of 30 microns or so. Therefore,
while high molecular weight may have been achieved by the process
of US'129, a low-gel polymer as defined herein is not
suggested.
[0029] An advantage of the present invention is the potential low
residence time (seconds versus minutes and hours) in the absence of
added steam or gas in the post condensation phase, during which
complete polymerization of the N66 is achieved starting from a
pre-polymer or suitable material. The potential for a significant
process simplification and the ability to quickly complete the
partial and/or complete polyamidation of the N66 via reactive
extrusion provides opportunities for numerous other process
simplifications such as the ability to polymerize compounds and
blend polymers continuously inline--i.e., in the same process
step.
[0030] The removal of volatile components from the process by
application of vacuum increases polymer quality and reduces the
propensity for gelation and gassing in applications like molding,
film, and fiber production. The process of the invention will also
reduce the propensity for undesirable side reactions, such as
crosslinking, since volatile organic compounds are absent and the
process residence time is significantly reduced. The inventive
process with the reduced residence time reduces gel formation in
the resulting polymer.
[0031] Narrow residence time distribution along with "reduced or
short" residence time are two separate concepts. While short
residence time is discussed herein, narrow residence time is also
applicable to the present invention. The distribution curve of a
narrow residence time is narrowed (or tightened) and aids in the
improvement of gel formation (or lack of gel formation).
Specifically, it is the long tail of high residence times that
leads to gel, and it is possible to have a long tail even if the
average residence time is short. Twin screw extruders are well
known to have a narrow residence time distribution due to the fact
that it has all-wiped surfaces precluding any dead zones.
[0032] This inventive system is compact, simple, and does not
require inventory of low RV material used to produce high RV
material unlike a conventional SSP (Solid State Polymerization)
process. Another advantage of this system is the vacuum component,
which removes volatiles and other impurities that reduce the
propensity of crosslinking and gel formation, thus increasing
polymer quality.
[0033] In addition, the present invention will result in improved
performance in operations such as fiber spinning, molding, and film
production in that the volatiles that react upon re-extrusion are
not present and thus cannot react.
BRIEF DESCRIPTION OF DRAWINGS
[0034] The figures illustrate potential applications of the present
invention and represent exemplary embodiments and are not intended
to limit the description of the present invention as otherwise
described herein.
[0035] FIG. 1 illustrates an embodiment of a preferred
polymerization system of the present invention with vacuum
capability.
[0036] FIG. 2 illustrates a conventional high molecular weight
solid state finishing process with high inventory.
[0037] FIGS. 3A-3D illustrate gel bodies associated with filament
breaks in Nylon 66 fiber.
[0038] FIG. 4 is a histogram illustrating an RV distribution of a
polymer having a nominal RV of 85 made by a preferred process of
the present invention.
[0039] FIG. 5 is a histogram illustrating an RV distribution of a
polymer having a nominal RV of 85 made by an SSP process utilizing
the apparatus of FIG. 2.
DETAILED DESCRIPTION OF EMBODIMENTS
[0040] The invention is described in detail below in connection
with the Figures for purposes of illustration, only. The invention
is defined in the appended claims. Terminology used herein is given
its ordinary meaning consistent with the definitions set forth
below. Vacuum, for example, is expressed in mm Hg at 0.degree.
C.
[0041] As used in the specification and claims, the singular forms
"a", "an" and "the" include plural references unless the context
clearly dictates otherwise. For example, the term "an article" may
include a plurality of articles unless the context clearly dictates
otherwise.
[0042] "Consisting essentially of" and like terminology refers to
the recited components and excludes other ingredients which would
substantially change the basic and novel characteristics of the
composition or article. Unless otherwise indicated or readily
apparent, a composition or article consists essentially of the
recited or listed components when the composition or article
includes 90% or more by weight of the recited or listed components.
That is, the terminology excludes more than 10% unrecited
components. Any polymeric composition of the present invention may
consist essentially of the recited components.
[0043] As used herein, "polyamides", "copolyamides" and like
terminology refers to compositions containing polyamides. Exemplary
polyamides and polyamide compositions are described in Kirk-Othmer,
Encyclopedia of Chemical Technology, Vol. 18, pp. 328-371 (Wiley
1982), the disclosure of which is incorporated by reference.
Briefly, polyamides are products that contain recurring amide
groups as integral parts of the main polymer chains. Linear
polyamides are of particular interest and may be formed from
condensation of bifunctional monomers as is well known in the art.
Polyamides are frequently referred to as nylons. Particular
polymers and copolymers and their preparation are seen in the
following patents: U.S. Pat. No. 4,760,129, entitled "Process for
Preparing Highly Viscous Polyhexamethyleneadipamide", to Haering et
al.; U.S. Pat. No. 5,504,185, entitled "Process for Production of
Polyamides, Polyamides Produced by Said Process and Polyamide Film
or Sheet", to Toki et al.; U.S. Pat. No. 5,543,495, entitled
"Process for Increasing the Molecular Weight of Polyamides and
Other Condensation Polymers", to Anolick et al.; U.S. Pat. No.
5,698,658, entitled "Linear Very High Molecular Weight Polyamides
and Process for Producing Them", to Dujani et al.; U.S. Pat. No.
6,011,134, entitled "Method for Manufacturing Poly(Hexamethylene
Adipamide) from Monomethyladipate and Hexamethylenediamine", to
Marks et al.; U.S. Pat. No. 6,136,947, entitled "Process and Device
for the Standardized Continuous Production of Polyamides", to
Wiltzer et al.; U.S. Pat. No. 6,169,162, entitled "Continuous
Polyamidation Process", to Bush et al.; "Polyamide Chain Extension
Process and Related Polyamide Product", to Zahr U.S. Pat. No.
7,138,482, entitled "Production Method of Polyamide", to Tanaka et
al.; U.S. Pat. No. 7,381,788, entitled "Method for Continuous
Production of Polyamide", to Tsujii et al.; and U.S. Pat. No.
8,759,475, entitled "Continuous Production of Polyamides", to
Thieny et al.
[0044] Percents, parts per million (ppm) and the like refer to
weight percent or parts by weight based on the weight of the
composition unless otherwise indicated.
[0045] Process temperatures refer to extruder set points unless
otherwise indicated.
[0046] Those with ordinary skill in the art will appreciate that
the elements in the Figures are illustrated for simplicity and
clarity and are not necessarily drawn to scale. For example, the
dimensions of some of the elements in the Figures may be
exaggerated, relative to other elements, in order to improve the
understanding of the present invention.
[0047] There may be additional components described in the
foregoing application that are not depicted on one of the described
drawings. In the event such a component is described, but not
depicted in a drawing, the absence of such a drawing should not be
considered as an omission of such design from the
specification.
[0048] Polymers of the current invention may be made into
monofilaments or multifilaments. Polymers of the current invention
may be made into blown films and cast films. The polymers can also
be made or used as gel-free sheet extrusion for subsequent thermal
forming and injection molded parts such as cable ties.
