U.S. patent application number 11/601140 was filed with the patent office on 2007-05-24 for fluoropolymer composition.
Invention is credited to Ralph Munson Aten, Heidi Elizabeth Burch, Sundar Kilnagar Venkataraman.
Application Number | 20070117929 11/601140 |
Document ID | / |
Family ID | 37831813 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070117929 |
Kind Code |
A1 |
Burch; Heidi Elizabeth ; et
al. |
May 24, 2007 |
Fluoropolymer composition
Abstract
A melt-mixed composition of non-melt flowable
polytetrafluoroethylene (PTFE) and melt-fabricable perfluoropolymer
is provided that exhibits thixotropy at increasing shear rate in
the molten state and high elongation at break even at PTFE
concentrations well above 4 wt %, based on the combined weight of
the PTFE and the perfluoropolymer, e.g. at least 200% up to at
least 30 wt % PTFE, the composition also exhibiting the structure
of a dispersion of submicrometer-size particles of the PTFE in a
continuous phase of the melt-fabricable perfluoropolymer.
Inventors: |
Burch; Heidi Elizabeth;
(Parkersburg, WV) ; Venkataraman; Sundar Kilnagar;
(Avondale, PA) ; Aten; Ralph Munson; (Chadds Ford,
PA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
37831813 |
Appl. No.: |
11/601140 |
Filed: |
November 16, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60738351 |
Nov 18, 2005 |
|
|
|
Current U.S.
Class: |
525/199 |
Current CPC
Class: |
C08F 2/00 20130101; C08L
27/18 20130101; Y10S 525/902 20130101; C08F 259/08 20130101; C08L
2205/025 20130101; C08L 51/003 20130101; C08L 2205/02 20130101;
C08F 114/26 20130101; C08F 114/26 20130101; C08F 2/00 20130101;
C08L 27/18 20130101; C08L 27/18 20130101; C08L 27/18 20130101; C08L
51/003 20130101; C08L 51/003 20130101; C08L 2666/02 20130101 |
Class at
Publication: |
525/199 |
International
Class: |
C08L 27/12 20060101
C08L027/12 |
Claims
1. Melt-mixed composition comprising non-melt flowable
polytetrafluoroethylene and melt-fabricable perfluoropolymer, said
polytetrafluoroethylene constituting at least about 4 wt % of the
combined weight of said polytetrafluoroethylene and said
melt-fabricable perfluoropolymer, said composition exhibiting
thixotropy when being subjected to increasing shear in the molten
state.
2. The melt-mixed composition of claim 1 wherein said thixotropy is
characterized by a reduction in melt viscosity upon increasing the
shear rate applied to said composition in the molten state from
about 10 s.sup.-1 to about 100 s.sup.-1 that is at least about 10%
greater than the reduction in melt viscosity at the same shear
rates for the melt-fabricable perfluoropolymer by itself, as
determined by the capillary rheometer method.
3. The melt-mixed composition of claim 2 wherein said reduction is
at least 100%.
4. The melt-mixed composition of claim 1 exhibiting an elongation
at break at least as high as that for said melt-fabricable
perfluoropolymer by itself.
5. The melt-mixed composition of claim 4 wherein said elongation at
break exists for said composition wherein said
polytetrafluoroethylene constitutes at least up to about 15 wt % of
the combined weight of said polytetrafluoroethylene and said
melt-fabricable perfluoropolymer.
6. The melt-mixed composition of claim 1 exhibiting an elongation
at break of at least about 200% at up to about 30 wt % of said
polytetrafluoroethylene, based on the combined weight of said
polytetrafluoroethylene and said melt-fabricable
perfluoropolymer.
7. The melt-mixed composition of claim 6 wherein said elongation at
break is at least 250%.
8. The melt-mixed composition of claim 1 wherein said
polytetrafluoroethylene constitutes up to about 75 wt % based on
the combined weight of said polytetrafluoroethylene and said
melt-fabricable perfluoropolymer.
9. Melt-mixed composition comprising a dispersion of
submicrometer-size particles comprising non-melt flowable
polytetrafluoroethylene in a continuous phase comprising
melt-fabricable perfluoropolymer, said dispersion exhibiting
thixotropy when being subjected to increasing shear in the molten
state.
10. The composition of claim 9 wherein said thixotropy is
characterized by a reduction in melt viscosity upon increasing the
shear rate applied to said composition in the molten state from
about 10 s.sup.-1 to about 100 s.sup.-1 that is at least about 10%
greater than the reduction in melt viscosity at the same shear
rates for the melt-fabricable perfluoropolymer by itself, as
determined by the capillary rheometer method.
11. The melt-mixed composition of claim 9 exhibiting an elongation
at break at least as high as that for said melt-fabricable
perfluoropolymer by itself.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to fluoropolymer compositions of
polytetrafluoroethylene and other perfluoropolymers.
[0003] 2. Description of Related Art
[0004] U.S. 2004/0242783 A1 discloses a blend of
tetrafluoroethylene/hexafluoropropylene copolymer, commonly called
FEP, and polytetrafluoroethylene (PTFE), the PTFE imparting the
improved extrusion property of reduced cone breaks during melt draw
down extrusion coating of wire. The PTFE content of the blend is
disclosed to be 0.03 to 2 parts by weight based on 100 parts by
weight of the copolymer. When the amount of PTFE is more than 2
parts by weight, two disadvantageous results are disclosed: the
melt viscosity of the blend increases significantly and the molded
article tends to become brittle [0027]. These are the same effects
as adding filler to a polymer, except that in the case of adding
PTFE to FEP, the disadvantageous effects arise with even small
additions of the PTFE to the FEP.
SUMMARY OF THE INVENTION
[0005] The present invention includes the discovery of
melt-fabricable perfluoropolymer compositions containing PTFE in
much greater amounts than 2 parts by weight per 100 parts by weight
of the perfluoropolymer that have desirable viscosities for melt
fabrication and that do not become brittle. According to one
embodiment, the present invention is a melt-mixed composition
comprising non-melt flowable polytetrafluoroethylene (PTFE) and
melt-fabricable perfluoropolymer, said PTFE constituting at least 4
wt % of the combined weight of said PTFE and said melt-fabricable
perfluoropolymer, said composition exhibiting thixotropy when being
subjected to increasing shear in the molten state. Thus, the
compositions of the present invention exhibit reduced melt
viscosity at increasing shear rate. Under shear conditions used in
melt mixing involved in melt fabrication, the melt viscosity of the
composition becomes low enough to enable the compositions to be
melt-fabricated, notwithstanding that the PTFE is non melt
flowable, i.e. the PTFE has such a high viscosity in the molten
state that it does not flow and therefore is not melt-processable.
Preferably this thixotropy is characterized by a reduction in melt
viscosity upon increasing the shear rate applied to the molten
dispersion from about 10 s.sup.-1 to 100 s.sup.-1 that is at least
about 10% greater, preferably at least about 100% greater, than the
reduction in melt viscosity at the same shear rates for the
melt-fabricable perfluoropolymer by itself, as determined by the
capillary rheometer method described later herein. The thixotropy
imparted to the perfluoropolymer by non-melt flowable PTFE is a
surprising result and exists for high contents for the PTFE, e.g.
up to about 65 wt % thereof and even up to about 75 wt % thereof,
based on the combined weight of the PTFE and the melt-fabricable
perfluoropolymer.
[0006] The melt-fabricable perfluoropolymer component of the
composition of the present invention imparts melt-fabricability to
the composition. Thus, the composition is melt-fabricable by such
processes as extrusion and injection molding to form strong, tough
products. Some indicia of this strength and toughness are the
tensile and flexural properties of the composition disclosed
herein.
[0007] The fact that the composition of the present invention is
not brittle is evident from the fact that it exhibits a high
elongation at break. Preferably, its elongation at break is at
least about 200%, more preferably at least about 250%, at up to at
least about 30 wt % PTFE in the composition, based on the combined
weight of the PTFE and perfluoropolymer. Most preferably, the
composition exhibits an elongation at break that is at least as
high as that of the melt-fabricable perfluoropolymer by itself,
indicating that the presence of the PTFE is not making the
composition brittle. This effect extends well above the 4 wt % PTFE
content of the composition, preferably at least to up to about 15
wt % PTFE, based on the combined weight of the PTFE and
melt-fabricable perfluoropolymer. The composition of the present
invention also exhibits properties indicating that the PTFE in the
composition is reinforcing the composition, rather than acting as a
filler. For example, both the tensile strength and elongation at
break can be greater than for the melt-fabricable perfluoropolymer
by itself. Another indicia of the composition of the present
invention not being brittle arises from its exhibiting an MIT Flex
Life of at least 500 cycles, preferably at least 1000 cycles, i.e.
film made from the composition by the MIT Flex Life test procedure
is flexed over upon itself repeatedly without breaking. Preferably
the MIT flex life of the composition is at least as great as for
the perfluoropolymer by itself. The melt-mixed composition of this
embodiment of the present invention can be considered as a melt
blend of the PTFE and perfluoropolymer components.
[0008] According to another embodiment, the present invention is a
melt-mixed composition comprising a dispersion of
submicrometer-size particles comprising non-melt flowable
polytetrafluoroethylene (PTFE) in a continuous phase comprising
melt-fabricable perfluoropolymer, said dispersion exhibiting
thixotropy when subjected to increasing shear in the molten state.
The continuous phase being the melt flowable perfluoropolymer is
confirmed by the melt fabricability of the melt mixed composition.
Articles molded from the composition are transparent to
translucent, rather than opaque as are articles molded from
PTFE.
[0009] Both the non-brittle and thixotropic attributes described
above are believed to arise from this novel dispersion/continuous
phase structure, wherein the PTFE is present in such small particle
size within the perfluoropolymer continuous phase. This novel
structure exists at compositions containing less than 4 wt % PTFE
although this is a preferred minimum amount. For example, the
melt-mixed dispersion composition can contain as little as about
0.1 wt % PTFE based on the combined weight of the PTFE and
perfluoropolymer, and the dispersion structure can exist for
amounts of PTFE greater than about 50 wt %, e.g. up to about 65 wt
% PTFE and even up to about 75 wt %, based on the combined weight
of the PTFE and perfluoropolymer, because of the small size of the
PTFE particle. All of the thixotropy, elongation at break, tensile
strength, and MIT Flex Life parameters applied to the first
mentioned embodiment of the present invention also apply to this
embodiment.
[0010] The melt-mixed nature of the compositions of the present
invention means that it has been preferably heated to the state at
which both the PTFE and the perfluoropolymer are molten and then
the molten mass is subjected to mixing of the two polymers
together, such as may occur in the typical melt fabrication
processes of extrusion or injection molding. In the case of
extrusion, the extruded, melt-mixed product can be molding pellets
for further melt fabrication into final product or final product.
Thus, the compositions of the present invention can be in any form
such as the molding pellet or final product form formed by melt
fabrication process.
[0011] It is surprising that compositions of the present invention
can contain much more PTFE than 2/100 parts by weight and exhibit
the properties described above, denoting melt-fabricability and
absence of brittleness. U.S. 2004/0242783 A1 discloses the uses of
a multi-screw kneader in an attempt to homogeneously disperse the
small amount of PTFE in the FEP [0029] and the use of pre-mixing to
improve the degree of dispersion of the PTFE [0042], indicating
that the dispersion achieved by the multi-screw kneader by itself
is deficient. In Example 1 of '783, powders of the PTFE and
copolymer are mixed together, followed by kneading in a twin-screw
extruder to produce molding pellets, which are then melt-draw down
extruded, using a single screw extruder, as a coating onto wire.
The PTFE powder used in the premixing has an average particle size
of 450 micrometers. The particle size of the FEP is not disclosed
in Example 1, but the aqueous emulsion polymerization to obtain
this copolymer is disclosed. This copolymer is recovered from
emulsion polymerization by coagulation, which provides a dry powder
particles that are agglomerates of the FEP particles of the
emulsion. The FEP particles of the emulsion are the primary
particles, which are submicrometer-size in average particle size so
as to be in the emulsion state. The agglomerates of the primary
particles are the secondary particles, which are typically hundreds
of time larger in diameter than the primary particles. The 450
micrometer PTFE fine powder particles used in the Example are
secondary particles. Thus, in Example 1, secondary particles of the
FEP and of the PTFE are pre-mixed before kneading in a twin screw
extruder.
[0012] The novel structure of the melt-mixed compositions of the
present invention, wherein the PTFE is dispersed in a continuous
phase of the melt-fabricable perfluoropolymer, is obtained by
carrying out the melt mixing on a mixture of submicrometer-size
particles of the PTFE and of the melt-fabricable perfluoropolymer.
Thus, these polymers are present as a mixture of primary particles,
rather than secondary particles. This mixture can be achieved by
providing the PTFE primary particle inside the perfluoropolymer
particle, i.e. in the form of a core/shell polymer. Alternatively,
each of the polymers can be provided in the form of aqueous
dispersions, e.g. from the aqueous dispersion polymerization
process for making each of them, followed by mixing these
dispersions together to form the mixture of primary particles of
each polymer. Core/shell polymer is preferred, because of the
greater intimacy of the two polymers and the ability of the molten
perfluoropolymer to integrate the particles together without
needing to overcome the incompatibility between the melt-fabricable
perfluoropolymer and the PTFE. Thus, the melt mixing converts the
mixture of primary particles that already exists into the
composition of the present invention, whether considered as a melt
blend or as a dispersion as described above.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The PTFE and melt-fabricable perfluoropolymer components of
the melt-mixed compositions of the present invention will be
described individually hereinafter, sometimes with reference to
their being supplied to the melt-mixed composition as a core/shell
polymer. This description of polymer components, however, also
applies to the supply of these polymers to the melt-mixed
composition from separate sources, e.g. from the combination of
core/shell polymer (PTFE core/perfluoropolymer shell) and
separately supplied perfluoropolymer or from separately supplied
PTFE and perfluoropolymer.
[0014] With respect to the PTFE component, the non-melt flowability
of the PTFE can also be characterized by high melt creep viscosity,
sometimes called specific melt viscosity, which involves the
measurement of the rate of elongation of a molten sliver of PTFE
under a known tensile stress for 30 min., as further described in
and determined in accordance with U.S. Pat. No. 6,841,594,
referring to the specific melt viscosity measurement procedure of
U.S. Pat. No. 3,819,594. In this test, the molten sliver made in
accordance with the test procedure is maintained under load for 30
min, before the measurement of melt creep viscosity is begun, and
this measurement is then made during the next 30 minutes of applied
load. The PTFE preferably has a melt creep viscosity of at least
about 1.times.10.sup.6 Pas, more preferably at least about
1.times.10.sup.7 Pas, and most preferably at least about
1.times.10.sup.8 Pas, all at 380.degree. C. This temperature is
well above the first and second melt temperatures of PTFE of about
343.degree. C. and 327.degree. C., respectively. The difference
between non-melt flowability of the PTFE core and the melt
flowability of the melt-fabricable perfluoropolymer shell is
apparent from the melt flow rate (MFR) test procedure of ASTM D
1238-94a. In this procedure, the MFR is determined by the rate in
g/10 min that perfluoropolymer that flows through a defined orifice
under a specified load at a specified temperature, usually
372.degree. C. The PTFE used in the present invention has no melt
flow (zero MFR). The high melt creep viscosity of the PTFE present
in the core of the core/shell polymer also means that the PTFE is
sinterable, i.e. a molded article, unsupported by the mold
(free-standing), of the PTFE can be heated above the melting point
of the PTFE to coalesce the PTFE particles without the molded
article flowing to lose its shape. The PTFE used in the present
invention is also often characterized by standard specific gravity
(SSG), which is the ratio of weight in air of a PTFE specimen
prepared in a specified manner to an equal volume of water at
23.degree. C. as further described in U.S. Pat. No. 4,036,802 and
ASTM D 4894-94. The lower the SSG, the higher the molecular weight
of the PTFE. The specimen preparation procedure as disclosed in
ASTM D-4894-94 includes compression molding the test specimen,
removing the compression molded test specimen from the mold, and
sintering the specimen in air, i.e. free standing, at 380.degree.
