U.S. patent application number 11/601365 was filed with the patent office on 2007-05-24 for fluoropolymer blending process.
Invention is credited to Ralph Munson Aten, Heidi Elizabeth Burch, Sundar Kilnagar Venkataraman.
Application Number | 20070117930 11/601365 |
Document ID | / |
Family ID | 38001946 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070117930 |
Kind Code |
A1 |
Venkataraman; Sundar Kilnagar ;
et al. |
May 24, 2007 |
Fluoropolymer blending process
Abstract
Aqueous dispersions on non-melt flowable PTFE and
melt-fabricable perfluoropolymer are mixed together, followed by
recovery of the resultant mixture of the dispersed particles of
each of these polymers, and melt mixing of the resultant mixture of
particles to obtain a blend in which the PTFE particles form the
disperse phase and the perfluoropolymer forms the continuous phase,
the melt mixing producing advantageous melt viscosities and the
blend exhibiting advantageous physical properties even at high PTFE
contents of about 30 wt %.
Inventors: |
Venkataraman; Sundar Kilnagar;
(Avondale, PA) ; Aten; Ralph Munson; (Chadds Ford,
PA) ; Burch; Heidi Elizabeth; (Parkersburg,
WV) |
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: |
38001946 |
Appl. No.: |
11/601365 |
Filed: |
November 16, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60737950 |
Nov 18, 2005 |
|
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|
Current U.S.
Class: |
525/199 ;
264/211.21 |
Current CPC
Class: |
B29K 2027/18 20130101;
B29C 45/0001 20130101; B29C 48/022 20190201; C08L 27/18 20130101;
C08L 2205/02 20130101; C08J 3/201 20130101; B29C 48/06 20190201;
C08J 2327/18 20130101; C08L 27/18 20130101; C08L 2666/04
20130101 |
Class at
Publication: |
525/199 ;
264/211.21 |
International
Class: |
C08L 27/12 20060101
C08L027/12 |
Claims
1. Process for melt fabricating perfluoropolymer, comprising
forming a mixture of submicrometer-size particles of
non-melt-flowable polytetrafluoroethylene and melt-fabricable
perfluoropolymer, melt blending said mixture to form a dispersion
of said particles of non-melt flowable polytetrafluoroethylene in a
continuous phase of said melt-fabricable perfluoropolymer, and
molding the resultant melt blend into an article.
2. The process of claim 1 wherein said melt blending and molding is
done by extrusion or injection molding.
3. The process of claim 1 wherein said article is pellets for
subsequent molding into a final article.
4. The process of claim 1 wherein said article is a final
article.
5. The process of claim 1 wherein said non-melt flowable
polytetrafluoroethylene constitutes at least about 0.1 wt % of the
combined weight of said non-melt-flowable polytetrafluoroethylene
and said melt-fabricable perfluoropolymer.
6. The process of claim 1 wherein said non-melt flowable
polytetrafluoroethylene constitutes at about 4 to 40 wt % of the
combined weight of said non-melt-flowable polytetrafluoroethylene
and said melt-fabricable perfluoropolymer.
7. The process of claim 1 wherein said mixture of
submicrometer-size particles of non-melt-flowable
polytetrafluoroethylene and melt-fabricable perfluoropolymer is
formed by mixing together an aqueous dispersion of
submicrometer-size particles comprising non-melt flowable
polytetrafluoroethylene and an aqueous dispersion of
submicrometer-size particles comprising melt-fabricable
perfluoropolymer and separating the resultant mixture of said
submicrometer-size particles from the resultant mixture of said
aqueous dispersions.
8. The process of claim 1 wherein said submicrometer-size particles
of non-melt flowable polytetrafluoroethylene are core/shell
particles, said polytetrafluoroethylene being present in said core
and at least a portion of said perfluoropolymer being present in
said shell.
9. The process of claim 1 wherein all of said melt-fabricable
perfluoropolymer in said melt mixture comes from said
submicrometer-size particles thereof.
10. The process of claim 7 wherein said separating is done by
co-coagulating said mixed-together aqueous dispersions and drying
the resultant mixture of submicrometer-size particles.
11. The process of claim 1 wherein said melt blending is carried
out in a single screw extruder.