Monofilaments may be used in 3D printing applications, ink
applications, etc. These polymers are applicable to any application
that requires excellent uniaxial or biaxial drawing technology and
applications needing high strength fibers such as for use in
industrial fabrics.
High Molecular Weight Nylon 6,6 Polymer
[0049] The high molecular weight polymer disclosed herein comprises
a polyamide polymer with relative viscosity (RV) greater than 50,
uniform viscosity with a standard deviation of typically less than
1, gel content less than 50 ppm, optical defects of less than (or
no greater than) 2000 parts per million (ppm).
[0050] In particular, disclosed herein is a high molecular weight
polyamide polymer, generally comprising a relative viscosity
greater than 50 as measured in a 90% strength formic acid solution;
consistent viscosity with a standard deviation of less than 1.0. a
gel content less than 50 ppm as measured by insolubles larger than
10 micron; and preferably an optical defect level of less than
2,000 parts per million (ppm) as measured by optical scanning of
pellets with an Optical Control System (OCS GMBH) analyzer. The
resulting polymer may or may not be heat stabilized.
[0051] In any embodiment, the RV of the nylon 6,6 polymer may be
greater than 50, 60, 70, 80, and 90. The RV is measured in 90%
strength formic acid. In another embodiment, the uniform viscosity
of the polymer standard deviation may be less than 1.2, 1.1, 1.0,
0.9. The polymer may include the gel content and the optical defect
level less than 50 ppm and 2000 ppm, respectively. In an
embodiment, the gel content and the contamination are preferably
less than 50 ppm and 100 ppm, respectively. The gel content is
measured by insolubles larger than 10 micron. The resulting fiber
formed from the polymer has a low pack pressure rise pack life of
greater than 15 days. In an embodiment, the resulting fiber formed
from the polymer has a pack life of more than 10, 20, 30, and also
more than 40 days.
[0052] The polymerization process optionally includes use of one or
more polyamidation catalysts such as hypo-phosphorus acid and salts
thereof. Specific examples include sodium hypophosphite, mono
sodium phosphate (MSP), manganese hypophosphite, and benzene
phosphinic acid (also called phenyl phosphinic acid or PPA).
[0053] The polymer may contain one or more additives such as
fiberglass, waxes, minerals, carbon fiber, fiber reinforcement,
heat stabilizers, color concentrates, impact modifiers, and flame
retardant additives. The polymer may also contain other commonly
used additives which are known to people skilled in this art.
Additives for Nylon 6 and Nylon 66 (depending on end use) may
include Alkylenediamine and monocarboxylic acids or primary or
secondary monoamines. Other additives and modifiers noted below may
be used in connection with any embodiment of the present
invention.
[0054] Canadian Pat. No. 963594 discloses heat-stable nylon 66
fibers with improved dyeability by adding sodium hypophosphite and
diphenylamine into the nylon salt solution before polymerization
reaction. U.S. Pat. No. 4,113,708 discloses a method using
phenylphosphinic acid to reduce the formation of ammonia during the
melt preparation of polyamide. Ger. Offen. DE 2158014 discloses a
method to stabilize nylon 66 by adding alkali metal hypophosphite
into amides and adipate before polymerization. Japanese Pat. Apps.
JP 89-179,534 and JP 90-111015 disclose a method for the
manufacturing of polyamides by first polymerizing diacids with
diamine in the presence of a hypophosphite to give an oligomer then
melt polymerizing the oligomer in the presence of a polyethylene
wax. Great Britain Pat. App. GB 6648485 discloses a heat and light
stabilizing additive for polyamide by adding sodium hypophosphite
and phenols containing at least one hydrocarbon radical and a
radical containing a COOH group or a derivative, to polyamide after
or during polycondensation. In Japanese Pat. App. JP 89-212160, the
polymerization additives contain manganese hypophosphite,
hexamethylenediamine, and triazine compounds, which are added to
reactants as fire retardants.
[0055] Hypophosphites have also been used as additives to modify
the properties of polyamide and/or copolyamide after the completion
of the polymerization reaction: a low-temperature antioxidant from
a halogenated hydroxyl ammonium compound, hydrosulfide, bisulfite,
phosphorus, and phosphate and a reducing agent from metal
hypophosphite and ammonium hypophosphite. Ger. Offen. DE 3636023
discloses a granulated thermoplastics for hot-melt adhesives by
mixing copolyamides with refined paraffin and sodium hypophosphite.
Japanese Pat. App. JP 85-198900 discloses a polyamide resin
composition by blending polyamides with modified polyolefin resins
and metal salts of H.sub.3PO.sub.4, H.sub.3PO.sub.2 and
H.sub.3PO.sub.2. Japanese Pat. App. JP 81-34897 discloses a method
for surface-sensitizing polyamide with sodium hydroxide and sodium
hypophosphite. Japanese Pat. App. JP 78-97229 discloses using
sodium hypophosphite as a heat stabilizer for copolyamide. Belg. BE
875530 discloses nonflammable polyester, polyamide and
polyester-polyamide compositions by mixing polymers or copolymers
with phosphinate salts. Japanese Pat. App. JP 90-208135 discloses a
polyhexamethyleneadipamide with restricted three-dimensional
structure. Copper acetate, potassium iodide or sodium hypophosphite
is added to the final polymerized product as stabilizers. Japanese
Pat. App. JP 90-116874 discloses mixing of sodium hypophosphite or
calcium acid hypophosphite with polyamide, to prevent
discolorization. Japanese Pat. App. JP 88-331806 discloses the use
of hypophospherous acid or hypophosphite as anti-coloring agent for
polyamide fillers. Japanese Pat. App. JP 88-273373 discloses an
injection moulded aliphatic polyamide container comprising a
polyamide and additives selected from orthophosphorous acid,
hypophosphorous acid, alkali metal salts and alkaline salts. Eur.
Pat. App. EP 88-305493 discloses a method by which sodium
hypophosphite and a cross-linking agent are added to a linear
aliphatic polyamide to improve its melt viscosity.
[0056] It has been found that the stain resistance of certain
polyamides can be improved by salt-blending the polyamide precursor
with a cationic dye modifier, such as 5-sulfoisophthalic acid or
salts or other derivatives; or copper iodide may be used to
stabilize the polyamide for electrical/electronic and automotive
molding applications.
[0057] While not necessarily needed for many embodiments of the
present invention, chain extenders may be used if so desired.
Suitable chain extender compounds include bis-N-acyl bislactam
compounds, isophthaloyl bis-caprolactam (IBC), adipoyl
bis-caprolactam (ABC), terphthaloyl bis-caprolactam (TBS), and
mixtures thereof.