C. The non-melt flowability of the PTFE enables this sintering to
be carried out without the test specimen losing its compression
molded shape and dimensions.
[0015] The PTFE can be the granular type or the fine powder type,
made by suspension or aqueous dispersion polymerization,
respectively. The PTFE can be homopolymer of tetrafluoroethylene or
a copolymer thereof with a small amount of comonomer, such as
hexafluoropropylene or perfluoro(alkyl vinyl ether), preferably
wherein the alkyl group contains 1 to 5 carbon atoms, that improves
the sinterability of the TFE, to obtain such improvement as reduced
permeability and greater flex life, as compared to the TFE
homopolymer. This type of PTFE is sometimes referred to as modified
PTFE. Examples of modified PTFE are disclosed in U.S. Pat. Nos.
3,142,665, 3,819,594, and 6,870,020. For simplicity and because the
modified PTFE exhibits the same non-melt flow, high melt creep
viscosity of PTFE homopolymer, this type of PTFE is included in the
term polytetrafluoroethylene or PTFE used herein.
[0016] The non-melt flowable PTFE used in the present invention is
to be distinguished from low molecular weight PTFE, which because
of its low molecular weight has melt flowability but not
melt-fabricability. This melt flowable PTFE, which has an MFR that
is measurable by ASTM D 1238-94a, is obtained by direct
polymerization under conditions that prevent very long polymer
chains from forming, or by irradiation degradation of non-melt
flowable PTFE. Such melt flowable PTFE is commonly called PTFE
micropowder. It is not considered as being melt fabricable because
the article molded from the melt is useless, by virtue of extreme
brittleness. Because of its low molecular weight (relative to
non-melt-flowable PTFE), it has no strength. An extruded filament
of the PTFE micropowder is so brittle that it breaks upon
flexing.
[0017] With respect to the melt-fabricable perfluoropolymer
component of the melt-mixed composition of the present invention,
as indicated by the prefix "per" in perfluoropolymer, the
monovalent atoms bonded to the carbon atoms making up the polymer
are all fluorine atoms. Other atoms may be present in the polymer
end groups, i.e. the groups that terminate the polymer chain. The
perfluoropolymer is a perfluoroplastic, not a
perfluoroelastomer.
[0018] If the non-melt flowable PTFE and melt-fabricable
perfluoropolymer is supplied to the melt-mixed composition of the
present invention as core/shell polymer, the PTFE forms the core
and the perfluoropolymer forms the shell.
[0019] The melt flow rate (MFR) of the perfluoropolymers used in
the present invention can vary widely, depending on the proportion
of PTFE, the melt-fabrication technique desired for the core/shell
polymer or melt-mixed composition, as the case may be, and the
properties desired in the melt-fabricated article. Thus, MFRs for
the melt-fabricable perfluoropolymer can be in the range of about
0.1 to 500 g/10 min, but will usually be preferred as about 0.5 to
100 g/10 min, and more preferably 0.5 to 50 g/10 min., as measured
according to ASTM D-1238-94a and following the detailed conditions
disclosed in U.S. Pat. No. 4,952,630, at the temperature which is
standard for the resin (see for example ASTM D 2116-91a and ASTM D
3307-93 that are applicable to the most common melt-fabricable
perfluoropolymers, both specifying 372.degree. C. as the resin melt
temperature in the Plastometer.RTM.). The amount of polymer
extruded from the Plastometer.RTM. in a measured amount of time is
reported in units of g/10 min in accordance with Table 2 of ASTM D
1238-94a. If the perfluoropolymer is present as the shell of
core/shell polymer, the MFR of the perfluoropolymer in the shell is
determined by carrying out the polymerization of the
perfluoromonomers used to form the perfluoropolymer by themselves,
i.e. no core, using the same recipe and polymerization conditions
used to form the shell, to obtain perfluoropolymer that can be used
in the MFR determination. The higher the MFR of the
perfluoropolymer, the greater is the tendency to generate smoke
when the polymer is subjected to the NFPA-255 burn test, thus
failing such test. The shell can have high MFR, e.g. greater than
20 g/10 min without the article melt-fabricated from the core/shell
polymer or separately-supplied PTFE and perfluoropolymer components
failing the NFPA-255 burn test, because in the presence of the
PTFE, the article molded from the melt-mixed composition, notably
as dispersed submicrometer-size particles in a perfluoropolymer
continuous phase does not flow, and thus, does not drip to cause
smoke generation.
[0020] Even when the core/shell polymer exhibits an MFR of 0 g/10
min, i.e. there is no flow of the polymer when measured by ASTM D
1238-94a at the temperature that is standard for the
melt-fabricable perfluoropolymer, the core/shell polymer can still
be melt-fabricable. The thixotropy exhibited by the core/shell
polymer and by the mixture of separately supplied PTFE and
perfluoropolymer components, as submicrometer-size particles when
subjected to the higher shear associated with melt fabrication
enables the melt mixing and melt fabrication to occur.
[0021] The melt-fabricability of the melt-mixed compositions of the
present invention can also be characterized by their melt
flowability, which enables the melt fabrication to be carried out.
In this regard, these compositions preferably have a melt viscosity
of no more than about 5.times.10.sup.5 Pas, more preferably, no
more than about 1.times.10.sup.5 Pas, and most preferably, no more
than about 5.times.10.sup.4 Pas, all at a shear rate of 100
s.sup.-1 and melt temperature in the range of about 350.degree. C.
to 400.degree. C. The determination of melt viscosities disclosed
herein, unless otherwise indicated, is by dividing shear stress
applied to the polymer melt by shear rate applied to the polymer
melt as disclosed on p. 31 of F. N. Cogswell, Polymer Melt
Rheology, A Guide for Industrial Practice, published by Woodhead
Publishing (1996). As a practical matter, the equivalent melt
viscosities are obtained simply by readout from the computer
accompanying the rheometer used to determine shear rate and shear
stress. The melt viscosity of the melt-fabricable perfluoropolymer
by itself is such that the above mentioned melt viscosities for the
polymer mixture are obtained. The melt viscosity of the
perfluoropolymer component by itself can also be characterized by
the above mentioned melt viscosities.
[0022] Examples of melt-fabricable perfluoropolymers that can be
used in the shell of the polymer of the core/shell polymer or as
separately supplied polymer include the copolymers of
tetrafluoroethylene (TFE) with one or more polymerizable
perfluorinated comonomers, such as perfluoroolefin having 3 to 8
carbon atoms, such as hexafluoropropylene (HFP), and/or
perfluoro(alkyl vinyl ether) (PAVE) in which the linear or branched
alkyl group contains 1 to 5 carbon atoms. Preferred PAVE monomers
include perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl
ether) (PEVE), perfluoro(propyl vinyl ether) (PPVE), and
perfluoro(butyl vinyl ether) (PBVE). The copolymer can be made
using several PAVE monomers, such as the TFE/perfluoro(methyl vinyl
ether)/perfluoro(propyl vinyl ether) copolymer, sometimes called
MFA by the manufacturer. The preferred perfluoropolymers are
TFE/HFP copolymer in which the HFP content is about 5-17 wt %, more
preferably TFE/HFP/PAVE such as PEVE or PPVE, wherein the HFP
content is about 5-17 wt % and the PAVE content, preferably PEVE,
is about 0.2 to 4 wt %, the balance being TFE, to total 100 wt %
for the copolymer. The TFE/HFP copolymers, whether or not a third
comonomer is present, are commonly known as FEP. TFE/PAVE
copolymers, generally known as PFA, have at least about 2 wt %
PAVE, including when the PAVE is PPVE or PEVE, and will typically
contain about 2-15 wt % PAVE. When PAVE includes PMVE, the
composition is about 0.5-13 wt % perfluoro(methyl vinyl ether) and
about 0.5 to 3 wt % PPVE, the remainder to total 100 wt % being
TFE, and as stated above, may be referred to as MFA. The low melt
viscosity of these copolymers relative to the high melt creep
viscosity of the PTFE, provides the melt flowability to the
perfluoropolymer for its melt fabricability, and the
perfluoropolymer composition itself provides the strength required
for the practical utility of the article melt fabricated from the
perfluoropolymer. The melt flow difference between the
melt-fabricable perfluoropolymer being characterizable by melt
viscosity and MFR and the non-melt flowable PTFE being
characterizable by melt creep viscosity and SSG is great. The
melt-fabricable perfluoropolymer cannot be characterized by either
melt creep viscosity or by SSG. In the melt creep viscosity test,
the sliver of molten perfluoropolymer melts, flows, and breaks
during the 30 minutes initial heating under load at 380.degree. C.,
so there is no sliver remaining for the melt creep determination
during the second 30 minutes of heating. In the SSG test, the
specimen (polymer) melts and flows during the 380.degree. C.
heating (sintering for non-melt flowable PTFE), undermining the
integrity of the specimen for the SSG determination. Of course, the
non-melt flowability of the PTFE used in the present invention,
enables the melt creep viscosity and SSG determinations to be made
on such PTFE.
[0023] The perfluoropolymer comonomer content of core/shell polymer
and of the melt-fabricable perfluoropolymer by itself is determined
by infrared analysis on compression molded film made from the
core/shell polymer particles in accordance with the procedures
disclosed in U.S. Pat. No. 4,380,618 for the particular
fluoromonomers (HFP and PPVE) disclosed therein. The analysis
procedure for other fluoromonomers are disclosed in the literature
on polymers containing such other fluoromonomers. For example, the
infrared analysis for PEVE is disclosed in U.S. Pat. No. 5,677,404.
The perfluoropolymer shell composition is made to have a
composition that is predictable from copolymerization to make the
perfluoropolymer by itself. The perfluoropolymer composition of the
core/shell polymer used in the present invention, however, is
determined on the entire core/shell polymer. The composition of the
shell is calculated by subtracting the weight of the TFE consumed
to make the PTFE core. The perfluoromonomer content other than TFE
of the core/shell polymer is preferably at least 1.5 wt % based on
the total weight of the TFE and perfluoromonomer in the core/shell
polymer, but concentrated in the shell. The perfluoropolymer
content of the melt-mixed composition when the perfluoropolymer is
separately supplied to the composition, i.e. not as core/shell
polymer, is the weight % of the perfluoropolymer component used to
form the melt mixed composition, based on the combined weights of
the PTFE and perfluoropolymer components used to form the
composition.
[0024] The combination of the non-melt flowable PTFE core and melt
fabricable perfluoropolymer shell of the polymer results in a
core/shell polymer that is also melt fabricable. Although the
presence of the non-melt flowable PTFE core may lower the MFR of
the overall polymer as compared to the MFR of the perfluoropolymer
shell, and may even render the MFR not measurable by ASTM D
1238-94a, the thixotropy exhibited by the polymer when subjected to
sufficient shear in the molten state enables the resultant melt
blend to be melt fabricated. This effect extends over the entire
range of core/shell compositions. At least about 0.1 wt % PTFE core
is required before the thixotropic effect is appreciable. The
maximum amount of PTFE core is preferably up to that amount that
enables the core to be the dispersed phase when the core/shell
polymer is melt-mixed, such as occurs in extrusion or injection
molding. The thixotropic phenomenon observed for the core/shell
polymer also exists for the melt-mixed composition wherein the PTFE
and perfluoropolymer components are separately supplied as
submicrometer-size particles, except that the thixotropy obtained
from the core/shell polymer is greater than for the melt-mixed
composition derived from the separately supplied polymers. In
either case, the preferred reduction in viscosity is at least about
100%, and more preferably at least about 500% greater than the
viscosity reduction for the perfluoropolymer by itself when the
shear rate is increased from about 10 s.sup.-1 to about 100
s.sup.-1. These shear rates are expressed in terms of "about",
because of limitations in the operation of the rheometer used to
measure them. The rheometer includes a variable speed piston that
provides the volumetric flow rate (Q) of molten polymer through the
rheometer orifice and various orifice sizes, the selection of which
provides the radius r in the equation: shear rate (.gamma.)=4
Q/.pi.r.sup.3. With particular rheometers it may be difficult to
adjust the piston speed and orifice size such that the exact shear
rates of 10 s.sup.-1 and 100 s.sup.-1 are obtained. The shear rates
used in the Examples were 11.9 s.sup.-1 and 101 s.sup.-1.
Typically, the rheometer can be operated so that the shear rates
are 10 s.sup.-1.+-.3 s.sup.-1 and 100 s.sup.-1.+-.5 s.sup.-1. In
absolute terms, the preferred reduction in melt viscosity by the
core/shell polymer of the present invention is at least about 200
Pas, more preferably at least about 400 Pas at the shear rates
specified above.
[0025] The advantage of thixotropy discovered by the present
invention extends to higher shear rates than 100 s.sup.-1 enabling
the melt-mixed composition of the present invention to be extruded
at a faster rate by melt-draw down extrusion than the
melt-fabricable perfluoropolymer by itself. Alternatively, the melt
cone formed in melt-draw-down extrusion can have a lower draw-down
ratio (DDR) than the usual DDR of 80 to 100:1, to improve
concentricity of the wall thickness of the extrudate, applied for
example as jacketing on FEP insulated communications cable,
especially such cable used in plenums of buildings. DDR is the
ratio of the cross-sectional area of the annular die opening to the
cross-sectional area of the final shape and size of the extrudate,
e.g. the plenum cable jacket just described.
[0026] Within the above composition range, various improvements in
physical properties exist. Preferably, the non-melt flowable
polytetrafluoroethylene content is about 4 to 50 wt % based on the
combined weight of the non-melt flowable polytetrafluoroethylene
and melt-fabricable perfluoropolymer. As the PTFE wt % increases
from 2 wt % based on the combined weight of the PTFE and
perfluoropolymer components of the composition, the elongation
and/or tensile strength increase, indicating reinforcement of the
perfluoropolymer continuous phase by the dispersed core particles.