12. The process of claim 1 wherein the mixture of
submicrometer-size particles of non-melt-flowable
polytetrafluoroethylene and melt-fabricable perfluoropolymer is
characterized by a reduction in melt viscosity upon increasing
shear rate from about 10 s.sup.-1 to about 100 s.sup.-1 that is at
least 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.
13. The process of claim 12 wherein said reduction in melt
viscosity is obtained when said particles of non-melt flowable
polytetrafluoroethylene constitute at least 4 wt % of the combined
weight of said particles of said non-melt-flowable
polytetrafluoroethylene and said melt-fabricable
perfluoropolymer.
14. The process of claim 1 wherein the mixture of
submicrometer-size particles of non-melt-flowable
polytetrafluoroethylene and melt-fabricable perfluoropolymer is
characterized by an elongation of at least 200% when said particles
of non-melt flowable polytetrafluoroethylene constitute at least 4
wt % of the combined weight of said particles of non-melt-flowable
polytetrafluoroethylene and said melt-fabricable
perfluoropolymer.
15. The process of claim 1 wherein the tensile strength of said
mixture is as least as great as that of said melt-fabricable
perfluoropolymer by itself.
16. Process for melt fabricating perfluoropolymer, comprising
forming a mixture of submicrometer-size particles of
non-melt-flowable polytetrafluoroethylene and melt-fabricable
perfluoropolymer, melt blending said mixture, and molding the
resultant melt blend into an article, said mixture of
submicrometer-size particles of non-melt-flowable
polytetrafluoroethylene and melt-fabricable perfluoropolymer being
characterized by either elongation at break of at least 75% and/or
viscosity reduction of at least 10% when shear is increased from
about 10 s.sup.-1 to 100 s.sup.-1, both with respect to the
elongation and viscosity reduction of the perfluoropolymer by
itself, at a composition at which said polytetrafluoroethylene
constitutes at least 4 wt % of the combined weight of said
polytetrafluoroethylene and said perfluoropolymer.
17. Molded article obtained by the process of claim 16.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to the blending together of
polytetrafluoroethylene and melt-fabricable perfluoropolymer.
[0003] 2. Description of Related Art
[0004] US 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 FEP composition by itself is
selected to provide good crack resistance for molded articles, and
the PTFE has a high enough molecular weight so that the molded
article does not have poor crack resistance. 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]. The FEP and PTFE are
blended together by melt kneading. Before kneading, the copolymer
and PTFE can be pre-mixed to improve the degree of dispersion of
the PTFE [0042]. In Example 1, 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-extruded, using a
single screw extruder, as a coating onto wire. The PTFE powder has
an average particle size of 450 micrometers. The particle size of
the copolymer is not disclosed in Example 1, but the aqueous
emulsion polymerization to obtain this copolymer is disclosed. The
copolymer is recovered from emulsion polymerization by coagulation,
which provides a dry powder particles having an average size about
the same as the PTFE powder.
SUMMARY OF THE INVENTION
[0005] It has been discovered that much greater proportions of PTFE
can be blended not only with FEP but with melt-fabricable
perfluoropolymers in general without the resultant blend losing its
melt fabricability and without causing articles molded from the
resultant melt blend to be brittle. The only limitation on the
greater proportion of PTFE that can be incorporated into the blend
is that the PTFE is the disperse phase in the melt blend and the
melt-fabricable perfluoropolymer is the continuous phase.
[0006] The present invention can be described as the process for
melt fabricating perfluoropolymer, comprising forming a mixture of
submicrometer-size particles of both non-melt-flowable
polytetrafluoroethylene (PTFE) and melt-fabricable
perfluoropolymer, melt blending said mixture to form a dispersion
of said particles of non-melt flowable PTFE in a continuous phase
of said melt-fabricable perfluoropolymer, and molding the resultant
melt blend into an article. Preferably, the non-melt flowable PTFE
constitutes at least about 0.1 wt %, preferably at least about 0.5
wt %, based on the combined weight of the non-melt flowable PTFE
and melt-fabricable perfluoropolymer to obtain appreciable
advantage from the PTFE component. Good physical properties can
exist when the non-melt flowable PTFE constitutes up to about 75 wt
% based on the aforesaid combined weight. This composition and
others disclosed herein for use in the present invention apply both
to the mixture of polymer particles and to the melt blend, wherein
the melt fabricable perfluoropolymer is the continuous phase and
therefore is no longer in particulate form. The composition used to
form the mixture is considered to be the same as the composition of
the melt blend and thus of the article molded therefrom.