[0058] Further, the polymer is converted to filaments. The
filaments have tenacity greater than 9.0 grams per denier (gpd) and
elongation greater than 18%. In an embodiment, the tenacity of the
filaments may be greater than 9.0, 9.5, 10, 10.5, 11, 11.5, and
12.0 gpd and the elongation of the filaments may be greater than
18, 19, 20, 21, 22, 23, 24, and 25%. The filaments drawn have
broken filaments less than 2 per 16 pounds (lbs) bobbin. More
preferred, the broken filaments drawn are less than 2 per 20 lbs, 2
per 22 lbs, 2 per 24 lbs, and 2 per 26 lbs bobbin. Further, the
polymer is used in several applications such as fibers, air bags,
and other industrial applications.
Process of Melt Polymerization
[0059] There are various routes or options to produce the high
molecular weight polyamides of the invention with low RV standard
deviation and substantially free of gels and optical defects.
Typically, variations in the in-line process of the invention such
as temperature requirements, types of extruders, modifiers, vacuum
elements, etc., and operation thereof are within the knowledge of a
skilled person. There are multiple methods for making the desired
polymer. The teachings herein represent the production of a
customizable polyamide polymer having the desired RV with little to
no gel content or fines present, low optical defects and also the
ability to make resulting fibers and yarns of high yield and high
tenacity.
[0060] Generally speaking, known processes for production of
polyamides is to make a low molecular weight polymer and then
increase the molecular weight by various means, typically involving
high temperatures and relatively long residence times. Low
molecular weight products are made by batch (autoclave) processes
(Ref: Big Chemical Encyclopedia, V19, [c.272]. D. B. Jacobs and J.
Zimmerman, in C. E. Schildknecht and I. Skeist, eds. Polymerisation
Processes, High Polymers, Vol. XXIX. Wiley-Interscience, New York,
1977, pp. 424, 467. A very detailed review of nylon-6,6
polymerization. [c.277], or continuous processes (reference: U.S.
Pat. No. 6,472,501). This low molecular weight polymer is then fed
to a solid state polymerization process which builds the molecular
weight to the desired high level. (see FIG. 2).
[0061] In order to produce the products of the present invention, a
low molecular weight polymer may be made via a batch autoclave
process and subsequently processed via solid state polymerization
(SSP) to high molecular weight. Autoclave processes are known to
yield polyamides with relatively low gel content, but the batch
process is prone to high variability in RV. To compensate for the
inherent RV variability in the batch autoclave process, one needs
to select a portion of the autoclave batch that would have a
narrower RV variation, discarding a large portion of material prior
to SSP processing, providing a relatively expensive and effort
intensive procedure.
[0062] One alternative is to make a low molecular weight polymer
via a continuous polymerization process and subsequently processing
the polymer via SSP to high molecular weight. Continuous processes
are known to yield very uniform RV, but conventional continuous
processes are prone to high gel content due to inherent dead zones
and non-uniform residence time leading to polymer degradation and
gelation. To compensate for the inherent gel production in the
continuous polymerization process, one may need to inspect every
pellet of product for gels and to remove them. Optical/pneumatic
automated processing may be employed; however here again providing
a relatively expensive and effort intensive procedure which is
prone to variability in product quality.
[0063] A third option (Option 3) is the inventive process described
in detail herein involving vacuum finishing technology at
relatively short residence times, which does not require
substantial inventory as do the procedures discussed immediately
above.
[0064] Option 3 is a process for producing a high molecular weight
polyamide polymer by vacuum finishing technology. The vacuum
finishing removes volatile components and allows for a resulting
high molecular weight polymer to be produced that is substantially
gel free. The absence of volatile materials and gels makes a more
pure polymer especially well-suited for the production of molded
parts, fibers, and films. The lower level of volatiles also reduces
gassing and voids in the fiber filaments, molded parts and films,
thus resulting in products with superior physical properties and
productivity. The absence of gels will also improve spin pack life
due to less contaminants, molded part and film defects, and
productivity. The inventive process occurs in the absence of added
or injected steam or gas during the second part of the reactor or
polymerization process (reactive extrusion under vacuum in a twin
screw extruder). The input temperature for the initial polymer feed
is approximately 285.degree. C. rising to a maximum of about
350.degree. C., preferably less than 310.degree. C., and most
preferably less than 290.degree. C., at the exit point of an
extruder. The residence time for the polymer held in the extruder
is less than 60 seconds, more preferably less than 30 seconds and
even more preferably less than 20 seconds. The vacuum for the
process is about 26-28 inches mercury, and a catalyst (e.g. PPA
(phenyl phosphoric acid)) can be used if desired. It has been found
that the reaction steam or vapor is removed from the extruder
within about thirty (30) seconds. The polymer leaving the extruder
can optionally be fed to a pipe for additional reaction, designated
as the Residence Time Block (RTB), sometimes referred to herein as
a residence time dwell vessel. The RTB is optionally provided with
tube inserts to ensure a uniform residence time distribution and
uniform melt temperature. A tube insert may be a static mixer
insert; Various configurations and types of tube inserts are
commercially available from Koch Heat Transfer Company and their
use is discussed in Chemical Engineering Process, September 2012,
pages 19-25; Shilling, Richard, L. The residence time in the RTB
can vary from 30 second to 5 minutes and up to 10 minutes. The melt
temperature can be from 290.degree. C. preferably and could be up
to a maximum of 350.degree. C.
[0065] In a preferred process of the present invention, the
combination of heat and mechanics (the movement of polymer through
the extruder) remove the water produced in the polymerization
reaction. However, it is an object to minimize the water formation
by (unlike prior art) not injecting additional steam into the
reaction at the post condensation stage. Steam is added at the
initial polymerization step. It has been found that the reaction
proceeds without the additional steam typically disclosed in the
prior art. This is turn leads to greater efficiency of the
equipment and more continuous operations since the ports are less
likely to become clogged or plugged. While U.S. Pat. No. 5,543,495
(1996 patent, assigned to DuPont) discloses production of high
molecular weight Nylon 6,6, it does so with the use of added steam
and catalyst throughout the process. The RV of the product was
shown to increase, however there was no discussion regarding gel or
impurity formation.
[0066] The present invention is better understood by reference to
the following test methods, additional definitions, attached
Figures and following examples.
Test Methods
[0067] The mechanical and chemical properties of the polymer and
the drawn filaments were measured using the following test
methods:
[0068] Relative viscosity (RV) of nylons refers to the ratio of
solution or solvent viscosities measured in a capillary viscometer
at 25.degree. C. (ASTM D 789). The solvent is formic acid
containing 10% by weight water and 90% by weight formic acid. The
solution is 8.4% by weight polymer dissolved in the solvent.