This reinforcement extends to much greater amounts of PTFE, e.g. up
to 20 wt % PTFE, more preferably up to 30 wt % PTFE, and most
preferably up to at least 40 wt % PTFE, 4 wt % PTFE core being the
preferred minimum, all percents being based on the combined weight
of the PTFE and perfluoropolymer components. Alternatively, either
the perfluoropolymer composition or its MFR can be adjusted to
optimize melt flow either for high production rate melt fabrication
or the production of intricately molded shapes, while still
retaining adequate physical properties for the particular utility
intended. For example, the elongation at break of the melt-mixed
composition of the present invention is preferably at least about
200% for compositions containing up to at least about 30 wt % PTFE,
preferably at least about 40 wt %, based on the combined weight of
the PTFE and perfluoropolymer components. The foregoing
characterizations of the compositions apply to core/shell polymer
when used to supply the polymer components to the composition, and
to these components when separately supplied, notwithstanding the
fact that core/shell polymer typically provides the best results in
terms of reduction in melt viscosity with increasing shear and such
physical properties as elongation at break and tensile
strength.
[0027] The process of melt blending the core/shell polymer or
separately supplied PTFE and perfluoropolymer components can also
be described by the advantageous property results, e.g. the melt
blend exhibiting thixotropy characterized by the at least about
10%, at least about 100%, or at least about 500% reductions in melt
viscosity when the shear rate is increased from about 10 s.sup.-1
to about 100 s.sup.-1 by the capillary rheometer method.
Alternatively or in combination with this thixotropy, the
melt-mixed composition, whether from core/shell polymer or
separately supplied polymers, preferably exhibits an elongation of
at least about 200% up to at least 30 wt % of the core/shell
polymer being the PTFE, more preferably, at least 40 wt % of the
composition being the PTFE. Example 50 discloses greater than 200%
elongation for a composition containing about 75 wt % PTFE. The
resultant melt blend and articles molded therefrom can also have
the structure described above, wherein the PTFE is dispersed as
submicrometer-size particles in continuous phase of the
melt-fabricable perfluoropolymer.
[0028] The core/shell polymer useful in the present invention can
be made by aqueous dispersion polymerization. In one embodiment,
the non-melt flowable PTFE core is prepared in a polymerization
that is separate from the polymerization forming the shell
melt-fabricable perfluoropolymer, and this core is used to seed the
polymerization of the perfluoromonomer forming the melt-fabricable
perfluoropolymer shell onto the core. In another embodiment, the
core is formed in situ, followed by the polymerization to form the
shell on the core. In this embodiment, the non-melt flowable PTFE
core is formed by polymerization of TFE. Then the TFE (and
initiator) feed to the polymerization reactor is stopped. The
polymerization reaction is allowed to complete itself, and the
transition to the copolymerization to form the shell polymer will
depend on the copolymerization system being used. For example, the
TFE remaining in the reactor can be permitted either to be consumed
by the PTFE polymerization or to be vented off, and in either case,
the copolymerization system for the shell polymer is then
established. Alternatively, the copolymerization system for the
shell polymer is established while maintaining the TFE
concentration in the reactor constant. Then the copolymerization to
form the shell is started. Addition of the comonomer along with
additional TFE to the reactor will depend on the comonomer. When
the comonomer is HFP, the total amount will typically be added at
the commencement of the copolymerization reaction. When the
comonomer is PAVE, it too may be added at the commencement of the
copolymerization reaction or co-fed to the reactor with the TFE
feed to the copolymerization reaction. Agitation of the aqueous
medium and initiator addition may be stopped when the initial
charge of comonomer is fed to the reactor to avoid premature
coagulation of the PTFE core. The copolymerization to form the
shell is carried out to obtain the relative amount of shell polymer
desired and particle size of the core/shell polymer.
[0029] The core/shell polymer is preferably formed by first forming
the core in situ, followed by the copolymerization to form the
shell. This provides a better integration of the shell with the
core, by chemical bonding between the core and the shell, wherein
the shell copolymer acts as a compatibilizing agent with other
melt-fabricable perfluoropolymer (from other core/shell polymer
particles or from independently supplied melt-fabricable
perfluoropolymer), enabling the matrix of the blend to be formed
and the PTFE core particles to become dispersed upon melt mixing
without causing disruptions within the matrix that would detract
from physical properties.
[0030] The polymerization to form the non-melt flowable PTFE core,
whether by seed polymerization, by suspension or aqueous dispersion
polymerization or in situ is conventional polymerization to form
the non-melt flowable PTFE. The polymerization to form the shell is
also conventional aqueous dispersion polymerization. Examples of
initiators used in both polymerizations include ammonium
persulfate, potassium persulfate, bis(perfluoroalkane carboxylic
acid) peroxide, azo compounds, permanganate oxalic acid system, and
disuccinic acid peroxide. Examples of dispersing agents used in the
aqueous dispersion polymerizations include ammonium
perfluorooctanoic and perfluoroalkyl ethane sulfonic acid salts,
such as the ammonium salt.
[0031] A typical aqueous dispersion polymerization process as known
in the art involves the steps of precharging an aqueous medium to a
stirred autoclave, deoxygenating, pressurizing with TFE to a
predetermined level, adding modifying comonomer if desired,
agitating, bringing the system to desired temperature, e.g.,
60.degree.-100.degree. C., introducing initiator, adding more TFE
according to predetermined basis, and regulating temperature.
Initiator addition, at the same or different rate, may continue
throughout the batch or only for part of the batch. Recipe and
operating parameters not fixed by the equipment are commonly
selected in order that temperature is maintained approximately
constant throughout the polymerization. This same general procedure
is followed for polymerizing the perfluoromonomers to make the
melt-fabricable perfluoropolymer, except that the polymerization
temperature and order of addition of the TFE and the other
perfluoromonomer will depend on the identity of the additional
perfluoromonomer. Examples of general procedures for making
melt-fabricable perfluoropolymer are disclosed in U.S. Pat. No.
5,677,404 (FEP) and U.S. Pat. No. 5,932,673 (PFA). The transition
between the polymerization to make the core and the polymerization
to make the shell can be varied as will be shown in the Examples
herein. The timing of the transition is set in order to obtain the
weight proportion of PTFE core desired in the core shell polymer.
The weight % core can be determined by comparing the weight of TFE
consumed in the polymerization of the core with the weight of
perfluoromonomers, e.g. TFE plus HFP or perfluoro(alkyl vinyl
ether) consumed in the polymerization of the shell.
[0032] The particle size of the core/shell polymer or separately
polymerized PTFE or melt fabricable perfluoropolymer is small
enough that the polymer particles remain dispersed in the aqueous
medium until the polymerization reaction is completed, whereupon
the dispersed core/shell polymer particles can be intentionally
coagulated, by such conventional means as increased agitation from
the agitation applied during polymerization or by addition of
electrolyte. Alternatively, the coagulation can be done by
freeze/thaw method such as disclosed in U.S. Pat. No. 5,708,131
(Morgan).
[0033] Typically, the average as-polymerized polymer particle size
(diameter), referred to as RDPS (raw dispersion particle size) in
the Examples, will be less than one micrometer (submicrometer-size)
as determined by the laser light scattering method of ASTM D 4464.
Preferably the average polymer particle size is less than about 0.5
micrometer, more preferably less than about 0.3 micrometer, and
even more preferably, less than about 0.25 micrometer and most
preferably less than about 0.2 micrometer. These particle sizes
apply to the particles of PTFE, melt-fabricable perfluoropolymer,
and to the core/shell polymer used to form the composition of the
present invention. The smaller the average core/shell polymer
particle size, the more stable the aqueous dispersion of the
polymer particles, enabling the polymerization to be carried out to
higher polymer solids content before stopping the polymerization
and carrying out coagulation. The average particle size of the core
of the core/shell polymer will vary with overall size of the
core/shell polymer and the weight proportion of the core desired
and will in any event, be smaller than the particle size of the
core/shell polymer particles. Thus, since the core/shell polymer
particles are on average submicrometer-size, so will the core
particles be submicrometer-size when the core/shell polymer
particles are melt-mixed to form the continuous phase of
melt-fabricable perfluoropolymer with the core PTFE particles
dispersed therein. Similarly, for each of the core/shell polymer
average particle sizes cited above, the core particles will be
smaller. Thus, for the average core/shell polymer particle size of
less than about 0.3 micrometer, the average particle size of the
core therein will also be less than about 0.3 micrometers. The
particle size of the core in the core/shell polymer incorporated
into the melt blend is understood to be the size of the particles
of PTFE in the melt blend and articles melt-fabricated from the
melt blend, because of the non-melt flowability of the PTFE.
[0034] The as-polymerized core/shell polymer particle sizes
described above are the primary particles (sizes) of the polymer.
Coagulation of the aqueous dispersion of the core/shell primary
particles and co-coagulation of the mixed together separately
prepared aqueous dispersions of PTFE particles and particles of
melt-fabricable perfluoropolymer causes these particles to
agglomerate together, and upon drying to become a fine powder
having an average particle size depending on the method of
coagulation, but of at least about 300 micrometers, as determined
by the dry-sieve analysis disclosed in U.S. Pat. No. 4,722,122. The
agglomerates of primary particles and thus the particles of the
fine powder are often referred as secondary particles.
[0035] Thus, the core/shell polymer particles or the separately
supplied PTFE and melt-fabricable perfluoropolymer components used
in the present invention can be provided in several forms, as
primary particles and as secondary particles. When these particles
are melt-mixed (blended), the core/shell polymer and the
melt-fabricable perfluoropolymer component, as the case may be,
loses its particulate form to become a blend of the two polymers,
notably wherein the core or PTFE primary particle becomes the
dispersed phase and the melt-fabricable perfluoropolymer becomes
the continuous phase. The melt mixing can be part of the melt
fabrication process, such as occurs during extrusion or injection
molding. Typically, the melt blend will be extruded as molding
pellets, which can later be used for melt fabrication into the
final article. The core/shell polymer useful in the present
invention can be made by aqueous dispersion polymerization. In one
embodiment, the non-melt flowable PTFE core is prepared in a
polymerization that is separate from the polymerization forming the
shell melt-fabricable perfluoropolymer, and this core is used to
seed the polymerization of the perfluoromonomer forming the
melt-fabricable perfluoropolymer shell onto the core. In another
embodiment, the core is formed in situ, followed by the
polymerization to form the shell on the core. In this embodiment,
the non-melt flowable PTFE core is formed by polymerization of TFE.
Then the TFE (and initiator) feed to the polymerization reactor is
stopped. The polymerization reaction is allowed to complete itself,
and the transition to the copolymerization to form the shell
polymer will depend on the copolymerization system being used. For
example, the TFE remaining in the reactor can be permitted either
to be consumed by the PTFE polymerization or to be vented off, and
in either case, the copolymerization system for the shell polymer
is then established. Alternatively, the copolymerization system for
the shell polymer is established while maintaining the TFE
concentration in the reactor constant. Then the copolymerization to
form the shell is started. Addition of the comonomer along with
additional TFE to the reactor will depend on the comonomer. When
the comonomer is HFP, the total amount will typically be added at
the commencement of the copolymerization reaction. When the
comonomer is PAVE, it too may be added at the commencement of the
copolymerization reaction or co-fed to the reactor with the TFE
feed to the copolymerization reaction. Agitation of the aqueous
medium and initiator addition may be stopped when the initial
charge of comonomer is fed to the reactor to avoid premature
coagulation of the PTFE core. The copolymerization to form the
shell is carried out to obtain the relative amount of shell polymer
desired and particle size of the core/shell polymer.
[0036] The core/shell polymer is preferably formed by first forming
the core in situ, followed by the copolymerization to form the
shell. This provides a better integration of the shell with the
core, by chemical bonding between the core and the shell, wherein
the shell copolymer acts as a compatibilizing agent with other
melt-fabricable perfluoropolymer (from other core/shell polymer
particles or from independently supplied melt-fabricable
perfluoropolymer), enabling the matrix of the blend to be formed
and the PTFE core particles to become dispersed upon melt mixing
without causing disruptions within the matrix that would detract
from physical properties.
[0037] The polymerization to form the non-melt flowable PTFE core,
whether by seed polymerization, by suspension or aqueous dispersion
polymerization or in situ is conventional polymerization to form
the non-melt flowable PTFE. The polymerization to form the shell is
also conventional aqueous dispersion polymerization. Examples of
initiators used in both polymerizations include ammonium
persulfate, potassium persulfate, bis(perfluoroalkane carboxylic
acid) peroxide, azo compounds, permanganate oxalic acid system, and
disuccinic acid peroxide. Examples of dispersing agents used in the
aqueous dispersion polymerizations include ammonium
perfluorooctanoic and perfluoroalkyl ethane sulfonic acid salts,
such as the ammonium salt.
[0038] A typical aqueous dispersion polymerization process as known
in the art involves the steps of precharging an aqueous medium to a
stirred autoclave, deoxygenating, pressurizing with TFE to a
predetermined level, adding modifying comonomer if desired,
agitating, bringing the system to desired temperature, e.g.,
60.degree.-100.degree. C., introducing initiator, adding more TFE
according to predetermined basis, and regulating temperature.
Initiator addition, at the same or different rate, may continue
throughout the batch or only for part of the batch. Recipe and
operating parameters not fixed by the equipment are commonly
selected in order that temperature is maintained approximately
constant throughout the polymerization. This same general procedure
is followed for polymerizing the perfluoromonomers to make the
melt-fabricable perfluoropolymer, except that the polymerization
temperature and order of addition of the TFE and the other
perfluoromonomer will depend on the identity of the additional
perfluoromonomer. Examples of general procedures for making
melt-fabricable perfluoropolymer are disclosed in U.S. Pat. No.
5,677,404 (FEP) and U.S. Pat. No. 5,932,673 (PFA). The transition
between the polymerization to make the core and the polymerization
to make the shell can be varied as will be shown in the Examples
herein. The timing of the transition is set in order to obtain the
weight proportion of PTFE core desired in the core shell polymer.
The weight % core can be determined by comparing the weight of TFE
consumed in the polymerization of the core with the weight of
perfluoromonomers, e.g. TFE plus HFP or perfluoro(alkyl vinyl
ether) consumed in the polymerization of the shell.
[0039] The particle size of the core/shell polymer is small enough
that the polymer particles remain dispersed in the aqueous medium
until the polymerization reaction is completed, whereupon the
dispersed core/shell polymer particles can be intentionally
coagulated, by such conventional means as increased agitation from
the agitation applied during polymerization or by addition of
electrolyte. Alternatively, the coagulation can be done by
freeze/thaw method such as disclosed in U.S. Pat. No. 5,708,131
(Morgan).
[0040] Typically, the average as-polymerized core/shell polymer
particle size (diameter), referred to as RDPS (raw dispersion
particle size) in the Examples, will be less than one micrometer
(submicrometer-size) as determined by the laser light scattering
method of ASTM D 4464. Preferably the average polymer particle size
is less than about 0.5 micrometer, more preferably less than about
0.3 micrometer, and even more preferably, less than about 0.25
micrometer and most preferably less than about 0.2 micrometer.
These particle sizes apply to the particles of PTFE,
melt-fabricable perfluoropolymer, and to the core/shell polymer
used to form the composition of the present invention. The smaller
the average core/shell polymer particle size, the more stable the
aqueous dispersion of the polymer particles, enabling the
polymerization to be carried out to higher polymer solids content
before stopping the polymerization and carrying out coagulation.