[0007] In a preferred embodiment, the forming of the mixture of
submicrometer-size particles is done by mixing together an aqueous
dispersion of submicrometer-size particles comprising the PTFE and
an aqueous dispersion of submicrometer-size particles comprising
the melt-fabricable perfluoropolymer and separating the resultant
mixture of said submicron particles from the resultant mixture of
said aqueous dispersions, i.e. from the aqueous media of the
combined dispersions. The separation step is conveniently carried
out by co-coagulating the mixed-together aqueous dispersions and
drying the resultant mixture of submicrometer-size particles. The
mixture of submicrometer-size particles remains after the
separation step. If co-coagulation of the mixed dispersions is used
as the separation step, the resultant agglomerates contain
particles of both polymers, i.e. particles of both polymers are
agglomerated together. Upon drying, the agglomerates become a
powder, which is typically referred to as secondary particles and
which upon melt mixing, disperses the primary particles of the
non-melt flowable PTFE into the continuous phase of the
melt-fabricable perfluoropolymer formed during the melt mixing. The
450 micrometer average particle size for the PTFE of Example 1 of
US 2004/0242783 A1 is a typical secondary particle size. In the
melt mixing, the PTFE particles retain their particulate identity,
while the perfluoropolymer particles melt and flow together to lose
their particulate identity, to form the continuous phase of the
melt blend. 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.
[0008] The melt fabrication process of the present invention
preferably starts with the mixing together of the primary
dispersion-polymerized particles of the two polymers. In contrast,
US 2004/0242783 A1, practices the mixing together of secondary
particles of each polymer. The kneading in a twin screw extruder as
required in '783 is not required in the present invention. Melt
blending can be carried out in a single screw extruder that would
be used for extrusion or for polymer melting in injection molding.
The significance of the difference between these procedures is the
ability to incorporate a greater proportion of the PTFE into the
melt blend with the perfluoropolymer to obtain surprisingly
advantageous results. This is true even for PTFE contents of at
least about 4 wt % based on combined weight non-melt flowable PTFE
and melt-fabricable perfluoropolymer.
[0009] With respect to melt properties, the melt blend produced in
accordance with the present invention is thixotropic, i.e. the melt
viscosity of the blend decreases (becomes more fluid) with
increasing shear. In this regard, the mixture of submicrometer-size
particles of non-melt-flowable PTFE and melt-fabricable
perfluoropolymer is preferably characterized by a reduction in melt
viscosity upon increasing shear rate 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 described hereinafter.
[0010] With respect to physical properties, the absence of
brittleness in articles melt fabricated in accordance with the
present invention is indicated preferably by the mixture of
submicrometer-size particles of non-melt-flowable PTFE and
melt-fabricable perfluoropolymer being characterized by an
elongation at break of at least about 200%, preferably at least
250%, as determined by tensile testing in accordance with ASTM D
638-03 as further described hereinafter. More preferably the
elongation at break is at least 75% of that of the melt-fabricable
perfluoropolymer by itself, more preferably at least 85% thereof.
As determined by the same ASTM test, the tensile strength of the
mixture is preferably at least about 75% of that of the
melt-fabricable perfluoropolymer by itself, more preferably at
least about 85% thereof. As shown in the Examples, mixtures
containing much greater amounts of the PTFE component than 2 parts
by weight of PTFE/100 parts of the FEP exhibit elongation at break
and/or tensile strength that is at least as high as that for the
perfluoropolymer composition by itself.
[0011] Contrary to the expectation from US 2004/0242783 A1, these
thixotropy and elongation attributes exist for compositions
containing at least about 4 wt % of the PTFE component, based on
combined weight as described above, as well as for lesser amounts,
e.g. as little as 0.5 wt % PTFE. The maximum amount of PTFE in the
composition at which these attributes will exist will depend on the
particular melt-fabricable perfluoropolymer, and will extend up to
at least about 15 wt % of the PTFE component, more preferably up to
about at least about 25 wt %, and most preferably, at up to least
about 30 wt % of the PTFE component, based on combined weight.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The present invention starts with creating a mixture of the
two components: submicrometer-size particles of non-melt flowable
PTFE and submicrometer-size particles of melt-fabricable
perfluoropolymer.