[0069] The relative viscosity, (.eta..sub.r), is the ratio of the
absolute viscosity of the polymer solution to that of the formic
acid:
.eta..sub.r=(.eta..sub.p/.eta..sub.r)=(f.sub.r.times.d.sub.p.times.t.sub-
.p)/.eta..sub.f
where: d.sub.p=density of formic acid-polymer solution at
25.degree. C., t.sub.p=average efflux time for formic acid-polymer
solution, s .eta..sub.f=absolute viscosity of formic acid,
kPa.times.s(E+6 cP) f.sub.r=viscometer tube factor, mm.sup.2/s
(cSt)/s=.eta..sub.r/t.sub.3 A typical calculation for a 50 RV
specimen:
.eta..sub.r=(f.sub.r.times.d.sub.p.times.t.sub.p)/.eta..sub.f
where f.sub.r=viscometer tube factor, typically 0.485675 cSt/s
d.sub.p=density of the polymer--formic solution, typically 1.1900
g/ml t.sub.p=average efflux time for polymer--formic solution,
typically 135.00 s .eta..sub.f=absolute viscosity of formic acid,
typically 1.56 cP giving an RV of
.eta..sub.r=(0.485675 cSt/s.times.1.1900 g/ml.times.135.00 s)/1.56
cP=50.0
[0070] The term t.sub.3 is the efflux time of the S-3 calibration
oil used in the determination of the absolute viscosity of the
formic acid as required in ASTM D789.
[0071] The Table below compares the ASTM D789 RV test method with
other standard viscosity measurements.
[0072] Conversion Chart for Relative Viscosity Test Methods:
Relative Viscosity
TABLE-US-00001 ASTM D789 JIS K 6920-2 ISO 307 Formic Acid (90%)
Sulfuric Acid (98%) Sulfuric Acid (95.7%) 40 2.5 2.4 45 2.7 2.5 50
2.8 2.7 55 2.9 2.8 60 3.0 2.9 65 3.1 3.0 70 3.2 3.1 75 3.3 3.1 80
3.4 3.2 85 3.5 3.3
Standard Deviation and RV Standard Deviation
[0073] The products of the invention are characterized by a
precision Relative Viscosity greater than 50 as measured in a 90%
strength formic acid solution as noted above, wherein the precision
Relative Viscosity has an RV Standard Deviation of less than or
equal to 1.25. The RV Standard Deviation of a material is the
standard deviation in Relative Viscosity of a material taken on at
least 15 randomly selected samples of that material. Preferably,
the randomly selected samples are randomly selected from a quantity
of 5 lbs or more of well mixed product. Still more preferably, at
least 25 samples are selected and analyzed from a well mixed
quantity of 10 lbs or more of product.
[0074] The standard deviation of a sample is defined as
follows:
s N = 1 N i = 1 N ( x i - x _ ) 2 , ##EQU00001##
where {x.sub.1, x.sub.2, . . . , x.sub.N} are the observed values
of the sample items and x is the mean value of these observations,
while the denominator N stands for the size of the sample: this is
the square root of the sample variance, which is the average of the
squared deviations about the sample mean.
[0075] Denier (ASTM D 1577) is the linear density of a fiber as
expressed as weight in grams of 9000 meters of fiber. The
monofilament is conditioned at 55.+-.2% relative humidity, and
75.degree..+-.2.degree. F. on the bobbin for 24 hours when the
monofilament has aged more than ten days since being made. A 0.9
meter sample of monofilament is weighed and denier is calculated as
the weight of a 9000 meter sample in grams. Denier times (10/9) is
equal to decitex (dtex). Denier, and tenacity tests performed on
samples of staple fibers are at standard temperature and relative
humidity conditions prescribed by ASTM methodology. Specifically,
standard conditions mean a temperature of 70+/-2.degree. F.
(21+/-1.degree. C.) and relative humidity of 65%+/-2%.
[0076] Tensile Properties such as tenacity, breaking strength and
elongation of the monofilament or multi filament were determined in
accordance with ASTM D 885M. Before tensile testing of -spun
monofilaments, the monofilament is conditioned on the package
(bobbin) for a minimum specified period at 55.+-.2% relative
humidity and 75.degree..+-.20.degree. F. This period is (unless
otherwise specified) is 24 hours when the filament has aged more
than ten days since spinning. Sample is air stripped prior to
testing. A recording is used to characterize the stress/stain
behavior of the conditioned monofilament. Samples are gripped in
air-activated clamps maintained at at least 40 psi pressure.
Samples are elongated to break while continuously recording
monofilament stress as a function of strain. Initial gauge length
is 10 inches (25.4 cm), and cross head speed is maintained at a
constant 6 inches (15.3 cm)/minute. Those of skill in the art will
appreciate that while the polymeric invention is may be primarily
for use in monofilaments, multifilaments can be made from the
customizable polymers. Breaking strength is recorded as the maximum
load in pounds or kilogram force and elongation is logged as the
strain in percentage prior to rupture of the sample. Tenacity is
calculated from the break strength divided by the denier (after
correcting for any adhesive on the filament) and is expressed as
grams per denier (gf/d).
Insoluble Material Test and Gel Content Parameter
[0077] The Insoluble Material Test is carried out wherein a
representative sample the product polymer (preferably at least 50
grams) is dissolved in an appropriate solvent, in this case 90%
formic acid at 25.degree. C. and processed as follows. The
resulting polymer--formic acid solution is filtered (EMD Millipore
47 mm diameter type AN1H04700 polypropylene filter with a 10 micron
pore size) and then the filter washed with fresh formic acid
solution at 25.degree. C. to remove any remaining polymer. A
further washing with reagent grade methanol is performed and the
filter and material remaining thereon are then dried to a constant
weight. The difference in the post-filtration weight and the tare
weight of the filter prior to use is taken to be the mass of
insoluble material in the product. Concentrations and sample size
utilized are selected to allow one to obtain an amount of insoluble
material that can be weighed with precision. Insoluble materials
may include gels, environmental contaminants, metals, degraded
additives, and other process and non-process related contamination.
Since it is found that the insolubles correlate closely in most
cases with gel content, the results are expressed as a Gel Content
Parameter in 90% formic acid at 25.degree. C. as is seen in the
following example calculation.
Typical Procedure/Sample Calculations:
[0078] 88.0 grams of resin is dissolved in 800 mL of 90% formic
acid 10% water at 25.degree. C. and vacuum filtered.
[0079] Filter starting weight is 85.000 milligrams.
[0080] After rinsing with clean formic and methanol and drying to
constant weight the filter with gels weighs 86.375 mg.
[0081] (86.375-85.000)/1000 mg/g)/88.0).times.1E6=15.6 ppm
insolubles or a Gel Content Parameter of 15.6 ppm.
Optical Control System (OCS) Measurements and Average Optical
Defect Level
[0082] Optical defects are measured by way of a vendor supplied
test based on the equipment employed (Optical Control Systems GmbH,
model PS-25 C). The unit utilizes a high speed CCD camera, with a
focusing/magnification lens, recording at 30 frames per second
(fps). The camera resolution is 63 micrometers (.mu.m) per pixel.