The average particle size of the core of the core/shell polymer
will vary with overall size of the core/shell polymer and the
weight proportion of the core desired and will in any event, be
smaller than the particle size of the core/shell polymer particles.
Thus, since the core/shell polymer particles are on average
submicrometer-size, so will the core particles be
submicrometer-size when the core/shell polymer particles are
melt-mixed to form the continuous phase of melt-fabricable
perfluoropolymer with the core PTFE particles dispersed therein.
Similarly, for each of the core/shell polymer average particle
sizes cited above, the core particles will be smaller. Thus, for
the average core/shell polymer particle size of less than about 0.3
micrometer, the average particle size of the core therein will also
be less than about 0.3 micrometers. The particle size of the core
in the core/shell polymer incorporated into the melt blend is
understood to be the size of the particles of PTFE in the melt
blend and articles melt-fabricated from the melt blend, because of
the non-melt flowability of the PTFE.
[0041] The as-polymerized core/shell polymer particle sizes
described above are the primary particles (sizes) of the polymer.
Coagulation of the aqueous dispersion of the core/shell primary
particles and co-coagulation of the mixed together separately
prepared aqueous dispersions of PTFE particles and particles of
melt-fabricable perfluoropolymer causes these particles to
agglomerate together, and upon drying to become a fine powder
having an average particle size depending on the method of
coagulation, but of at least about 300 micrometers, as determined
by the dry-sieve analysis disclosed in U.S. Pat. No. 4,722,122. The
agglomerates of primary particles and thus the particles of the
fine powder are often referred as secondary particles.
[0042] Thus, the core/shell polymer particles or the separately
supplied PTFE and melt-fabricable perfluoropolymer components used
in the present invention can be provided in several forms, as
primary particles and as secondary particles. When these particles
are melt-mixed (blended), the core/shell polymer and the
melt-fabricable perfluoropolymer component, as the case may be,
loses its particulate form to become a blend of the two polymers,
notably wherein the core or PTFE primary particle becomes the
dispersed phase and the melt-fabricable perfluoropolymer becomes
the continuous phase. The melt mixing can be part of the melt
fabrication process, such as occurs during extrusion or injection
molding. Typically, the melt blend will be extruded as molding
pellets, which can later be used for melt fabrication into the
final article. In all these melt-mixed forms, the
dispersion/continuous phase structure of the melt-mixed composition
is present as indicated by the melt fabricability of the melt
mixture. The melt blending or melt fabrication process which
includes melt mixing is typically carried out at a temperature
above the melting temperature of the polytetrafluoroethylene, which
is about 343.degree. C. for the first melt and about 327.degree. C.
for subsequent melts and which is above the melting temperature of
the melt-fabricable perfluoropolymer. Thus, the melt mixing
temperature will typically be at least about 350.degree. C.
[0043] all these melt-mixed forms, the dispersion/continuous phase
structure of the melt-mixed composition is present as indicated by
the melt fabricability of the melt mixture. The melt blending or
melt fabrication process which includes melt mixing is typically
carried out at a temperature above the melting temperature of the
polytetrafluoroethylene, which is about 343.degree. C. for the
first melt and about 327.degree. C. for subsequent melts and which
is above the melting temperature of the melt-fabricable
perfluoropolymer. Thus, the melt mixing temperature will typically
be at least about 350.degree. C.
[0044] The core/shell polymer useful in the present invention can
be prepared as a concentrate, i.e. of relatively high PTFE content
for dilution with separately supplied melt-fabricable
perfluoropolymer by itself. Separately supplied melt fabricable
perfluoropolymer is perfluoropolymer that is not supplied by the
core/shell polymer. Submicrometer-size PTFE particles can also be
separately supplied. The resultant melt blend causes the
melt-fabricable perfluoropolymer from the shell of the core/shell
polymer to melt mix with the separately supplied melt-fabricable
perfluoropolymer to become indistinguishable as they form together
the continuous phase for the dispersed particles of non-melt
flowable polytetrafluoroethylene. Preferably the dilution of the
core/shell polymer with additional (independently supplied)
melt-fabricable perfluoropolymer involves the mixing together of
aqueous dispersions of each polymer, followed by co-coagulation of
the intermixed dispersion, resulting in an intimate mixing of the
primary particles of the polymer from each dispersion with one
another. This provides the best overall results for the viscosity
of the melt blend in melt mixing accompanying melt fabrication and
for the physical properties of articles molded from the blend.
Co-coagulation of the mixed dispersions results in the formation of
agglomerates, which contain both primary particles of core/shell
polymer and separately supplied melt-fabricable perfluoropolymer.
The particle sizes of these primary particles and agglomerates
(fine powder when dried) are the same as disclosed above with
respect to the core/shell polymer particles and agglomerates
thereof by themselves. The independently supplied melt-fabricable
perfluoropolymer should be compatible with the perfluoropolymer of
the shell of the core/shell polymer. By compatible is meant that
the melt-fabricable perfluoropolymers become indistinguishable in
melt mixing and cooling in forming the continuous phase as
described above. Preferably the monomers making up the
melt-fabricable perfluoropolymer of the shell and the independently
supplied perfluoropolymer are either the same or in the homologous
series. The shell polymer and the independently supplied
perfluoropolymer are considered to be the same even there may be
small difference in concentration of the same perfluoromonomer
and/or difference in MFR as occurs from small differences in the
polymerization process producing the shell polymer as compared to
producing the perfluoropolymer by itself. This provides the
indistinguishability of the shell perfluoropolymer and
independently supplied perfluoropolymer resulting from melt mixing
to form the continuous phase of the blend. The most common
melt-fabricable perfluoropolymers, FEP and PFA are incompatible
with one another, one indicia of which is that magnified frozen
(that is non-molten) cross-sections of the cooled melt blend reveal
domains of each perfluoropolymer being present when viewed under
polarized light.
[0045] In this embodiment, in which the core/shell polymer is a
concentrate, the core/shell polymer by itself can be
melt-fabricable by itself. Alternatively, the core/shell polymer
can have little-to-no melt-fabricability by virtue of high PTFE
core content. In this embodiment, the melt-fabricability of the
blend of core/shell polymer and separate melt-fabricable
perfluoropolymer is enabled by the latter, and the combination of
the melt-fabricable perfluoropolymer from the shell of the
core/shell polymer and the perfluoropolymer supplied by itself
provides the continuous phase of the melt-mixed blend.
[0046] In the embodiment wherein the core/shell polymer is a
concentrate, the proportions of core/shell polymer and
independently supplied perfluoropolymer are chosen to provide in
melt blend combination any of the core/shell compositions described
above.
[0047] In the embodiment wherein the non-melt flowable PTFE and
melt-fabricable perfluoropolymer are separately supplied to form
the melt-mixed composition, each of these components are preferably
made by aqueous dispersion polymerization by polymerization methods
well known in the art. The polymerization conditions and
ingredients described above can be used, except that shell
polymerization over a core is not used in this embodiment. Thus, an
aqueous dispersion of the non-melt flowable PTFE and an aqueous
dispersion of the melt-fabricable perfluoropolymer will be
obtained. In each dispersion, the polymer particles will be
submicrometer-size. The core/shell polymer particle sizes and
measurement thereof described above for the core/shell polymer
applies to each of these aqueous dispersions. These dispersions are
mixed together, the result being a mixture of primary particles of
each polymer in the resultant aqueous dispersion. Co-coagulation of
this dispersion mixture, as described above for the core/shell
polymer, forms agglomerates, each containing primary particles of
each polymer. The same is true after drying the agglomerates to
form fine powder having an average particle size typically at least
about 300 micrometers. Melt-mixing of this fine powder to form a
composition of the present invention, as in the case of core/shell
polymer, provides a mixture of primary particles of each polymer as
the starting material for the melt mixing step. The conditions of
shear rate in the melt-mixing process are not critical because the
mixture of primary particles of each polymer component has already
been established prior to melt mixing. Because a high shear rate is
associated with high productivity, the shear rate used for the melt
mixing will preferably be high, e.g. at least about 75
s.sup.-1.
EXAMPLES
Test Procedures
[0048] The procedures for determining melt creep viscosity,
standard specific gravity (SSG), melt flow rate (MFR), core/shell
polymer composition, and average core/shell polymer particle size
(RDPS) reported in the Examples are disclosed earlier herein. The
determination of melt viscosity is also disclosed earlier herein.
All of the core/shell polymers and separate melt-fabricable
perfluoropolymers disclosed in the Examples exhibited a melt
viscosity less than about 5.times.10.sup.4 Pas at 350.degree. C.
and shear rate of 101 s.sup.-1.
[0049] The thixotropy of the melt blends described in the Examples
is determined by capillary rheometry method of ASTM D 3835-02 in
which the melt temperature of the polymer in the rheometer is
350.degree. C. This method involves the extrusion of molten polymer
through the barrel of a Kayeness.RTM. capillary rheometer at a
controlled force to obtain the shear rate desired. The results are
reported in the Examples as melt viscosity change (reduction),
.DELTA..eta. in Pas in increasing the shear rate on the molten
polymer from 11.9 s.sup.-1 to 101 s.sup.-1. The determination of
melt viscosity using the rheometer has been discussed hereinbefore.
The melt viscosities are determined at the two shear rates, and the
viscosity difference is determined by subtracting the melt
viscosity at 101 s.sup.-1 from the melt viscosity at 11.9
s.sup.-1.
[0050] The elongation at break and tensile strength parameters
disclosed hereinbefore and values reported in the Examples are
obtained by the procedure of ASTM D 638-03 on dumbbell-shaped test
specimens 15 mm wide by 38 mm long and having a web thickness of 5
mm, stamped out from 60 mil (1.5 mm) thick compression molded
plaques.
[0051] The procedure for measuring MIT Flex Life is disclosed at
ASTM D 2176 using a 8 mil (0.21 mm) thick compression molded film,
unless otherwise indicated.
[0052] The compression molding of the plaques and film used in
these tests was carried out on fine powder under a force of 20,000
lbs (9070 kg) at a temperature of 350.degree. C. to make 6.times.6
in (15.2.times.15.2 cm) compression moldings. In greater detail, to
make the 60 mil thick plaque, the fine powder was added in an
overflow amount to a chase which was 55 mil (1.4 mm) thick. The
chase defines the 6.times.6 in sample size. To avoid sticking to
the platens of the compression molding press, the chase and fine
powder filling are sandwiched between two sheets of aluminum foil.
The press platens are heated to 350.degree. C. This sandwich is
first pressed for 5 min at about 200 lb (91 kg) to melt the fine
powder and cause it to coalesce, followed by pressing at 10000 lb
(4535 kg) for 2 min, followed by 20000 lb (9070 kg) for 2 min,
followed by release of the pressing force, removal of the
compression molding from the chase and sheets of aluminum foil, and
cooling in air under a weight to prevent warping. The film used in
the MIT test used the same procedure except that the chase was 8
mil (0.21 mm) thick and defining a 4.times.4 in. (10.2
cm.times.10.2 cm) square cavity. The film samples used in the MIT
test were 1/2 in. (1.27 cm) strips cut from the compression molded
film. Compression molding of the core/shell polymer coagulated and
dried into fine powder produces the dispersion of the PTFE core in
a continuous matrix of the shell perfluoropolymer. The compression
molding is necessary to give the test specimen strength. If the
powder were merely coalesced by heating at the temperature of the
compression molding, to simulate the fusing of a coating, the
resultant coalesced article would have little strength.
[0053] The Examples disclose the formation of melt blends of
non-melt flowable PTFE and melt-fabricable perfluoropolymer,
starting from core/shell polymer, from core shell polymer used as a
concentrate for dilution by additional perfluoropolymer, and from
separately supplied polymers. The properties of these melt blends
are applicable to, i.e. carry over into, the melt fabricated
articles obtained therefrom.
Comparative Example A
[0054] This Example shows the polymerization to form a typical
high-performing FEP by itself for comparison with Examples
preparing core/shell polymer in which FEP is the shell.
[0055] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water and 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water. With
the reactor paddle agitated at 46 rpm, the reactor was heated to
60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 103.degree. C. After the temperature had become steady
at 103.degree. C., HFP was added slowly to the reactor until the
pressure was 444 psig (3.1 MPa). Ninety-two milliliters of liquid
PEVE was injected into the reactor. Then TFE was added to the
reactor to achieve a final pressure of 645 psig (4.52 MPa). Then 40
mL of freshly prepared aqueous initiator solution containing 1.04
wt % of ammonium persulfate (APS) and 0.94 wt % potassium
persulfate (KPS) was charged into the reactor. Then, this same
initiator solution was pumped into the reactor at 10 mL/min for the
remainder of the polymerization. After polymerization had begun as
indicated by a 10 psig (70 kPa) drop in reactor pressure,
additional TFE was added to the reactor at a rate of 24.5 lb (11.1
kg)/125 min until a total of 24.5 lbs (11.1 kg) of TFE had been
added to the reactor after kickoff. Furthermore, liquid PEVE was
added at a rate of 1.0 mL/min for the duration of the reaction. The
total reaction time was 125 min after initiation of polymerization.
At the end of the reaction period, the TFE feed, PEVE feed, and the
initiator feed were stopped, and the reactor was cooled while
maintaining agitation. When the temperature of the reactor contents
reached 90.degree. C., the reactor was slowly vented. After venting
to nearly atmospheric pressure, the reactor was purged with
nitrogen to remove residual monomer. Upon further cooling, the
dispersion was discharged from the reactor at below 70.degree. C.
Solids content of the dispersion was 36.81 wt % and raw dispersion
particle size (RDPS) was 0.167 .mu.m. After coagulation, the
polymer was isolated by filtering and then drying in a 150.degree.
C. convection air oven. This polymer was stabilized by heating at
260.degree. C. for 1.5 hr in humid air containing 13 mol % water.
The TFE/HFP/PEVE terpolymer (FEP) had a melt flow rate (MFR) of
37.4 g/10 min, an HFP content of 10.5 wt %, a PEVE content of 1.26
wt %, and a melting point of 260.degree. C. For this material, the
viscosity change (reduction), .DELTA..eta., is 101 Pas. The FEP
exhibited a tensile strength and elongation at break of 2971 psi
(20.8 MPa) and 310%, respectively.
Example 1
[0056] Core/shell polymer when the shell polymer is FEP and the
proportion of core to shell is widely varied, is made in this
Example. A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water and 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water. With
the reactor paddle agitated at 46 rpm, the reactor was heated to
60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 103.degree. C. After the temperature had become steady
at 103.degree. C., the pressure of the reactor was raised to 250
psig (1.75 MPa) using TFE. Fifty milliliters of an initiating
solution consisting of 1.04 wt % APS and 0.94 wt % KPS in water was
injected to the reactor, then this same initiator was added at 0.5
mL/min. After polymerization had begun as indicated by a 10 psig
(70 kPa) drop in reactor pressure, additional TFE was added at 0.2
lb (90.7 g)/min for 10 min. After 2 lbs (9070 g) of TFE had been
fed after initiation, the TFE feed was stopped, then the reactor
contents were agitated for 10 minutes with the initiator still
being fed. The agitator and initiator pumps were stopped, then 1280
mL of HFP were added to the reactor. Agitation was resumed and
initiation was resumed using the same solution at a rate of 10
mL/min. The reactor pressure was raised to 600 psi (4.1 MPa) with
TFE. An aliquot of 92 mL of PEVE was added to the reactor, then 1
mL/min PEVE and 0.2 lb (90.87 g)/min TFE were added over the
remainder of the reaction. After an additional 20 lb (9070 g) of
TFE were reacted, the PEVE injection was stopped. Two more lbs (907
g) of TFE were fed, for a total of 24 lb (10.9 kg) TFE for the
batch, then the batch was terminated in a manner similar to
Comparative Example A. Solids content of the dispersion of the
resultant TFE/HFP/PEVE copolymer was 35.8 wt % and raw dispersion
particle size (RDPS) was 0.246 .mu.m. The polymer was finished in a
manner similar to Comparative Example A. Details of the composition
of this core/shell polymer and its properties are presented as
Example 1 in Table 1.