[0013] With respect to the non-melt flowable PTFE component, the
non-melt flowability aspect 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 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 Pa.circle-solid.s, more preferably at least about
1.times.10.sup.7 Pa.circle-solid.s, and most preferably at least
about 1.times.10.sup.8 Pa.circle-solid.s, all at 380.degree. C.
This temperature is well above the first and second melt
temperatures of PTFE of 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
the rate in g/10 min at which perfluoropolymer flows through a
defined orifice under a specified load at a specified temperature,
usually 372.degree. C. Since the PTFE used in the present invention
has no melt flow in general and when subjected to the procedure of
ASTM D 1238-94a, has a zero MFR, the melt characteristic of the
PTFE is not determined by this ASTM procedure. 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
together 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 D4894-94 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.
[0014] 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) 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.
[0015] 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.
[0016] With respect to the perfluoropolymer component of the
mixture used in 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. The melt flow rate
(MFR) of the perfluoropolymers used in the present invention can
vary widely, depending on the proportion of non-melt flowable PTFE
component, the melt-fabrication technique desired for the mixture
of polymer components, and the properties desired in the
melt-fabricated article. Thus, MFRs for the melt-fabricable
fluoropolymer 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 fluoropolymers, 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. 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 perfluoropolymer component
can have high MFR, e.g. greater than 20 g/10 min, without the
article melt-fabricated from the polymer mixture used in the
present invention failing the NFPA-255 burn test, because the
presence of the PTFE component as dispersed particles in the
continuous phase of melt-fabricable perfluoropolymer making up the
molded article does not flow, and thus, does not drip to cause
smoke generation.
[0017] Examples of perfluoropolymers that can be used in the
polymer mixture used in the present invention 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 as
indicated by the inability of the melt-fabricable perfluoropolymer
to be characterizable by melt creep viscosity or by SSG. 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 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
either of the melt creep viscosity or SSG determinations to be made
on such PTFE.
[0018] The perfluoroolefin or PAVE comonomer composition of the
perfluoropolymer component is determined by infrared analysis on
compression molded film made from the perfluoropolymer 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.
[0019] The combination of the non-melt flowable PTFE and melt
fabricable perfluoropolymer components (submicrometer-size
particles) used in the present invention results in a polymer
mixture that is also melt fabricable. One attribute of melt
flowability, enabling melt fabricability, is that the polymer
mixture used the present invention exhibits a melt viscosity of
preferably no more than about 5.times.10.sup.5 Pa.circle-solid.s,
more preferably, no more than about 1.times.10.sup.5
Pa.circle-solid.s, and most preferably, no more than about
5.times.10.sup.4 Pa.circle-solid.s, 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.
[0020] As discussed above, the melt fabricability of the
perfluoropolymer component can be characterized by its MFR.
Although the presence of the non-melt flowable PTFE component may
lower the MFR of the overall melt blend of the polymers as compared
to the MFR of the perfluoropolymer by itself, and may even render
the MFR not measurable by ASTM D 1238-94a, the thixotropy exhibited
by the polymer blend when subjected to sufficient shear in the
molten state, enables the resultant melt blend to be melt
fabricated into articles by the typical melt fabrication techniques
of extrusion and injection molding. The melt viscosity of the
polymer blend, as discussed above, reflects the thixotropic effect,
because of its determination at a much higher shear rate than is
encountered in the MFR determination. The thixotropic effect
extends over the entire range of polymer mixture compositions. At
least about 0.5 wt % of the PTFE component is required before the
thixotropic effect is appreciable. The maximum amount of PTFE
component is preferably up to that amount beyond which the PTFE is
no longer the dispersed phase when the polymer mixture is melt
mixed (blended), such as occurs in extrusion or injection molding.
Preferably the 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.)=4Q/.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 polymer
mixture used in the present invention is at least about 200 Pas,
more preferably at least about 400 Pas at the shear rates specified
above.
[0021] The advantage of thixotropy discovered by the present
invention extends to higher shear rates than 100 s.sup.-1 enabling
the polymer mixture 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.