The camera is placed at a 90 degree angle perpendicular above the
sample transport system. This transport system utilizes a vibrating
platform to transport the sample past the camera field-of-view at a
constant rate. This platform, being of a pure white material, also
serves as the background for the analysis. Typical sample size is
0.5 to 5 kilograms (kg) of 2.5-3 mm cylindrical pellets. Placed
between the sample field-of-view and the camera is an annular light
source emitting visible light typically between 400 and 700
nanometers (nm). The light source can be a fluorescent type ring
bulb or an array of light emitting diodes (LEDs). The camera images
the sample field-of-view through a ring shaped opening in the light
source. As the sample is transported through the field-of-view, the
camera system records the reflected visible image. This software
also characterizes these defects based on color (up to five
different categories) and size of the defects. The defect sizes are
categorized into as many as 10 different categories from 63 .mu.m
up to the maximum size selected. The apparent diameter is
calculated from which a defect volume can be derived. The system
expresses the Average Optical Defect Level in ppm (assuming
constant density) based on defects in a size range of from 25
microns to 5 mm. The analysis detects gels, black specks, fish
eyes, holes, and wrinkles, scratches, coating voids, water drops,
oil stains, insects, die lines, contaminations and bubbles. This
method of determining defect levels is specified generally for
films instead of pellets in ASTM 7310, but is otherwise
substantially the same.
[0083] Referring to FIG. 1, there is shown a preferred apparatus 10
for producing the polyamide products of the present invention.
Apparatus 10 includes an evaporator 12, a plug flow reactor 14,
provided with a decompressor/flasher, a heat exchanger 16, a phase
separator 18, a finishing vessel 20, a twin screw extruder 22
provided with a motor 24, a residence time block or residence time
dwell vessel 26, as well as a pelletizer 28.
[0084] In operation, a nylon salt solution is fed to evaporator 12
where the solution is concentrated and fed with catalyst to plug
flow reactor 14 where the nylon is polymerized to an RV of about
3-20. The polymer is decompressed and maintained in the melt and
heated with of heat exchanger 16 before being fed to phase
separator 18 where volatiles are removed as shown.
[0085] From the phase separator, the low molecular weight nylon is
fed to finishing vessel 20 where moisture is removed and the nylon
further polymerized to an RV of about 30-45. In vessel 20, the
polymer melt is blanketed with an inert gas and/or steam. After
vessel 20, the melt, preferably including catalyst, is fed to twin
screw extruder 22 driven by motor 24. The twin screw extruder is
operated under high vacuum to remove moisture and other volatiles,
typically over 600 mm Hg vacuum, and the low molecular weight nylon
is further polymerized thereon for a relatively short residence
time in the extruder; typically, less than 1 minute, in order to
raise the RV of the polymer melt to more than 50, possibly or
suitably above 75 or so. The twin screw extruder is typically
operated at a barrel set point temperature between 275 or
285-350.degree. C. Preferably closer to 285.degree. C. is preferred
such as from 280-290.degree. C. Higher molecular weights, such as
an RV of 75 or more are achieved with higher extruder temperatures
such as above 300.degree. C. or so. Optionally, the extruder is
operated below 300.degree. C. and the polymer melt is fed to
residence time dwell vessel 26 where the material further
polymerizes before being fed to a pelletizer 28, such that an RV of
75 or more can be achieved utilizing a melt temperature of below
300.degree. C.
[0086] Polyamides with a precision RV of 80 and above and low gel
content are readily prepared in apparatus 10 as is seen in the
Examples which follow.
[0087] The inventive apparatus 10 is an alternative to an SSP
process which requires a large inventory of material as is
appreciated from FIG. 2.
[0088] In FIG. 2 there is shown an SSP apparatus 30, including a
wet chip silo 32, feed hoppers 34, 35 and a crystallizer 38.
Further provided is an SSP tower 40, as well as product hoppers 42,
44, heat and moisture regulators indicated at 46, 48, as well as a
moisture regulating silo 50 and a buffer hopper 52.
[0089] In operation, polymer chip prepared by a commercialized
process having an RV value of 35-45 or so is fed to SSP tower 40
and held at a temperature of from 150.degree. C. to 190C for a
residence time of 1-48 hours to increase molecular weight. Despite
the added expense in terms of equipment and inventory, the SSP
process described above produces a polyamide with a higher RV
Standard Deviation than apparatus 10 described above. Gel Content
Parameters and Optical Defect Levels are also difficult to control,
depending upon the quality of the low molecular weight material fed
to the SSP tower and the degree of control exercised over the SSP
process.
[0090] Product quality is reflected, in part, by the Gel Content
Parameter which is correlates to insolubles in the product and may
relate to the appearance and fiber-forming characteristics of the
product depending on gel size and gel levels on a volumetric basis.
Gels are believed to generate largely by way of thermal 10
degradation of polymer, catalyst and additives in the system.
Without being bound by any particular theory, it is believed
gelation is a function of time and temperature. The
photomicrographs of FIG. 3 are line drawings of bright field and
fluorescent photomicrographs respectively of first (FIGS. 3A, 3B)
and second (3C, 3D) filament breaks observed during fiber
manufacture from Nylon 6,6. The breaks had gel material as
evidenced by the fluorescence and that they were insoluble in
formic acid. The observed gel bodies were in a size range of about
18 microns and less. It is seen from FIG. 3 that the gel bodies are
often associated with breaks during high speed manufacture and are
a likely cause of many breaks.
[0091] Product quality is also reflected, in part, by the RV
Standard Deviation which unexpectedly correlates closely with the
processability of the polyamide into fiber, film and molded parts
as is seen in the examples which follow. Nylon 6,6 with an RV of 85
made by way of the process and apparatus of FIG. 1 had an RV
Standard Deviation of 0.83, while Nylon 6,6 with an RV of 85 made
by way of the process and apparatus of FIG. 2 had an RV Standard
Deviation of 1.4, about a 75% higher level of variability. Details
on the RV distributions of the products appear in FIGS. 4, 5.
Example 1
Continuous Polycondensation of PA66 (Also Known as N66)
[0092] Continuous polycondensation of PA66 was carried out starting
with utilizing the apparatus of FIG. 1, concentrating the AH-salt
solution at a pressure of 2 bar to approximately 85% solids. This
hot salt solution is then fed into the polycondensation reactor
that is jacket heated in three stages from 204 to 270.degree. C.,
with the solution temperature increasing to 230.degree. C. with a
pressure of 18.5 bar. The precondensate is removed from the sump of
the reactor end by an extrusion pump and pressed onto a
decompressor/flasher that has been heated to 290.degree. C., with a
final pressure of only 1 bar. The prepolymerisate then flows
through a phase separator followed by a finisher, so that the last
remaining traces of water evaporate and the precondensate takes the
temperature of 275.degree. C. The extrusion pump presses the
material through the polymer pipe to the twin screw extruder with
barrel zone temperature set at 275.degree. C. The temperature is
raised to and held for approximately 20-30 seconds at 350.degree.