Examples 2-6
[0057] Examples 2 through 6 were prepared in a manner similar to
Example 1, with the proportions of core and shell altered by
changing the relative amounts of TFE fed during each phase of
polymerization. Details are given in Table 1 below. TABLE-US-00001
TABLE 1 PTFE HFP PEVE HFP PEVE Tensile Example Core, Content,
Content, Content in Content in MFR, Strength, Elongation
.DELTA..eta., Number wt % wt % wt % Shell, wt % Shell, wt % g/10
min MPa at Break, % Pa s 1 7.6% 6.84 1.37 7.41 1.48 0 26.7 357
12936 2 11.5% 6.42 1.43 7.25 1.62 0.4 23.8 393 6273 3 15.4% 6.41
1.47 7.57 1.74 0.7 21.3 358 6495 4 19.2% 6.18 1.69 7.65 2.09 0 24.9
394 9000 5 26.9% 5.83 1.81 7.98 2.48 0 20.9 338 9113 6 39.0% 5.08
1.30 8.34 2.12 0 17.3 235 10344
Each of the polymerizations was carried out to a solids content of
33.8 to 35.8 wt % and the RDPS of the polymer particles ranged from
194 to 261 nm (0.194 to 0.261 micrometers). As compared to the
reduction in melt viscosity of 101 Pas for typical FEP by itself
(Comparison Example A), the core/shell polymer of the present
invention exhibits a much greater melt viscosity reduction with
increasing shear, with the maximum reduction occurring at the
lowest core content tested. This thixotropy enables the core/shell
polymer, which exhibits very low MFR, to be melt fabricated when
subjected to the higher shear applied in the melt fabrication
process. With respect to physical properties, the tensile strength
and elongation at break of the core/shell polymer was better than
for the FEP by itself at PTFE core contents up to about 30 wt %,
and useful tensile strength and elongation at break exist for core
contents exceeding core content of about 40 wt %. The best
combination of optimum thixotropy and physical properties occur in
the range of about 4 to 20 wt % PTFE core when the perfluoropolymer
in the shell is FEP. The low-to-no MFR shown for the core/shell
polymers in Table 1 is beneficial to the utility of articles
melt-fabricated from the core/shell polymers. Such articles
(dispersion of PTFE core in perfluoropolmer continuous phase
obtained by melt mixing the core/shell polymer) when exposed to
high heat such as in a building fire will resist flowing and
dripping to thereby remain non-smoking.
Comparative Example B
[0058] This Example shows the polymerization of PTFE by itself
essentially under the same conditions used for polymerizing TFE to
make the non-melt flowable PTFE in the core/shell polymer of
Examples 1-6 above and in the Examples to follow to verify that the
PTFE is non-melt flowable.
[0059] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water, 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water, and
1.0 g Krytox.RTM. 157 FSL, available from E.I. du Pont de Nemours
and Company, Inc. Krytox.RTM. 157 FSL is a perfluoropolyether
carboxylic acid as further described in Table 1 of U.S. Pat. No.
6,429,258. With the reactor paddle agitated at 46 rpm, the reactor
was heated to 60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 103.degree. C. After the temperature had become steady
at 103.degree. C., the pressure of the reactor was raised to 250
psig (1.75 MPa) using TFE. Fifty milliliters of an initiating
solution consisting of 1.04 wt % APS and 0.94 wt % KPS in water was
injected to the reactor, then this same initiator was added at 0.5
mL/min. After polymerization had begun as indicated by a 10 psig
(0.07 MPa) drop in reactor pressure, additional TFE was added at
0.2 lb (90.8 g)/min for 7.5 min. After 1.5 lbs (681 g) of TFE had
been fed after initiation, the TFE and initiator feeds were stopped
and the reactor was vented. The contents of the reactor were cooled
to 80.degree. C. before being discharged. Solids content of the
dispersion was 4.78 wt % and raw dispersion particle size (RDPS)
was 0.089 .mu.m. After coagulation, the polymer was isolated by
filtering and then dried in a 150.degree. C. convection air oven.
The standard specific gravity (SSG) of the resulting PTFE
homopolymer, measured according to the method described in U.S.
Pat. No. 4,036,802, was determined to be 2.200. The results
demonstrate that the core of the core/shell polymer is non-melt
flowable PTFE because it has a measurable SSG. The PTFE also
exhibits a melt creep viscosity greater than 108 Pas at 380.degree.
C. and zero MFR.
Examples 7-11
[0060] These Examples show core/shell polymer with approximately
the same core content and with varying HFP and PEVE content for the
overall core/shell polymer and for the shell FEP.
[0061] PTFE dispersions were polymerized in the manner of
Comparative Example B, varying the amount of Krytox.RTM. 157 FSL
used as shown in Table 3. Rather than venting the reaction vessel,
however, the TFE feed was stopped, then the contents of the reactor
were stirred for 10 min with continuing initiator feed. After 10
min, the initiator feed was stopped, then the reactor pressure was
raised to 444 psig (3.1 MPa) with HFP. A 92 mL aliquot of PEVE was
injected, then TFE was added to the reactor to a pressure of 650
psig (4.6 MPa). For the remainder of the batch, an initiator
solution of 1.04 wt % APS and 0.94 wt % KPS was added at a rate of
10 mL/min, while PEVE was added at 1.0 mL/min. TFE was added at 0.2
lb (90.7 g)/min for 97.5 min. After an additional 19.5 lbs (8845 g)
of TFE were added, the TFE, initiator, and PEVE feeds were stopped,
and the reactor was cooled while maintaining agitation. When the
temperature of the reactor contents reached 90.degree. C., the
reactor was slowly vented. After venting to nearly atmospheric
pressure, the reactor was purged with nitrogen to remove residual
monomer. Upon further cooling, the dispersion was discharged from
the reactor at below 70.degree. C. After coagulation, the polymer
was isolated by filtering and then drying in a 150.degree. C.
convection air oven. This polymer was stabilized by heating at
260.degree. C. for 1.5 hr in humid air containing 13 mol % water.
Properties of these polymers are shown in Table 2. TABLE-US-00002
TABLE 2 Krytox .RTM. PTFE HFP PEVE HFP PEVE Tensile Example 157FSL
Core, Content, Content, Content in Content in MFR, Strength,
Elongation .DELTA..eta., Number Used, g wt % wt % wt % Shell, wt %
Shell, wt % g/10 min MPa at Break, % Pa s 7 0 6.5% 6.96 1.82 7.44
1.95 8.4 22.1 329 2197 8 1 6.4% 8.56 1.59 9.14 1.70 7.6 21.4 316
2079 9 2 6.4% 9.18 1.31 9.81 1.40 5.8 18.0 207 1710 10 4 6.2% 11.26
1.43 12.01 1.53 17.0 21.2 312 1202 11 5 6.4% 9.44 1.31 10.08 1.40
9.5 22.6 323 1800
The polymerizations shown in Table 2 were carried out to solids
concentrations of 36.2 to 38.7 wt % polymer solids, and the average
RDPS of the core/shell polymer was from 76 to 191 nm. As shown in
Table 2, both the MFR and melt viscosity as well as physical
properties of the core/shell polymer can be changed by changing the
shell compositions, using the same TFE, HFP and PEVE
comonomers.
Examples 12-14
[0062] These Examples show the effect of increasing polymerization
initiator concentration in the polymerization to form the FEP shell
so as to further increase MFR.
[0063] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water, 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water, and
5.0 g Krytox.RTM. 157 FSL. With the reactor paddle agitated at 46
rpm, the reactor was heated to 60.degree. C., evacuated and purged
three times with tetrafluoroethylene (TFE). The reactor temperature
then was increased to 103.degree. C. After the temperature had
become steady at 103.degree. C., the pressure of the reactor was
raised to 250 psig (1.75 MPa) using TFE. Fifty milliliters of an
initiating solution consisting of 1.04 wt % APS and 0.94 wt % KPS
in water was injected to the reactor, then this same initiator was
added at 0.5 mL/min. After polymerization had begun as indicated by
a 10 psig (0.07 MPa) drop in reactor pressure, additional TFE was
added at 0.2 lb (90.8 g)/min for 10 min. After 1.5 lbs (681 g) of
TFE fed after initiation, the TFE feed was stopped, then the
reactor contents were agitated for 10 minutes with the initiator
still being fed. The agitator and initiator pumps were stopped,
then the pressure of the reactor was raised to 444 psig (3.1 MPa)
with HFP. Agitation was restarted and initiation was resumed at a
rate of 10 mL/min using a solution as shown in Table 3. The reactor
pressure was raised to 650 psi (4.55 MPa) with TFE. An aliquot of
92 mL of PEVE was added to the reactor, then 1 mL/min PEVE and 0.2
lb (90.7 g)/min TFE were added over the remainder of the reaction.
After an additional 19.5 lbs (8845 g) of TFE were added, the TFE,
initiator, and PEVE feeds were stopped, and the reactor was cooled
while maintaining agitation. When the temperature of the reactor
contents reached 90.degree. C., the reactor was slowly vented.
After venting to nearly atmospheric pressure, the reactor was
purged with nitrogen to remove residual monomer. Upon further
cooling, the dispersion was discharged from the reactor at below
70.degree. C. After coagulation, the polymer was isolated by
filtering and then drying in a 150.degree. C. convection air oven.
This polymer was stabilized by heating at 260.degree. C. for 1.5 hr
in humid air containing 13 mol % water. The properties of these
polymers are shown in Table 3. TABLE-US-00003 TABLE 3 Shell
Initiator Shell Initiator PTFE HFP PEVE HFP PEVE Tensile Example
Solution APS Solution KPS Core, Content, Content, Content in
Content in MFR, Strength, Elongation .DELTA..eta., Number Conc., wt
% Conc. wt % Wt % wt % wt % Shell, wt % Shell, wt % g/10 min MPa at
Break, % Pa s 12 1.04 0.94 6.4% 9.41 1.32 10.05 1.41 9.8 22.2 269
2194 13 2.08 1.88 6.3% 11.17 1.23 11.92 1.31 20.9 19.5 320 1060 14
2.08 1.88 6.2% 11.59 1.35 12.35 1.44 28.0 17.3 274 627
The polymerizations shown in this Table produced polymer solids
contents of 38.2 to 39.3 wt % for the aqueous dispersion
polymerization medium, and average RDPS of 142 to 230 nm.
Examples 15-19
[0064] These Examples show the preparation of the core/shell
polymer using ammonium persulfate as the sole initiator, and
changing the initiator concentration at almost constant core
content to vary the MFR of the core/shell polymer.
[0065] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water, 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water, and
5.0 g Krytox.RTM. 157 FSL. With the reactor paddle agitated at 46
rpm, the reactor was heated to 60.degree. C., evacuated and purged
three times with tetrafluoroethylene (TFE). The reactor temperature
then was increased to 103.degree. C. After the temperature had
become steady at 103.degree. C., the pressure of the reactor was
raised to 250 psig (1.75 MPa) using TFE. Fifty milliliters of an
initiating solution consisting of 1.834 wt % APS in water was
injected to the reactor, then this same initiator was added at 0.5
mL/min. After polymerization had begun as indicated by a 10 psig
(0.07 MPa) drop in reactor pressure, additional TFE was added at
0.2 lb (90.8 g)/min for 10 min. After 1.5 lbs (681 g) of TFE fed
after initiation, the TFE feed was stopped, then the reactor
contents were agitated for 10 minutes with the initiator still
being fed. The initiator pump was stopped, then the pressure of the
reactor was raised to 444 psig (3.1 MPa) with HFP. Initiation was
resumed using a solution as shown in Table 4 at a rate of 10
mL/min. The reactor pressure was raised to 650 psi (4.55 MPa) with
TFE. An aliquot of 92 mL of PEVE was added to the reactor, then 1
mL/min PEVE and 0.2 lb (90.8 g)/min TFE were added over the
remainder of the reaction. After an additional 19.5 lbs (8853 g) of
TFE were added, the TFE, initiator, and PEVE feeds were stopped,
and the reactor was cooled while maintaining agitation. When the
temperature of the reactor contents reached 90.degree. C., the
reactor was slowly vented. After venting to nearly atmospheric
pressure, the reactor was purged with nitrogen to remove residual
monomer. Upon further cooling, the dispersion was discharged from
the reactor at below 70.degree. C. After coagulation, the polymer
was isolated by filtering and then drying in a 150.degree. C.
convection air oven. The properties of these polymers are shown in
Table 4. TABLE-US-00004 TABLE 4 Shell Initiator Solution APS PTFE
HFP PEVE HFP PEVE Tensile Concentration, Core, Content Content,
Content in Content in MFR, Strength, Elongation .DELTA..eta., Ex.
No. wt % wt % wt % wt % Shell, wt % Shell, wt % g/10 min MPa at
Break, % Pa s 15 1.89 6.32% 10.47 1.00 11.17 1.07 10.8 22.8 327
1005 16 2.37 6.21% 11.86 1.14 12.64 1.21 18.2 20.8 323 1093 17 2.84
6.25% 11.41 1.06 12.18 1.13 15.2 18.1 320 1168 18 3.31 6.19% 12.33
0.95 13.14 1.01 27.4 16.1 206 1170 19 3.79 6.17% 12.72 0.97 13.55
1.04 30.7 16.3 181 871
The polymerizations carried out for the Examples in Table 4
resulted in polymer solids contents ranging from 30.9 to 39.5 wt %
and average RDPS of 100 to 184 nm.
Comparative Example C
[0066] This Example shows the copolymerization to make a typical
high-performing PFA by itself for comparison with the core/shell
polymer containing essentially the same PFA.