[0022] Within the above composition range, various improvements in
physical properties exist. Preferably, the non-melt flowable PTFE
content is about 4 to 40 wt % based on the combined weight of the
non-melt flowable PTFE and melt-fabricable perfluoropolymer and
more preferably about 4 to 30 wt % as the PTFE content. As the PTFE
wt % increases from 2 wt % based on the combined weight of these
polymer components, the elongation and tensile strength increase,
indicating reinforcement of the perfluoropolymer continuous phase
by the dispersed PTFE particles. This reinforcement extends to much
greater amounts of PTFE component, e.g. up to at least about 15 wt
% of the PTFE, more preferably up to about 25, and most preferably
up to about 30 wt % of the PTFE, 4 wt % of the PTFE component being
the preferred minimum, all percents being based on the combined
weight of the PTFE and the perfluoropolymer components.
Alternatively, either the perfluoropolymer component or its MFR can
be selected to optimize melt flow either for high production rate
melt fabrication or the production of intricate molded shapes,
while still retaining adequate physical properties for the
particular utility intended. For example, the elongation at break
of the polymer mixture is preferably at least 200% for compositions
containing up to at least about 30 wt % of the PTFE component,
based on the combined weight of the PTFE and perfluoropolymer
components. As shown in one of the Examples, the elongation of at
least 200% can exist for compositions containing up to about 75 wt
% of the PTFE component.
[0023] The process of the present invention can also be
characterized by the novel properties obtained by the melt blending
of a mixture of molten polymer particles together. Thus, the
process for melt fabricating perfluoropolymer comprises forming a
mixture of submicrometer-size particles of non-melt-flowable
polytetrafluoroethylene and melt-fabricable perfluoropolymer, melt
blending said mixture, and molding the resultant melt blend into an
article, said mixture of submicrometer-size particles of
non-melt-flowable polytetrafluoroethylene and melt-fabricable
perfluoropolymer being characterized by elongation of at least 75%
of that of the perfluoropolymer by itself and/or viscosity
reduction of at least 10% greater than for the perfluoropolymer by
itself, when shear is increased from about 10 s.sup.-1 to 100
s.sup.-1 when said polytetrafluoroethylene constitutes at least 4
wt % of the combined weight of said polytetrafluoroethylene and
said perfluoropolymer. The preferred minimum thixotropy effects
(viscosity reductions) described above also apply to this
embodiment and to compositions wherein the PTFE component
constitutes up to at least 15 wt %, preferably at least 25 wt % and
more preferably at least 30 wt %, based on the combined weight of
the PTFE and perfluoropolymer components.
[0024] Another embodiment of the present invention is the molded
article obtained by any of the processes of the present invention
disclosed herein.
[0025] Each of the PTFE and perfluoropolymer components used in the
present invention are preferably made by aqueous dispersion
polymerization and each aqueous dispersion polymerization can be
done by conventional, known methods.
[0026] In one embodiment of the present invention, the PTFE
component is prepared as an aqueous dispersion-produced core/shell
polymer wherein the PTFE is the core and the shell is
melt-fabricable perfluoropolymer. At least a portion, if not all,
of the melt-fabricable perfluoropolymer is supplied to the polymer
mixture as the shell of the core/shell polymer. The remaining
portion to arrive at the perfluoropolymer amount desired in the
PTFE/perfluoropolymer mixture for melt blending may be separately
supplied melt-fabricable perfluoropolymer. Alternatively,
submicrometer-size PTFE particles can be separately supplied to the
polymer mixture to increase its PTFE content. 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 PTFE. 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 perfluoropolymer and the
independently supplied perfluoropolymer are considered to be the
same even though there may be small differences 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.
[0027] In the preparation of core/shell polymer, the non-melt
flowable PTFE core can be 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 perfluoromonomers 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 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. These addition techniques can be used when just the
perfluoropolymer by itself is being made. 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 in core/shell polymerization. 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.
[0028] The core/shell polymer if used to provide all or a portion
of the melt-fabricable perfluoropolymer to the polymer mixture 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 without causing disruptions within the matrix that would
detract from physical properties.
[0029] 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.