C. The extruder has two vacuum vents operating at 28'' Hg vacuum.
The exiting polymer is pumped through a strand die and
pelletized.
[0093] The Nylon 66 product polymer typically had a precision RV of
greater than 50 with a RV Standard Deviation of less than or equal
to 1.25, a Gel Content Parameter of less than 50 ppm as determined
by parts per million insolubles larger than 10 microns in 90%
formic acid at 25.degree. C.; and an Average Optical Defect Level
of less than 2,000 parts per million (ppm) as measured by optical
scanning of pellets.
Examples 2-5
[0094] Devolatilization experiments were carried out using a W/P
(Werner and Pfleiderer) 40 mm twin screw extruder of the class
shown in FIG. 1. The L/D of the extruder was 56 and the extruder
had 14 barrels. The screw was designed with a melting section and
two devolatilization zones. Vacuum vents (vent stuffers) were
provided to the extruder. Each of the vent stuffers was connected
to a liquid ring vacuum pump which was operated to maintain a
predetermined vacuum at these vents. The screws were designed to
produce melt seals upstream of each vacuum vent. A feeder was used
to precisely feed polymer pellets. A five-hole die (diameter 4 mm)
was mounted at the end of the extruder. Experiments were performed
at 125 to 200 lbs/hr. and between 300 and 500 rpm screw speeds. The
processing temperature was between 265 and 350.degree. C. The
strands were cooled using a 5' water bath and pelletized with a
strand pelletizer. A hand held electronic temperature probe from
EDL was used to measure the melt temperature at the exit of the
die.
[0095] Viscosity measurements (RV) were performed in formic acid.
Residence time was measured using colored pellets.
[0096] Details and results appear in the table immediately
below.
Devolatilization Using Twin Screw Extruder Only:
TABLE-US-00002 [0097] Average Residence Melt RV Optical Feed Rate
Time in the Temperature Feed Final Standard Defect Example (PPH)
RPM extruder (C.) RV RV Deviation Level 2 125 450 32 s 350 42 89
1.41 1670 3 150 450 30 s 340 42 80 2.12 1600 4 175 450 28 s 330 42
75 0.35 1440 5 200 450 24 s 322 42 66 1.90 1358
Examples 6-8
[0098] Following the procedure of Examples 2-5, a residence time
(RT) block (residence time dwell vessel) was attached at the end of
the extruder. The pelletizing die was mounted at the end of the RT
block. The diameter of the RTB pipe was 2.2.degree. and it was 3'
long. Low pressure drop (LPD) static mixers from Ross Engineering
were inserted into this RTB pipe. The main objective of this RTB
block is to make high RV polymers at lower melt temperature. The
higher temperature makes higher gels, black specks. It can be seen
from Examples 6,7 that the melt temperature is higher at lower rate
and same rpm. The higher melt temperature not only increases the RV
it also makes higher degradation products. A sufficient reaction
time was allowed inside the residence time block (RTB) to reach the
RV at equilibrium moisture content.
[0099] Details and results appear in the table immediately
below.
Devolatilization Using Twin Screw Extruder and RTB
TABLE-US-00003 [0100] Average Residence time RV Gel Optical Feed
Rate (Total) Melt Temp. Feed Final Standard Content Defect Example
(PPH) RPM Extruder + RTB (.degree. C.) RV RV Deviation Parameter
Level 6 125 450 112 s 315 42 88 1.17 16.4 1792 7 150 450 100 s 310
42 83 0.77 12.6 1360 8 150 450 98 s 318 46* 95 1.77 15.3 1920 *The
polymer had a different composition
Example 9
[0101] For purposes of comparison, Nylon 66 polymer was prepared by
feeding low molecular weight Nylon 66 polymer flake to an SSP
column as shown and described in connection with FIG. 2 and solid
state polymerized such that the final product had an RV of 85.
Examples 10-13
[0102] In these examples high tenacity Nylon 66 multifilament yarn
suitable for tire cord was spun from polymer made in accordance
with the reactive extrusion process generally described in Examples
1-8. Nylon 66 polymer in flake form containing 150 ppm Benzene
Phosphinic acid, 70 ppm copper in the form of copper bromide and
Potassium in the form of potassium bromide and potassium iodide and
having a RV of about 85 and balanced amine and carboxyl end groups
is melt-spun in a conventional manner to provide an as-spun
multifilament yarn. Nylon 66 polymer in flake form with the present
invention described in Examples 2-8 was fed from a separate silo
onto an extruder followed by a quench zone and draw zone. Draw
ratio is determined by the ratio of the highest and lowest roll
speeds. Results are compared with an SSP process where the Nylon 66
polymer in flake form produced from a continuous polymerization
line followed by solid state polymerization column in order to
increase the RV of the polymer to 85 (Example 9). The spinning
performance of the polymer flake from each method is determined by
the stress test where draw ratio is increased step by step and
average broken filament number for each step is recorded using
online broken filament detectors. Bobbins were collected from each
draw ratio and the physical properties of obtained yarn was tested.
The quality of the yarn is determined by number of average of
broken filament count during spinning when the yarn is drawn to an
extent to achieve 9.5 g/den. The results in the Table below show
that as-spun yarn which is produced from the Nylon 66 flake from
the process generally described in Examples 1-8 is capable of
obtaining 9.5 g/den tenacity at lower average broken filament count
per minute. The amount of gel that were not soluble in formic acid
is also reported. Polymer from the reactive vacuum extrusion
process shows less insolubles than polymer produced from continuous
polymerization followed by SSP process.
[0103] Details and results appear in the table immediately
below.
TABLE-US-00004 Gel Average Average Content Optical BFC/ Param- RV
Defect Exam- Polymer min at eter Standard Level ple Process RV 9.5
g/den (ppm) Deviation (ppm) 10 SSP 85 0.95 11.6 1.6 187 11 SSP 85
0.81 10.6 1.3 272 12 Reactive 85 0.35 8.4 0.6 1472 Vacuum Extrusion
13 Reactive 85 0.07 9.7 0.7 704 Vacuum Extrusion
[0104] The SSP material was superior in terms of optical defect
levels; but exhibited more filament breakage. Without intending to
be bound by any theory, it is believed the spinning performance
correlates more strongly with gel content and/or RV standard
deviation than optical defect levels.
[0105] Multifilament nylon yarns and their use in tire cord are
discussed at some length in U.S. Pat. No. 7,159,381, U.S. Pat. No.
4,720,943 and U.S. Pat. No. 4,416,935.
Listing of Preferred Embodiments of the Invention
[0106] There is thus provided in a First Polyamide Polymer
Embodiment of the present invention a high molecular weight
polyamide polymer, wherein the polyamide polymer is characterized
by a precision Relative Viscosity greater than 50 as measured in a
90% strength formic acid solution, wherein the precision Relative
Viscosity has an RV Standard Deviation of less than or equal to
1.25.