[0067] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 54
pounds (24.5 kg) of demineralized water and 240 mL 20 wt % solution
of ammonium perfluorooctanoate surfactant in water. With the
reactor paddle agitated at 50 rpm, the reactor was evacuated and
purged three times with tetrafluoroethylene (TFE). Ethane was added
to the reactor until the pressure was 8 in Hg (3.93 psig,
2.71.times.10.sup.-2 MPa), then 200 mL of perfluoro(ethyl vinyl
ether) (PEVE) were added. The reactor temperature then was
increased to 75.degree. C. After the temperature had become steady
at 75.degree. C., TFE was added to the reactor to achieve a final
pressure of 300 psig (2.07 MPa). An aliquot of 400 mL of a freshly
prepared aqueous initiator solution containing 0.2 wt % of ammonium
persulfate (APS) was charged to the reactor. This same initiator
solution was pumped into the reactor at 5 mL/min for the remainder
of the batch. After polymerization had begun, as indicated by a 10
psig (0.07 MPa) drop in reactor pressure, additional TFE was added
to the reactor at a rate of 0.167 lb/min (75.6 g/min) until a total
of 20 lb (9080 g) of TFE were added after kickoff. PEVE was added
at 2.0 mL/min for the duration of the batch, 120 min. At the end of
the reaction period, the TFE, PEVE, and initiator feeds were
stopped and the reaction vessel was vented. Solids content of the
dispersion was 29.7 wt %, and the raw dispersion particle size
(RDPS) was 0.172 .mu.m. After coagulation, the polymer was isolated
by filtering and then dried in a 150.degree. C. convection air
oven. This PEVE/TFE copolymer had a melt flow rate (MFR) of 29.0
g/10 min, a PEVE content of 3.01 wt %, a melting point of
302.degree. C., and an MIT flex life of 2463 cycles. The viscosity
change was 111 Pas. The copolymer also exhibited a tensile strength
of 3027 psi (21.2 MPa) and elongation at break of 349%.
Example 20
[0068] This Example shows the preparation of core/shell polymer in
which the shell is essentially the same PFA as Comparative Example
C.
[0069] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 54
pounds (24.5 kg) of demineralized water and 240 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water. With
the reactor paddle agitated at 50 rpm, the reactor was evacuated
and purged three times with tetrafluoroethylene (TFE). The reactor
temperature then was increased to 75.degree. C. After the
temperature had become steady at 75.degree. C., the pressure of the
reactor was raised to 300 psig (2.1 MPa) using TFE. Four hundred
milliliters of an initiating solution consisting of 0.2 wt % APS in
water was injected to the reactor, then this same initiator was
added at 5.0 mL/min. After polymerization had begun as indicated by
a 10 psig (0.07 MPa) drop in reactor pressure, additional TFE was
added at 0.2 lb (90.8 g)/min for 5 min. After 1 lb (454 g) of TFE
was fed after initiation, the TFE and initiator feeds were stopped,
then the reactor was slowly vented. After stopping agitation, the
reactor vapor space was evacuated. Agitation was resumed at 50 rpm,
then the contents were cooled to 25.degree. C. The agitator was
again stopped, then the pressure in the reactor was raised to 8 in
Hg (3.93 psig, 2.71.times.10.sup.-2 MPa) with ethane. After the
addition of ethane, the agitator was restarted at 50 rpm and the
contents of the reactor were warmed to 75.degree. C. A 200 mL
aliquot of perfluoro(ethyl vinyl ether) (PEVE) was added, then the
pressure in the reactor was raised to 250 psig (1.75 MPa) with TFE
(1.72 MPa). For the duration of the reaction, PEVE was added at 2
mL/min and initiation was resumed using the same solution at a rate
of 5 mL/min. The pressure of TFE in the reactor was continuously
adjusted to maintain a reaction rate of 0.167 lb TFE/min (75.7
g/min). After 19 lbs (8618 g) TFE reacted in 114 min, the reaction
was terminated by stopping TFE, initiator, and PEVE feeds, then
venting the reactor. Solids content of the dispersion was 26.3 wt
%, and the raw dispersion particle size (RDPS) was 0.192 .mu.m.
After coagulation, the polymer was isolated by filtering and then
dried in a 150.degree. C. convection air oven. This core/shell
polymer had a melt flow rate (MFR) of 8.6 g/10 min, a PEVE content
of 2.99 wt %, melting points of 301.degree. C. and 324.degree. C.,
and an MIT flex life of 5384 cycles. The copolymer also exhibited a
tensile strength of 3414 psi (23.5 MPa) and elongation at break of
392%. The PTFE core content was 4.9 wt %, and .DELTA..eta. was 2051
Pas. These results are included in Table 5. According to these
results, the core/shell polymer, exhibits a melt viscosity
reduction of almost 20.times. of the PFA by itself and improved
tensile strength and elongation at break.
Examples 21-24
[0070] Examples 21 through 24 were prepared as described in Example
20. The ratio of core polymer to shell polymer was altered by
changing the ratio of the TFE consumed by the first phase of the
polymerization and the TFE consumed by the second phase of the
polymerization. Specific details for Examples 20-24 are given in
Table 5 below. TABLE-US-00005 TABLE 5 PTFE PEVE PEVE Tensile
Example Core, Content, Content in MFR, Strength, Elongation MIT
Flex Life, .DELTA..eta., Number wt % wt % Shell, wt % g/10 min MPa
at Break, % cycles Pa s 20 4.9% 2.99 3.15 8.6 23.9 392 5384 2051 21
9.7% 2.70 3.00 2.3 25.8 392 22884 6772 22 19.5% 2.41 2.99 0.0 27.0
397 48748 15834 23 29.4% 1.86 2.64 0.0 26.6 411 14209 41128 24
39.6% 1.12 1.85 0.0 28.3 396 3752 3500
The polymerizations shown in Table 5 were carried out to a polymer
solids content of 18.4 to 28.5 wt % and average RDPS of 184 to 192
nm. The results in Table 5 show large reductions in melt viscosity
over the entire range of core/shell polymer compositions and
improved tensile strength and elongation also over the entire
range. Also surprising is the increase in MIT Flex Life, ranging
from an increase of at least about 150% to an increase more than
2000%. Preferably the core/shell polymer in which the shell is PFA
exhibits an increase in MIT Flex Life of at least 200% as compared
to the PFA by itself.
[0071] The PTFE/FEP core shell polymer also exhibited favorable MIT
Flex Life as compared to the same FEP by itself (1100 cycles)
through the range of 4 to 40 wt % PTFE core, i.e. the MIT flex life
of the PTFE/FEP core shell polymer was about the same at 10 wt %
PTFE core and above, while at lower PTFE content, a substantial
improvement was been found to exist. For example, at 7 wt % PTFE
core, the MIT Flex Life was 17000 cycles.
Comparative Example D
[0072] This Example shows the preparation of a different typical
PFA by itself for comparison with core/shell polymer.
[0073] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 54
pounds (24.5 kg) of demineralized water, 5.0 g Krytox.RTM. 157 FSL
(available from E.I. du Pont de Nemours and Co., Inc.), and 240 mL
20 wt % solution of ammonium perfluorooctanoate surfactant in
water. With the reactor paddle agitated at 50 rpm, the reactor was
evacuated and purged three times with tetrafluoroethylene (TFE).
Ethane was added to the reactor until the pressure was 8 in Hg
(3.93 psig, 2.71.times.10.sup.-2 MPa), then 200 mL of
perfluoro(propyl vinyl ether) (PPVE) were added. The reactor
temperature then was increased to 75.degree. C. After the
temperature had become steady at 75.degree. C., TFE was added to
the reactor to achieve a final pressure of 250 psig (1.75 MPa). An
aliquot of 400 mL of a freshly prepared aqueous initiator solution
containing 0.2 wt % of ammonium persulfate (APS) was charged to the
reactor. This same initiator solution was pumped into the reactor
at 5 mL/min for the remainder of the batch. After polymerization
had begun, as indicated by a 10 psig (0.07 MPa) drop in reactor
pressure, additional TFE was added to the reactor at a rate of
0.167 lb/min (75.6 g/min) until a total of 20 lb (9080 g) of TFE
were added after kickoff. PPVE was added at 2.0 mL/min for the
duration of the batch, 120 min. At the end of the reaction period,
the TFE, PPVE, and initiator feeds were stopped and the reaction
vessel was vented. When the reactor pressure reached 5 psig (0.035
MPa), the reactor was swept with nitrogen, then the reactor
contents were cooled to 50.degree. C. before the dispersion was
discharged from the reactor. Solids content of the dispersion was
28.9 wt %, and the raw dispersion particle size (RDPS) was 0.130
.mu.m. After coagulation, the polymer was isolated by filtering and
then dried in a 150.degree. C. convection air oven. This TFE/PPVE
copolymer had a melt flow rate (MFR) of 8.2 g/10 min, a PPVE
content of 3.66 wt %, melting points of 232 and 328.degree. C., and
an MIT flex life of 78583 cycles. The tensile strength of the PFA
was 3502 psi (24.5 MPa) and the elongation at break was 292%. The
viscosity change was 2658 Pas.
Example 25
[0074] This Example shows the preparation of core/shell polymer in
which the shell polymer is essentially the same as Comparison
Example D.
[0075] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 54
pounds (24.5 kg) of demineralized water, 5 g Krytox.RTM. 157FSL,
and 240 mL of a 20 wt % solution of ammonium perfluorooctanoate
surfactant in water. With the reactor paddle agitated at 50 rpm,
the reactor was evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 75.degree. C. After the temperature had become steady
at 75.degree. C., the pressure of the reactor was raised to 300
psig (2.1 MPa) using TFE. Four hundred milliliters of an initiating
solution consisting of 0.2 wt % APS in water was injected to the
reactor, then this same initiator was added at 5.0 mL/min. After
polymerization had begun as indicated by a 10 psig (0.07 MPa) drop
in reactor pressure, additional TFE was added at 0.2 lb (90.8
g)/min for 5 min. After 4 lb (1816 g) of TFE was fed after
initiation, the TFE and initiator feeds were stopped, then the
reactor was slowly vented. After stopping agitation, the reactor
vapor space was evacuated. Agitation was resumed at 50 rpm, then
the contents were cooled to 25.degree. C. The agitator was again
stopped, then the pressure in the reactor was raised to 8 in Hg
(3.93 psig, 2.71.times.10.sup.-2 MPa) with ethane. After the
addition of ethane, the agitator was restarted at 50 rpm and the
contents of the reactor were warmed to 75.degree. C. A 200 mL
aliquot of perfluoro(propyl vinyl ether) (PPVE) was added, then the
pressure in the reactor was raised to 250 psig (1.75 MPa) with TFE.
For the duration of the reaction, PPVE was added at 2 mL/min and
initiation was resumed using the same solution at a rate of 5
mL/min. The pressure of TFE in the reactor was continuously
adjusted to maintain a reaction rate of 0.167 lb TFE/min (75.7
g/min). After 16 lbs (8618 g) TFE reacted in 96 min, the reaction
was terminated by stopping TFE, initiator, and PPVE feeds, then
venting the reactor. Solids content of the dispersion was 29.3 wt
%, and the raw dispersion particle size (RDPS) was 0.105 .mu.m.
After coagulation, the polymer was isolated by filtering and then
dried in a 150.degree. C. convection air oven. This core/shell
polymer had no detectable melt flow rate (MFR) (0 g/10 min), a PPVE
content of 3.42 wt %, melting points of 306 and 326.degree. C., and
an MIT flex life of 72502 cycles. The core shell polymer also
exhibited a tensile strength of 4097 psi (28.7 MPa) and elongation
at break of 370%. The PTFE core content was 19.3 wt %, and
.DELTA..eta. was 19568 Pas. These results are included in Table
6.
Examples 26-45
[0076] Examples 26 through 45 were prepared as described in Example
25. The ratio of core polymer to shell polymer was altered by
changing the ratio of TFE consumed by the first and second phases
of the polymerization. Furthermore, the PPVE content in the polymer
was varied systematically by changing both the amount of PPVE
precharged to the reactor, as well as the rate at which it was
added during polymerization. Specific details are given in Table 6
below. TABLE-US-00006 TABLE 6 PPVE PPVE PTFE PPVE PPVE Tensile
Example Precharge, Addition Rate, Core, Content, Content in MFR,
Strength, Elongation MIT Flex Life, .DELTA..eta., Number mL mL/min
wt % wt % Shell, wt % g/10 min MPa at Break, % cycles Pa s 25 200 2
19.3% 3.42 4.24 0 28.7 370 72502 19568 26 200 2 4.8% 3.96 4.16 2.0
25.8 315 309473 6756 27 200 2 9.6% 3.70 4.09 0.6 28.1 366 116091
9468 28 200 2 14.4% 3.75 4.39 0.2 28.1 358 154775 10144 29 200 2
19.3% 3.48 4.32 0.0 28.6 386 92820 8996 30 200 2 24.2% 3.26 4.29
0.0 26.3 340 169801 8393 31 200 2 29.0% 3.30 4.65 0.1 27.3 383
57947 12542 32 200 2 33.9% 3.10 4.69 0.0 26.9 353 91448 7259 33 200
2 38.8% 2.89 4.73 0.0 21.2 419 48162 10561 34 200 2 43.7% 2.93 5.21
0.0 28.8 422 13812 14735 35 200 2 48.7% 2.66 5.18 0.0 29.1 415 7142
17139 36 211 2.1 4.8% 4.59 4.82 2.0 28.4 338 395879 8505 37 222 2.2
9.6% 3.80 4.21 0.7 28.9 351 254853 8815 38 234 2.4 14.4% 4.25 4.96
0.3 28.8 344 478394 9854 39 250 2.5 19.1% 4.74 5.86 0.2 28.7 363
116048 8058 40 267 2.7 23.9% 4.50 5.91 0.0 29.3 363 94574 9956 41
286 2.9 28.6% 4.57 6.40 0.0 30.5 368 130642 9403 42 308 3.1 33.3%
4.99 7.47 0.0 29.9 387 90304 6362 43 333 3.3 38.2% 4.58 7.40 0.0
29.2 395 33165 8244 44 364 3.6 42.6% 5.31 9.26 0.0 27.2 365 21712
10761 45 400 4 48.5% 3.00 5.83 0.0 25.3 354 127306 20049
The polymerizations of Table 6 were carried out to polymer solids
contents of 29.3 to 31.3 wt %, and average RDPS of 95 to 145 nm,
except that the core/shell polymer of example 45 had an average
RDPS of 250 nm which limited the polymerization to a solids content
of 16.9 wt %. For Examples 25 to 35, the amount of PPVE fed to the
polymerization remained constant, with the result being that as the
PTFE core content increased, the PEVE content of the overall
core/shell polymer decreased. For Examples 36 to 45, the PPVE feed
to the polymerization of the shell was increased as PTFE core
content increased, to approximately keep the PPVE content of the
overall core/shell polymer constant. This resulted in an increase
in PPVE content for the shell as the PTFE core content increased.
The results reported in Table 6 show that some of the core/shell
polymers of Examples 25-35 exhibit either better physical
properties or melt viscosity reduction or both than the
corresponding core/shell polymer of Examples 36-45, while for other
core/shell polymers, the corresponding ones in Examples 36-45 are
better. Adjustment of the amount of comonomer with the TFE in the
polymerization of the shell is another way to adjust the melt and
physical properties of the core/shell polymer. The results reported
in Table 6 also show melt viscosity reductions of more than 200%
greater than the melt viscosity reduction obtained for the PFA by
itself over the entire range of PTFE core contents. The tensile
strengths and elongations at break for the core/shell polymer were
also superior for the core/shell polymer.