[0030] Examples of initiators that can be used in both
polymerizations and in polymerization of the PTFE and
perfluoropolymer components by themselves, all aqueous dispersion
polymerizations, include ammonium persulfate, potassium persulfate,
bis(perfluoroalkane carboxylic acid) peroxide, azo compounds,
permanganate 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
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 and
pressure. Initiator addition, at a fixed or variable 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).
[0032] The particle size of the core/shell polymer or PTFE or
perfluoropolymer by itself is small enough (submicrometer in size)
that the polymer particles remain dispersed in the aqueous
polymerization medium until the polymerization reaction is
completed, whereupon the dispersed 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). The coagulation is preferably a co-coagulation of mixed
together aqueous dispersions of the polymers used to form the
polymer mixture staring material for the process of the present
invention, as will be further described below.
[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 D4464.
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 PTFE and perfluoropolymer particles and to the
core/shell polymer particles if used, to form the starting polymer
mixture in the process of the present invention. The submicrometer
size of the core/shell polymer establishes that both the core and
the shell are submicrometer in size. The smaller the average
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 particle size of
the PTFE component incorporated into the melt blend from the
polymer mixture 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. The
core/shell polymer provides submicrometer-size particles of both
the PTFE as the core and the melt-fabricable perfluoropolymer as
the shell. A single core/shell polymer particle is a mini-mixture
of these polymer components. A multiplicity of core/shell polymer
particles forms a mixture of submicrometer-size particles, which is
also a mixture of the PTFE and melt-fabricable perfluoropolymer
components.
[0034] The as-polymerized polymer particle sizes described in the
aqueous dispersions 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, which upon drying become a fine
powder mixture of these polymer particles, the agglomerated mixture
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] Preferably, the separately prepared aqueous dispersions of
the PTFE and perfluoropolymer components, or core/shell polymer and
perfluoropolymer components, as the case may be, are mixed together
to obtain mixture of the submicrometer-size polymer particles still
in aqueous dispersion form. When core/shell polymer is not used,
then all of the melt-fabricable perfluoropolymer is separately
supplied from the PTFE component. Co-coagulation of the mixed
dispersions results in the formation of agglomerates, which contain
primary particles of each polymer component intermixed with one
another. The agglomerates can be separated from the aqueous medium
by decanting, with or without filtration, followed by drying to
form the fine powder, called secondary particles as described
above. Typically, the polymer mixture used as the starting material
in the process of the present invention will be a mixture of fine
powder, the particles of fine powder each containing primary
particles of both polymer components.
[0036] For convenience, this fine powder can be subjected to melt
extrusion, involving melt blending of the mixture, to form pellets
as an intermediate molded article for further melt fabrication.
Alternatively, the first exposure of the polymer mixture can be
part of the melt fabrication process to form the final article,
such as extruded wire insulation, cable jacket, or injection molded
article. In either case, the melt blending involves the formation
of a molten mass of polymer and mixing this mass together as part
of the melting process. Typically, this melt blending will be
carried out at a temperature above the melting temperature of the
PTFE, and thus above the melting temperature of the melt-fabricable
perfluoropolymer, whether the melting temperature of the PTFE is
the first melt temperature (about 343.degree. C.) or second melt
temperature (about 327.degree. C.) of the PTFE, e.g. at a melt
temperature of at least 350.degree. C. As described above, the melt
blend becomes a dispersion of the PTFE component in a continuous
phase of the perfluoropolymer component, and this dispersion
relationship is carried over into the article molded from the melt
blend, and if the molded article is pellets, then into the final
article melt fabricated from the pellets.
EXAMPLES
Test Procedures
[0037] The procedures for determining melt creep viscosity,
standard specific gravity (SSG), melt flow rate (MFR), and average
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 polymer blends disclosed in
the Examples exhibited a melt viscosity less than about
5.times.10.sup.4 Pa.circle-solid.s at 350.degree. C. and shear rate
of 101 s.sup.-1.
[0038] 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 rate to obtain the shear rate desired. The results are
reported in the Examples as melt viscosity change (reduction),
.DELTA..eta. in Pa.circle-solid.s 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.