[0107] A Second Polyamide Polymer Embodiment is provided in the
form of a high molecular weight polyamide polymer, wherein the
polyamide polymer is characterized by: [0108] a Relative Viscosity
greater than 50 as measured in a 90% strength formic acid solution;
[0109] a Gel Content Parameter of less than 50 ppm as determined by
parts per million insolubles larger than 10 microns in 90% formic
acid at 25.degree. C.; and [0110] an Average Optical Defect Level
of less than 2,000 parts per million (ppm) as measured by optical
scanning of pellets.
[0111] A Third Polyamide Polymer Embodiment is provided in the form
of a high molecular weight polyamide polymer, wherein the polyamide
polymer is characterized by: [0112] a Relative Viscosity greater
than 50 as measured in a 90% strength formic acid solution; and
[0113] a Gel Content Parameter of less than 50 ppm as determined by
parts per million insolubles larger than 10 microns in 90% formic
acid at 25.degree. C.
[0114] Still yet a Fourth Polyamide Polymer Embodiment of the
present invention is provided in the form of a high molecular
weight polyamide polymer, wherein the polyamide polymer is
characterized by: [0115] a precision Relative Viscosity greater
than 50 as measured in a 90% strength formic acid solution, wherein
the precision Relative Viscosity has an RV Standard Deviation of
less than or equal to 1.25; [0116] a Gel Content Parameter of less
than 50 ppm as determined by parts per million insolubles larger
than 10 microns in 90% formic acid at 25.degree. C.; and [0117] an
Average Optical Defect Level of less than 2,000 parts per million
(ppm) as measured by optical scanning of pellets.
[0118] Additional embodiments include the following:
[0119] Polyamide Polymer Embodiment No. 5 is the polyamide polymer
of any of Polyamide Polymer Embodiments 1 through 4, wherein the
polymer is Nylon 6,6 polymer.
[0120] Polyamide Polymer Embodiment No. 6 is the polyamide polymer
of any of Polyamide Polymer Embodiments 1 through 4, wherein the
polymer is Nylon 6 polymer.
[0121] Polyamide Polymer Embodiment No. 7 is the polyamide polymer
of any of Polyamide Polymer Embodiments 1 through 4 wherein the
polymer is a random copolymer of Nylon 6,6 and Nylon 6.
[0122] Polyamide Polymer Embodiment No. 8 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
Relative Viscosity is greater than 70 as measured in a 90% strength
formic acid solution.
[0123] Polyamide Polymer Embodiment No. 9 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
Relative Viscosity is greater than 90 as measured in a 90% strength
formic acid solution.
[0124] Polyamide Polymer Embodiment No. 10 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
Relative Viscosity is in the range of from 50 to 200 as measured in
a 90% strength formic acid solution.
[0125] Polyamide Polymer Embodiment No. 11 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
Relative Viscosity is in the range of from 75 to 100 as measured in
a 90% strength formic acid solution.
[0126] Polyamide Polymer Embodiment No. 12 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
Relative Viscosity is in the range of from 80 to 97.5 as measured
in a 90% strength formic acid solution.
[0127] Embodiment No. 13 is the polyamide polymer of any of the
foregoing Polyamide Polymer Embodiments, wherein the precision
Relative Viscosity has an RV Standard Deviation of less than
1.0.
[0128] Polyamide Polymer Embodiment No. 14 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
precision Relative Viscosity has an RV Standard Deviation of less
than 0.9.
[0129] Polyamide Polymer Embodiment No. 15 is the polyamide polymer
of any of the foregoing Polyamide Polymer Embodiments, wherein the
precision Relative Viscosity has an RV Standard Deviation of from
0.5 to 1.25.
[0130] Polyamide Polymer Embodiment No. 16 is the polyamide polymer
of any of Polyamide Polymer Embodiments Nos. 1-5 and 8-15, wherein
the polymer is a Nylon 6,6 polymer made into a multifilament yarn
having the following characteristics: tenacity greater than 9.0
g/d; elongation greater than 18%, and broken filaments less than 2
per 20 lb bobbin.
[0131] Polyamide Polymer Embodiment No. 17 is the polyamide polymer
of Polyamide Polymer Embodiment No. 16, made into a multifilament
yarn having the following characteristics: tenacity greater than
9.0 g/d; elongation greater than 18%, and broken filaments less
than 1 or less than 0.5 broken filaments per 20 lb bobbin.
[0132] Polyamide Polymer Embodiment No. 18 is the Nylon 6,6
multifilament yarn according to Polyamide Polymer Embodiment Nos.
16 or 17, wherein the yarn is to incorporated into 1 or more of:
tires, airbags, seatbelts and industrial fabrics.
[0133] Polyamide Polymer Embodiment No. 19 is the polyamide polymer
of any Polyamide Polymer Embodiment Nos. 1-5 and 8-18, wherein
further the polyamide is a Nylon 6,6 polymer which exhibits a Gel
Content Parameter of less than 40 ppm as determined by parts per
million insoluble larger than 10 microns in 90% formic acid at
25.degree. C.
[0134] Polyamide Polymer Embodiment No. 20 is the polyamide polymer
of Polyamide Polymer Embodiment No. 19, wherein further the Nylon
6,6 polymer exhibits a Gel Content Parameter of less than 25 ppm as
determined by parts per million insoluble larger than 10 microns in
90% formic acid at 25.degree. C.
[0135] Polyamide Polymer Embodiment No. 21 is the polyamide polymer
of Polyamide Polymer Embodiment No. 19, wherein further the Nylon
6,6 polymer exhibits a Gel Content Parameter of from 1 ppm to less
than 10 ppm as determined by parts per million insoluble larger
than 10 microns in 90% formic acid at 25.degree. C.
[0136] Polyamide Polymer Embodiment No. 22 is the polyamide polymer
of Polyamide Polymer Embodiment No. 19, wherein further the Nylon
6,6 polymer exhibits a Gel Content Parameter of less than 10 ppm as
determined by parts per million insoluble larger than 10 microns in
90% formic acid at 25.degree. C.
[0137] Polyamide Polymer Embodiment No. 23 is the polyamide polymer
of any of Polyamide Polymer Embodiments Nos. 1-5 and 8-22, wherein
further the polyamide is as Nylon 6,6 polymer which exhibits an
Average Optical Defect Level of no greater than 1000 parts per
million (ppm).
[0138] Polyamide Polymer Embodiment No. 24 is the polyamide polymer
of Polyamide Polymer Embodiment No. 23, wherein further the Nylon
6,6 polymer exhibits an Average Optical Defect Level of no greater
than 500 parts per million (ppm).
[0139] Further aspects of the invention include processes for
making high molecular weight polyamides having any of the features
of Polyamide Polymer Embodiments 1 through 24 noted above.