[0077] Examples 20-45 show core/shell polymer compositions for PFA
shell, wherein the PTFE core content ranges from about 4 to about
50 wt %, exhibiting one or more favorable properties of reduced
melt viscosity with increased shear and high tensile strength and
high elongation at break.
Example 46
[0078] This Example shows the use of core/shell polymer of the
present invention as a concentrate, blended with melt-fabricable
perfluoropolymer by itself.
[0079] A core/shell PFA containing 38.4 wt % PTFE core polymerized
in the manner of Example 33 was coagulated by freezing, rinsed, and
dried overnight at 150.degree. C. One hundred fifty grams of this
powder were dry-blended with 150 g of pellets (about 3.5 mm in
diameter.times.about 3.5 mm in length) of a standard PFA made in
the manner of Comparative Example D. This standard PFA had an MFR
of 13.5 g/10 min and a PPVE content of 4.3 wt %. The resulting
blend was introduced to the mixing bowl of a 350 cm.sup.3-capacity
Haake Rheomix.RTM. batch intensive mixer that had been preheated to
350.degree. C. and was equipped with roller blades. The mixture was
blended at 50 rpm for 5 min to effect complete melting and mixing
of the two components. The resulting blend had a tensile strength
of 2900 psi (20.3 MPa), elongation at break is 316%, melt flow rate
is 0 g/10 min, and MIT Flex Life is 32,562 cycles. The viscosity
reduction of this blend is 26,875 Pas. Comparison of these results
with Example 29 shows that both the tensile strength and elongation
of the blend diminished from blend made solely of the core/shell
polymer, but that the thixotropy (reduction in melt viscosity) was
much greater.
Example 47
[0080] This Example shows the use of core/shell FEP polymer of the
present invention as a concentrate, blended with melt-fabricable
perfluoropolymer by itself.
[0081] A core/shell FEP containing 38.6 wt % PTFE core was
polymerized and finished in the manner of Example 7. Fifty grams of
this powder were dry-blended with 250 g of pellets of a standard
FEP (same dimensions as pellets of Example 46) in the manner of
Comparative Example A. This standard FEP had an MFR of 30 g/10 min,
an HFP content of 10.4 wt %, and a PEVE content of 1.2 wt %. The
resulting blend was introduced to the mixing bowl of a 350
cm.sup.3-capacity Haake Rheomix.RTM. batch intensive mixer equipped
with roller blades that had been preheated to 350.degree. C. The
mixture was blended at 50 rpm for 5 min to effect complete melting
and mixing of the two components. The resulting blend had a tensile
strength of 3087 psi (21.6 MPa), elongation at break is 311%, and
melt flow rate is 14.9 g/10 min, and MIT Flex Life is 2459 cycles.
The viscosity reduction of this blend is 1086 Pas, which is much
greater than for the FEP of Example A by itself, without sacrifice
in physical properties.
Comparative Example E
[0082] This example shows the preparation of a PTFE/FEP composition
of matter by melt blending of PTFE fine powder and FEP powder via
extrusion.
[0083] A dry blend composed of 7 wt % PTFE fine powder and the
remainder a compacted FEP powder polymerized in the manner of
Comparative Example A was prepared by tumbling. The agglomerated
PTFE fine powder had an average particle size of 475 micrometers, a
standard specific gravity (SSG) of 2.175, and an HFP content of
0.45 wt %. The compacted FEP powder had an average aggregate size
(diameter) of approximately 6 mm (pellets or powder), an MFR of 30,
an HFP content of 10.2 wt %, and a PEVE content of 1.2 wt %. The
resulting powder blend was fed at a rate of 25 lbs/hr (11.4 kg/hr)
to a 28 mm twin screw extruder operating at 350.degree. C. and 217
rpm. A general purpose screw configuration was utilized. The molten
output from the 28 mm twin screw extruder was pumped directly to a
11/2'' single screw extruder equipped with a general metering
screw. The single screw extruder operated at 350.degree. C. and
22.1 rpm. The resulting strand was quenched in a trough of cold
water, then cut into about 3 mm (length) pellets. This blend had a
tensile strength of only 1070 psi (7.5 MPa), elongation at break is
126%, and MFR is 3.59 g/10 min. Comparison of these results with
those of Example 11 reveals that the latter exhibits much better
physical properties (tensile strength of 22.6 MPa and elongation of
323%), indicating the importance of the dispersion of
submicrometer-size-size PTFE particles in the continuous phase of
FEP, obtained by starting with submicrometer-size-size core/shell
polymer particles in the melt blending process.
Comparative Example F
[0084] This example shows the preparation of a PTFE/FEP composition
of matter by melt blending of PTFE fine powder and FEP pellets via
extrusion.
[0085] A dry blend composed of 7 wt % PTFE fine powder and the
remainder to total 100 wt % of FEP pellets polymerized in the
manner of Comparative Example A was prepared by tumbling. The
agglomerated PTFE fine powder had an average particle size of 475
micrometers, a standard specific gravity (SSG) of 2.175, and an HFP
content of 0.45 wt %. The oblate spheroid FEP pellets were
approximately 3.5 mm in diameter, and have an MFR of 30.5, an HFP
content of 10.2 wt %, and a PEVE content of 1.2 wt %. The resulting
blend was fed at a rate of 20 lbs/hr (9080 g/hr) to a 28 mm twin
screw extruder operating at 350.degree. C. and 217 rpm. A general
purpose screw configuration was utilized. The molten output from
the 28 mm twin screw extruder was pumped directly to a 11/2'' (3.8
cm) single screw extruder equipped with a general metering screw.
The single screw extruder operated at 350.degree. C. and 22.1 rpm.
The resulting strand was quenched in a trough of cold water, then
cut into about 3 mm pellets. This blend had a tensile strength of
only 1121 psi (7.8 MPa), an elongation at break of 172%, an MFR of
15.1 g/10 min, and an MIT Flex Life of 2924 cycles. The viscosity
change of this blend is 674 Pas. Similar to Comparative Example E,
these properties are greatly inferior to those of Example 11.
Comparative Example G
[0086] This example shows the preparation of a PTFE/PFA composition
of matter by melt blending of PTFE fine powder and PFA pellets via
extrusion.
[0087] A dry blend composed of 20 wt % PTFE fine powder and the
remainder PFA pellets polymerized in the manner of Comparative
Example D was prepared by tumbling. The agglomerated PTFE fine
powder had an average particle size of 475 micrometers, a standard
specific gravity (SSG) of 2.175, and an HFP content of 0.45 wt %.
These standard PFA pellets were oblate spheroids approximately 3.5
mm in diameter, and had an MFR of 13.5 g/10 min and a PPVE content
of 4.3 wt %. The resulting blend was fed at a rate of 25 lbs/hr
(11.4 kg) to a 28 mm twin screw extruder operating at 350.degree.
C. and 210 rpm. A general purpose screw configuration was utilized.
The molten output from the 28 mm twin screw extruder was pumped
directly to a 11/2'' (3.8 cm) single screw extruder equipped with a
general metering screw. The single screw extruder operated at
350.degree. C. and 21.5 rpm. The resulting strand was quenched in a
trough of cold water, then cut into about 3 mm pellets. This blend
has a tensile strength of 1121 psi (7.8 MPa), an elongation at
break of 172%, and an MFR of 4.1 g/10 min. These properties are
greatly inferior to those of the core/shell compositional
counterpart, Example 25, which exhibited tensile strength of 28.7
MPa and elongation of 370%.
Example 48
[0088] This Example describes the blending of FEP aqueous
dispersion and ultra-high molecular weight PTFE aqueous dispersion.
Polymer C (non-melt flowable PTFE). The FEP is that which was
prepared according to Comparative Example A. The PTFE aqueous
dispersion was prepared as described below:
[0089] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with
19.5 kg of demineralized water, 600 grams of paraffin wax, 60 ml of
a 20 wt % solution of ammonium perfluorooctanoate dispersant (C-8),
10 ml of a 2 wt % oxalic acid solution, and 1 gram succinic acid.
With the reactor paddle agitated at 46 rpm, the reactor was heated
to 65.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 80.degree. C. After the temperature was steady at
80.degree. C., TFE was added slowly to the reactor until the
pressure was 2.75 MPa. Then 245 mL of freshly prepared aqueous
initiator solution containing 0.015% KMnO4 and 0.007% ammonium
phosphate in water were added to the reactor at the rate of 80
ml/min. Then, this same initiator solution was pumped into the
reactor at 5 mL/min. TFE was added at a rate sufficient to maintain
the pressure at 2.75 MPa. After 7.0 kg of TFE was added following
initial pressurizing with TFE, initiator solution addition was
stopped. The polymerization time to the stopping of initiator
addition was 57 min. After a total of 12.6 kg of TFE had been added
after initial pressureup, the TFE and the C-8 solution feeds were
stopped and the polykettle was vented. The length of the reaction,
measured from the start of the first initiator injection to the
termination of TFE feed, was 183 min. The contents were discharged
from the polykettle and the supernatant wax was removed. Solids
content of the raw dispersion was 39.3 wt % and RDPS was 289 nm. A
portion of the dispersion was diluted to 11 wt % solids and
coagulated in the presence of ammonium carbonate under vigorous
agitation conditions. The coagulated dispersion (fine powder) was
separated from the liquid and dried at 150.degree. C. for three
days. The PTFE resin had SSG of 2.159 and a melt creep viscosity of
at least 1.times.10.sup.11 Pas at 380.degree. C., which is the
upper limit of measurement.
[0090] The aqueous dispersions of Comparative Example A and the
PTFE prepared in the preceding paragraph were mixed together by
rolling at a rate of 5-10 rpm for 10 minutes, resulting in a blend
that was 7 wt % PTFE and 93 wt % FEP on a dry solids basis. The
polymer blend was isolated by freezing, filtering, and then drying
in a 150.degree. C. convection air oven. This polymer was
stabilized by heating at 260.degree. C. for 1.5 hr in humid air
containing 13 mol % water. The resulting blend had a melt flow rate
of 0 g/10 min and viscosity change (reduction), .DELTA..eta., of
6434 Pas. The blend also exhibited a tensile strength and
elongation at break of 2676 psi (18.45 MPa) and 219%,
respectively.
Example 49
[0091] This Example describes the blending of FEP aqueous
dispersion of Comparative Example A (FEP) and small particle high
molecular weight (non-melt flowable) PTFE made as described in the
following paragraph.
[0092] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water and 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water. With
the reactor paddle agitated at 46 rpm, the reactor was heated to
60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 103.degree. C. After the temperature had become steady
at 103.degree. C., the pressure of the reactor was raised to 250
psig (1.75 MPa) using TFE. Fifty milliliters of an initiating
solution consisting of 1.04 wt % APS and 0.94 wt % KPS in water was
injected to the reactor, then this same initiator was added at 0.5
mL/min. After polymerization had begun as indicated by a 10 psig
(0.07 MPa) drop in reactor pressure, additional TFE was added at
0.2 lb (90.8 g)/min for 7.5 min. After 1.5 lbs (681 g) of TFE fed
after initiation, the TFE and initiator feeds were stopped and the
reactor was vented. The contents of the reactor were cooled to
80.degree. C. before being discharged. Solids content of the
dispersion was 4.81 wt % and raw dispersion particle size (RDPS)
was 0.138 .mu.m. A portion of the dispersion was coagulated, then
after coagulation, the polymer was isolated by filtering and then
dried in a 150.degree. C. convection air oven. The standard
specific gravity (SSG) of the resulting PTFE homopolymer was 2.217.
The results demonstrate that the core is non-melt flowable PTFE
because it has a measurable SSG. The PTFE also exhibited a melt
creep viscosity greater than 108 Pas at 380.degree. C.
[0093] Aqueous dispersions of the FEP of Comparative Example A and
the PTFE prepared in the preceding paragraph were mixed by rolling
at a rate of 5-10 rpm for 10 minutes, resulting in a blend that was
7 wt % PTFE and 93 wt % FEP on a dry solids basis. The polymer
blend was isolated by freezing, filtering, and then drying in a
150.degree. C. convection air oven. This polymer was stabilized by
heating at 260.degree. C. for 1.5 hr in humid air containing 13 mol
% water. The resulting blend had a melt flow rate of 14.8 g/10 min
and exhibited a viscosity change (reduction), .DELTA..eta., of 756
Pas. The blend exhibited a tensile strength and elongation at break
of 2997 psi (20.66 MPa) and 306%, respectively.
Example 50
[0094] This Example describes the blending of PFA dispersions of
Comparative Example D and high molecular weight PTFE, which
contains HFP modifier, as prepared in the following paragraph. A
cylindrical, horizontal, water-jacketed, paddle-stirred, stainless
steel reactor having a length to diameter ratio of about 1.5 and a
water capacity of 10 gallons (37.9 L) was charged with 19.5 kg of
demineralized water, 709 grams of paraffin wax, 3.13 mL of a 20 wt
% solution of ammonium perfluorooctanoate dispersant (C-8), 98 mL
of a 0.1 wt % solution of FeCl.sub.2.4H.sub.2O in water, and 101 mL
of a 0.1 wt % solution of CuCl.sub.2.2H.sub.2O in water. With the
reactor paddle agitated at 46 rpm, the reactor was heated to
65.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 91.degree. C. After the temperature was steady at
91.degree. C., 520 mL of a 2 wt % solution of disuccinyl peroxide
in water was injected as rapidly as possible. Then, 29.5 mL of
hexafluoropropylene (HFP) was injected before the pressure of the
reactor was raised to 350 psig using TFE. After polymerization had
begun, as indicated by a 15 psig (0.07 MPa) drop in reactor
pressure, additional TFE was added to the reactor at a rate of 0.3
lb/min (0.14 kg/min) and the temperature of the reactor and its
contents were increased to 93.degree. C. After 1.5 lbs (0.68 kg) of
TFE had reacted, a 2 wt % aqueous solution of ammonium
perfluorooctanoate was added at a rate of 40 mL/min for the
remainder of the reaction. After 29.6 lbs (13.4 kg) of TFE had been
consumed, the reaction was terminated by stopping the TFE and the
C-8 solution feeds, and the contents of the reactor were allowed to
continue to react until the reactor pressure reached 175 psig (1.20
MPa). The length of the reaction, measured as time during which TFE
was being fed, was approximately 31 minutes. The contents were
discharged from the polykettle and the supernatant wax was removed.
Solids content of the raw dispersion was 42.3 wt % and RDPS was 205
nm. A portion of the dispersion was diluted to 11 wt % solids and
coagulated in the presence of ammonium carbonate under vigorous
agitation conditions. The coagulated dispersion (fine powder) was
separated from the liquid and dried at 150.degree. C. for three
days. The PTFE resin has SSG of 2.186, melt creep viscosity of
4.times.10.sup.9 Pas at 380.degree. C., and HFP content of 0.45 wt
%.
[0095] Aqueous dispersions of the PFA of Comparative Example D and
the PTFE prepared according to the preceding paragraph were mixed
together by rolling at a rate of 5-10 rpm for 10 minutes, resulting
in a blend that was 20 wt % PFA and 80 wt % of the PTFE prepared
according to the preceding paragraph, on a dry solids basis. The
polymer blend was isolated by freezing, filtering, and then drying
in a 150.degree. C. convection air oven. The resulting blend had a
melt flow rate of 1.0 g/10 min and exhibited a viscosity change
(reduction), .DELTA..eta., of 14805 Pas. The blend exhibited a
tensile strength and elongation at break of 3242 psi (22.35 MPa)
and 311%, respectively.