[0140] There is provided in a first Process Embodiment of the
present invention a method of making a high molecular weight
polyamide polymer with a precision Relative Viscosity and low gel
content comprising: [0141] (a) providing a first polyamide polymer
melt comprising a first polyamide polymer with a first Relative
Viscosity; [0142] (b) feeding the first polyamide polymer melt to a
twin screw extruder; [0143] (c) melt-processing the first polyamide
polymer melt under vacuum in the twin screw extruder to remove
steam and other volatiles therefrom, thereby increasing the
molecular weight of the polymer melt to provide a second polyamide
polymer melt comprising a second polyamide polymer with a second
Relative Viscosity, said second polyamide polymer being
characterized by either: (i) a precision Relative Viscosity greater
than 50 as measured in a 90% strength formic acid solution with an
RV Standard Deviation of less than or equal to 1.25; or (ii) a Gel
Content Parameter of less than 50 ppm as determined by parts per
million insoluble larger than 10 microns in a 90% formic acid
solution at 25.degree. C. and an Average Optical Defect level of
less than 2000 ppm as measured by optical scanning at 50 micron
resolution; [0144] (d) optionally feeding the second polymer melt
to a residence time dwell vessel and melt-processing the second
polymer melt in the residence time dwell vessel to provide a third
polyamide polymer melt comprising a third polyamide polymer with a
third Relative Viscosity higher than the second Relative Viscosity
of the second polyamide polymer, [0145] said third polyamide
polymer being characterized by either (i) a precision Relative
Viscosity greater than 50 as measured in a 90% strength formic acid
solution with an RV Standard Deviation of less than or equal to
1.25; or (ii) a Gel Content Parameter of less than 50 ppm as
determined by parts per million insoluble larger than 10 microns in
a 90% formic acid solution at 25.degree. C. and an Average Optical
Defect level of less than 2000 ppm as measured by optical scanning
at 50 micron resolution; and [0146] (e) recovering a product
polyamide polymer characterized by either: (i) a precision Relative
Viscosity greater than 50 as measured in a 90% strength formic acid
solution with an RV Standard Deviation of less than or equal to
1.25; or (ii) a Gel Content Parameter of less than 50 ppm as
determined by parts per million insoluble larger than 10 microns in
a 90% formic acid solution at 25.degree. C. and an Average Optical
Defect level of less than 2000 ppm as measured by optical scanning
at 50 micron resolution.
[0147] Process Embodiment No. 2 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment No.
1, wherein the polyamide polymer melt is to melt-processed in the
twin screw extruder at a temperature in the range of from
280.degree. C. to 350.degree. C.
[0148] Process Embodiment No. 3 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment No.
1, wherein the polyamide polymer melt is melt-processed in the twin
screw extruder at a temperature in the range of from 285.degree. C.
to 305.degree. C.
[0149] Process Embodiment No. 4 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, wherein the polyamide polymer melt is
melt-processed in the twin screw extruder under vacuum in the range
of 600 mm Hg vacuum to 725 mm Hg vacuum.
[0150] Process Embodiment No. 5 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, wherein the polyamide polymer melt is
melt-processed in the twin screw extruder under vacuum in the range
of from 650 mm Hg vacuum to 725 mm Hg vacuum.
[0151] Process Embodiment No. 6 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, wherein the polyamide polymer melt is
melt-processed in the twin screw extruder for a residence time in
the extruder of less than 60 seconds.
[0152] Process Embodiment No. 7 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, wherein the polyamide polymer melt is
melt-processed in the twin screw extruder for a residence time in
the extruder of less than 30 seconds.
[0153] Process Embodiment No. 8 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, wherein the polyamide polymer melt is
melt-processed in the twin screw extruder for a residence time in
the extruder of less than 20 seconds.
[0154] Process Embodiment No. 9 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, wherein the polyamide polymer melt is
melt-processed in the twin screw extruder for a residence time in
the extruder of from 10 seconds to 60 seconds.
[0155] Process Embodiment No. 10 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to any of the foregoing
Process Embodiments, comprising feeding the second polymer melt to
a residence time dwell vessel and melt-processing the second
polymer melt in the residence time dwell vessel to provide the
third polyamide polymer melt comprising a third polyamide polymer
with a third Relative Viscosity higher than the second Relative
Viscosity of the second polyamide polymer, said third polyamide
polymer being characterized by either: (i) a precision Relative
Viscosity greater than 50 as measured in a 90% strength formic acid
solution with an RV Standard Deviation of less than or equal to
1.25; or (ii) a Gel Content Parameter of less than 50 ppm as
determined by parts per million insoluble larger than 10 microns in
a 90% formic acid solution at 25.degree. C. and an Average Optical
Defect level of less than 2000 ppm as measured by optical scanning
at 50 micron resolution.
[0156] Process Embodiment No. 11 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment No.
10, wherein the polyamide polymer melt is melt-processed in the
residence time dwell vessel at a temperature in the range of from
280.degree. C. to 350.degree. C.
[0157] Process Embodiment No. 12 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment No.
10, wherein the polyamide polymer melt is melt-processed in the
residence time dwell vessel at a temperature in the range of from
285.degree. C. to 350.degree. C.
[0158] Process Embodiment No. 13 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment
Nos. 10, 11 or 12, wherein the polyamide polymer melt is
melt-processed in the residence time dwell vessel for a residence
time in the residence time dwell vessel of from 30 seconds to 5
minutes.
[0159] Process Embodiment No. 14 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment
Nos. 10, 11, 12 or 13, wherein the polyamide polymer melt is
melt-processed in the residence time dwell vessel for a residence
time in the residence time dwell vessel of at least 1 minute.
[0160] Process Embodiment No. 15 is the method of making a high
molecular weight polyamide polymer with a precision Relative
Viscosity and a low gel content according to Process Embodiment
Nos. 10, 11, 12, 13 or 14, wherein the polyamide polymer melt is
melt-processed in the residence time dwell vessel for a residence
time in the residence time dwell vessel of from 1.5 to 3
minutes.
[0161] The product polyamide product recovered from any of Process
Embodiments 1 through 15 may have any or all of the features and
combinations recited above in connection with the Polyamide Polymer
Embodiment Nos. 1 through 24.
[0162] While the invention has been described in detail,
modifications within the spirit and scope of the invention will be
readily apparent to those of skill in the art. Such modifications
are also to be considered as part of the present invention. In view
of the foregoing discussion, relevant knowledge in the art and
references discussed above in connection with the description of
the related art and detailed description of embodiments, the
disclosures of which are all incorporated herein by reference,
further description is deemed unnecessary. In addition, it should
be understood from the foregoing discussion that aspects of the
invention and portions of various embodiments may be combined or
interchanged either in whole or in part. Furthermore, those of
ordinary skill in the art will appreciate that the foregoing
description is by way of example only, and is not intended to limit
the invention.
* * * * *