Example 51
[0096] This Example describes the blending of PFA dispersions of
Comparative Example D and as-polymerized very small particle PTFE
(non-melt flowable PTFE) prepared as described in the following
paragraph.
[0097] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water, 330 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water, and
5.0 g Krytox.RTM. 157 FSL, available from E.I. du Pont de Nemours
and Company, Inc. Krytox.RTM. 157 FSL is a perfluoropolyether
carboxylic acid as further described in Table 1 of U.S. Pat. No.
6,429,258. With the reactor paddle agitated at 46 rpm, the reactor
was heated to 60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 103.degree. C. After the temperature had become steady
at 103.degree. C., the pressure of the reactor was raised to 250
psig (1.75 MPa) using TFE. Fifty milliliters of an initiating
solution consisting of 1.04 wt % APS and 0.94 wt % KPS in water was
injected to the reactor, then this same initiator was added at 0.5
mL/min. After polymerization had begun as indicated by a 10 psig
(0.07 MPa) drop in reactor pressure, additional TFE was added at
0.2 lb (90.8 g)/min for 7.5 min. After 1.5 lbs (681 g) of TFE fed
after initiation, the TFE and initiator feeds were stopped and the
reactor was vented. The contents of the reactor were cooled to
80.degree. C. before being discharged. Solids content of the
dispersion was 4.8 wt % and raw dispersion particle size (RDPS) was
0.008 .mu.m. A portion of the polymer was coagulated, and after
coagulation the polymer was isolated by filtering and then dried in
a 150.degree. C. convection air oven. The standard specific gravity
(SSG) of the resulting PTFE homopolymer was 2.121. The PTFE also
exhibited a melt creep viscosity greater than 108 Pas at
380.degree. C.
[0098] Aqueous dispersions of the PFA of Comparative Example D and
the PTFE prepared according to the preceding paragraph were mixed
together by rolling at a rate of 5-10 rpm for 10 minutes, resulting
in a blend that was 20 wt % of the PTFE prepared according to the
preceding paragraph and 80 wt % PFA on a dry solids basis. The
polymer blend was isolated by freezing, filtering, and then drying
in a 150.degree. C. convection air oven. The resulting blend has a
melt flow rate of 2.0 g/10 min and exhibited a viscosity change
(reduction), .DELTA..eta., of 4797 Pas. The blend exhibited a
tensile strength and elongation at break of 3693 psi (25.46 MPa)
and 419%, respectively.
Example 52
[0099] This Example shows the preparation of core/shell polymer
containing a very small amount of non-melt flowable PTFE.
[0100] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 50
pounds (22.7 kg) of demineralized water, 5.0 g Krytox.RTM. 157 FSL
(available from E.I. du Pont de Nemours and Company, Inc.), and 330
mL of a 20 wt % solution of ammonium perfluorooctanoate surfactant
in water. With the reactor paddle agitated at 46 rpm, the reactor
was heated to 60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor temperature then was
increased to 103.degree. C. After the temperature had become steady
at 103.degree. C., the pressure of the reactor was raised to 250
psig (1.75 MPa) using TFE. Fifty milliliters of an initiating
solution consisting of 1.04 wt % APS and 0.94 wt % KPS in water was
injected to the reactor, then this same initiator was added at 0.5
mL/min. After polymerization had begun as indicated by a 10 psig
(70 kPa) drop in reactor pressure, additional TFE was added at 0.2
lb (90.7 g)/min for approximately 1 min. After 0.21 lbs (95.3 g) of
TFE had been fed after initiation, the TFE feed was stopped, then
the reactor contents were agitated for 10 minutes with the
initiator still being fed. The agitator and initiator pumps were
stopped, then the pressure of the polykettle was increased to 444
psig with HFP. The agitator was restarted and initiation was
resumed using the same solution at a rate of 10 mL/min. The reactor
pressure was raised to 650 psi (4.5 MPa) with TFE. An aliquot of 92
mL of PEVE was added to the reactor, then 1 mL/min PEVE and 0.2 lb
(90.87 g)/min TFE were added over the remainder of the reaction.
After an additional 20.79 lb (9430 g) of TFE were reacted, the TFE,
initiator, and PEVE feeds were stopped, and the reactor was cooled
while maintaining agitation. When the temperature of the reactor
contents reached 90.degree. C., the reactor was slowly vented.
After venting to nearly atmospheric pressure, the reactor was
purged with nitrogen to remove residual monomer. Upon further
cooling, the dispersion was discharged from the reactor at below
70.degree. C. After coagulation, the polymer was isolated by
filtering and then drying in a 150.degree. C. convection air oven.
This polymer was stabilized by heating at 260.degree. C. for 1.5 hr
in humid air containing 13 mol % water. Solids content of the
dispersion of the resultant TFE/HFP/PEVE copolymer was 39.1 wt %
and raw dispersion particle size (RDPS) was 0.113 .mu.m. The PTFE
core content of this core/shell polymer was 0.87 wt %, the HFP
content was 12.2 wt % in the shell, and the PEVE content was 1.06
wt % in the shell, the remainder to total 100% by weight being TFE.
This material had a tensile strength of 3650 psi (25.2 MPa), an
elongation at break of 370%, a melt flow rate of 14.8 g/10 min, and
a viscosity reduction, .DELTA..eta., of 713 Pas, which is much
greater than for the FEP of Comparative Example A. The viscosity
change at about 0.1 wt % PTFE core is estimated to be about 2 times
greater than for the FEP by itself.
Examples 53-60
[0101] These Examples show the preparation of core/shell polymer in
which the core is non-melt flowable PTFE and the shell is the melt
processable PFA of Comparative Example C, wherein the core content
extends to high amounts.
[0102] A cylindrical, horizontal, water-jacketed, paddle-stirred,
stainless steel reactor having a length to diameter ratio of about
1.5 and a water capacity of 10 gallons (37.9 L) was charged with 54
pounds (24.5 kg) of demineralized water and 240 mL of a 20 wt %
solution of ammonium perfluorooctanoate surfactant in water. With
the reactor paddle agitated at 50 rpm, the reactor was evacuated
and purged three times with tetrafluoroethylene (TFE). The reactor
temperature then was increased to 75.degree. C. After the
temperature had become steady at 75.degree. C., the pressure of the
reactor was raised to 300 psig (2.07 MPa) using TFE. Eighty
milliliters of an initiating solution consisting of 0.2 wt % APS in
water were injected into the reactor. This same initiator was then
added at 5.0 mL/min. After polymerization had begun as indicated by
a 10 psig (0.07 MPa) drop in reactor pressure, TFE was added at 0.2
lb (90.8 g)/min for 5 min. After 1 lb (454 g) of TFE was fed after
initiation, the TFE and initiator feeds were stopped, then the
reactor was slowly vented. After stopping agitation, the reactor
vapor space was evacuated. Agitation was resumed at 50 rpm, then
the contents were cooled to 25.degree. C. The core has an MFR of
zero at 372.degree. C. The agitator was again stopped, then the
pressure in the reactor was raised to 8 in Hg (3.93 psig,
2.71.times.10.sup.-2 MPa) with ethane. After the addition of
ethane, the agitator was restarted at 50 rpm and the contents of
the reactor were warmed to 75.degree. C. A 200 mL aliquot of
perfluoro(ethyl vinyl ether) (PEVE) was added, then the pressure in
the reactor was raised to 250 psig (1.75 MPa) with TFE (1.72 MPa).
For the duration of the reaction, PEVE was added at 2 mL/min and
initiation was resumed using the same solution at a rate of 5
mL/min. The pressure of TFE in the reactor was continuously
adjusted to maintain a reaction rate of 0.167 lb TFE/min (75.7
g/min). After 19 lbs (8618 g) TFE reacted in 114 min, the reaction
was terminated by stopping TFE, initiator, and PEVE feeds, then
venting the reactor. Solids content of the dispersion was 30.2 wt
%, and the raw dispersion particle size (RDPS) was 0.099 .mu.m.
After coagulation, the polymer was isolated by filtering and then
dried in a 150.degree. C. convection air oven. This core/shell
polymer had a melt flow rate (MFR) of 4.1 g/10 min, a PEVE content
of 2.93 wt %, and a 50 mil MIT flex life of 851 cycles. The
core/shell polymer also exhibited a tensile strength of 4075 psi
(28.1 MPa) and elongation at break of 353%. The viscosity change
was 7780 Pas. These results are included as Example 53 in Table 7
below along with results for additional core shell/polymers with
increasing core content, wherein the core is PTFE homopolymer. The
ratio of core polymer to shell polymer was altered by changing the
ratio of the TFE consumed by the first phase of the polymerization
and the TFE consumed by the second phase of the polymerization.
Tensile strength and elongation at break were measured both on
compression molded plaques by the procedure disclosed under Test
Procedures and on strands extruded by the Kayeness capillary
rheometer described under Test Procedures operating at 350.degree.
C. and 4 s.sup.-1. TABLE-US-00007 TABLE 7 PTFE PEVE PEVE Tensile
Elongation 1.27 mm Tensile Elongation Comparative Core, Content,
Content in MFR, Strength, at Break, % MIT Flex Life, .DELTA..eta.,
Strength, at Break, % Ex. No. wt % wt % Shell, wt % g/10 min MPa
plaque plaque cycles Pa s MPa strand strand 53 4.85 2.93 3.08 4.12
28.1 353 851 7780 29.2 519 54 9.71 2.85 3.16 3.34 28.3 353 1016
8667 27.2 487 55 19.51 2.45 3.04 0 28.4 372 1796 9533 30.1 334 56
29.4 2 2.83 0 28.7 384 2560 5180 16.7 72 57 49.33 1.35 2.66 0 22
275 2070 19283 31.3 559 58 59.27 1.21 2.98 0 20.4 298 460 (1) (1)
(1) 59 74.42 0.77 3 0 16.6 219 233 16500 18.5 153 60 89.76 0.28
2.61 0 15.5 96 116 17237 8.7 46 (1) not measured
To shorten the time for MIT Flex Life testing the testing reported
in Table 3 was done on 1.27 mm thick compression molded films,
which gives a much smaller MIT Flex Life, which would be much
larger if the testing were done on 0.21 mm thick films. As shown in
Table 7, as the core content for the PTFE homopolymer core increase
to about 75 wt %, the tensile properties are still reasonably
significant to have value. For example at about 75 wt % core
content, elongation is still above 200%. The tensile properties of
the melt extruded strand are also reasonably high to have
value.
Example 61
[0103] This Example is directed to improved injection molding and
resulting from the use of the melt-mixed composition of the present
invention, wherein the composition is obtained by melt mixing
core/shell polymer.
[0104] PFA 1 used in this Example is a copolymer of
tetrafluoroethylene (TFE) and perfluoro(propyl vinyl ether) (PPVE),
with PPVE content of 4.1 wt %, and MFR of 29.2 g/10 min.
[0105] PFA 2 used in this Example is the same copolymer as PFA 1
except that the PPVE content of 4.2 wt %, and the MFR is 12.6 g/10
min.
[0106] State 1 is a core/shell polymer having 4.78 wt %
polytetrafluoroethylene core and a PFA shell of composition like
that of PFA 1. State 1 MFR is 8 g/10 min.
[0107] State 2 is a core/shell polymer having 4.81 wt %
polytetrafluoroethylene (PTFE) core and a PFA shell of composition
like that of PFA 2. State 2 has an MFR of 4 g/10 min.
[0108] All of these polymers were in the form of extruded/cut
pellets. In the pellets of the state 1 and state 2 polymers, the
core was present as dispersed submicrometer-size particles in a
matrix of the PFA from the shell made by melt mixing the core/shell
polymer in an extruder.
Test Procedures for this Example
[0109] Flex Life--The procedure for measuring MIT Flex Life is in
accordance with ASTM D 2176, and the MIT Flex Life values reported
in this Example were measured on a 50 mil (1.27 mm) thick film
compression molded in the same way as disclosed under Test
Procedures at the beginning of the Examples for the compression
molding of 60 mil (1.5 mm) thick plaque, except that the thickness
of the chase to mold the 1.27 mm thick film was 50 mils (1.27 mm)
thick. Use of the thicker film (thicker than the 0.21 mm thick film
in the preceding Examples) shortens the time required in the flex
test, thereby resulting in much smaller MIT flex life numbers
(cycles). The MIT Flex Life determined on the 1.27 mm thick film
can be described as the 1.27 mm MIT Flex Life.
[0110] Injection Moldability--The "snake flow" test measures the
flowability of polymer at shear rates typical of those used in
injection molding. A molten polymer sample was injected into a mold
having a rectangular channel 12.7 mm by 2.54 mm, the channel being
serpentine in shape. The distance that injected polymer travels in
the channel is an index of polymer melt flowability. For
convenience, the weight of the polymer in the channel ("Shot
Weight") is reported.
[0111] The equipment used was a Nissei Injection Molding Machine,
Model FN4000. Temperature profile: rear 350.degree. C., Center
350.degree. C.; Front 355.degree. C.; Nozzle 360.degree. C.; mold
temperature 180.degree. C.; injection pressures 80 MPa and 120
MPa.
Injection Molding
[0112] The core/shell polymers, State 1 and State 2 are compared
with PFA 1 and PFA 2 at injection pressure of 80 MPa. State 2 and
PFA 2 are also compared at 120 MPa. Tables 8 and 9 summarize the
results for injection pressures of 80 MPa and 120 MPa respectively.
TABLE-US-00008 TABLE 8 Injection Pressure 80 MPa Polymer PFA 1
State 1 PFA 2 State 2 MFR 29.2 8 12.6 4 Shot Wt. (g) 9.6 9.0 6.0
4.5
[0113] TABLE-US-00009 TABLE 9 Injection Pressure 120 MPa Polymer
PFA 2 State 2 MFR 12.6 4 Shot Wt. (g) 13.1 12.7
[0114] The results show that the melt-mixed compositions of the
invention have melt flowability under injection molding conditions
much higher than would be predicted from their low MFR, thus
exhibiting the shear thinning (thixotropic) behavior of core/shell
polymer of the present invention. One aspect of the importance of
being able to obtain high shot weight with low MFR core/shell
polymer is shown by the flex life test results.
[0115] The 1.27 mm MIT Flex Life was measured on State 1 and State
2 core/shell polymers of the invention and on PFA 1 and PFA 2.
Results are summarized in Table 10. TABLE-US-00010 TABLE 10 Polymer
PFA 1 State 1 PFA 2 State 2 MFR 29.2 8 12.6 4 Flex Life (cycles)
139 362 418.sup.(1) 880 .sup.(1)The MIT flex life for 8 mil (0.21
mm) thick film of PFA 2 is about 15,000 cycles.
[0116] The flex life test results show that States 1 and 2
melt-mixed compositions, while behaving like polymers of much
higher melt flow rate in melt processing, also behave like polymers
of low melt flow rate by exhibiting high MIT Flex Life.
* * * * *