[0039] The elongation at break and tensile strength parameters
disclosed hereinbefore and values reported in the Examples are
obtained from the mixture of PTFE and melt-fabricable
perfluoropolymer components by the procedure of ASTM D 638-03 on
dumbbell-shaped test specimens formed from the mixture, 15 mm wide
by 38 mm long and having a web width of 5 mm, stamped out from 60
mil (1.5 mm) thick compression molded plaques of the mixture.
[0040] The compression molding of the plaques 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 10,000 lb
(4535 kg) for 2 min, followed by 20,000 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
quenching in air, the plaque being under a weight to prevent
warping. Compression molding of the core/shell polymer coagulated
and 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.
[0041] The procedure for measuring MIT Flex Life is disclosed at
ASTM D 2176 using a 8 mil (0.21 mm) thick compression molded film.
The disclosures of the MIT Flex Life parameter and values herein
are with reference to and are obtained using a 0.21 mm thick
compression molded film, unless otherwise indicated.
Preparation of Polymer Components
Polymer A (FEP)
[0042] 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). Forty
milliliters 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 (0.07 MPa) 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. A portion of
the dispersion was coagulated to produce material for testing.
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.,
was 101 Pa.circle-solid.s. The FEP exhibited a tensile strength and
elongation at break of 2971 psi (20.8 MPa) and 310%, respectively.
This FEP is a typical high-performing FEP.
Polymer B (PTFE Micropowder)
[0043] 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
lbs (22.7 kg) of demineralized water, 200 mL of 20 wt % ammonium
perfluorooctanoate in water, and 2.0 g Krytox.RTM. 157 FSL
(available from E.I. du Pont de Nemours & Co., Inc.). With the
reactor paddle agitated at 70 rpm, the reactor was heated to
60.degree. C., evacuated and purged three times with
tetrafluoroethylene (TFE). The reactor and its contents were cooled
to 25.degree. C. and ethane was added to achieve a pressure change
of 27 in Hg (91 kPa). The temperature of the reactor and its
contents were increased to 90.degree. C. After the temperature had
become steady at 90.degree. C., tetrafluoroethylene (TFE) was added
to the reactor until the pressure was 250 psig (1.72 MPa). Then 90
mL of freshly prepared aqueous initiator solution containing 1.0 wt
% of ammonium persulfate (APS) was charged into the reactor. Then,
this same initiator solution was pumped into the reactor at 10
mL/min for 70 minutes. 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 26 lb (11.8 kg)/110 min until
a total of 26.0 lbs (11.8 kg) of TFE had been added to the reactor
after kickoff. The total reaction time was 110 min after initiation
of polymerization. At the end of the reaction period, the TFE feed
was stopped, and the TFE remaining in the reactor was allowed to
react down to a pressure of 35 psig (0.24 MPa). When the reactor
pressure reached 35 psig, the reactor was slowly vented. After
venting to atmospheric pressure, the contents of the reactor were
cooled and the dispersion was discharged from the reactor at below
70.degree. C. Solids content of the dispersion was 36.32 wt % and
raw dispersion particle size (RDPS) was 0.110 .mu.m. A portion of
the dispersion was coagulated to produce material for testing. The
polymer was isolated by filtering and then drying in a 150.degree.
C. convection air oven. This material had a melt flow rate (MFR) of
2.15 g/10 min using the procedure of ASTM D 1238-94a, except that
the diameter of the extrusion orifice was 0.031 inch (0.79 mm)
instead of the usual 0.082 in (2.09 mm). The fact that this PTFE
exhibited an MFR means that this PTFE was melt flowable.
Polymer C (Non-Melt Flowable PTFE)
[0044] 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% KMnO.sub.4 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 pressure up, the TFE and the C-8 solution feeds were
stopped and the reactor 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 reactor 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 in an air oven 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 Pa.circle-solid.s at
380.degree. C., which is the upper limit of measurement, indicating
ultra-high molecular weight, and a zero MFR.
Polymer D (Non-Melt Flowable Modified PTFE)
[0045] 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.24 H.sub.2O in
water, and 101 mL of a 0.1 wt % solution of CuCl.sub.22 H.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 reactor 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 in an air oven at 150.degree.
C. for three days. The PTFE resin has SSG of 2.186, melt creep
viscosity of 4.times.10.sup.9 Pa.circle-solid.s at 380.degree. C.,
an HFP content of 0.45 wt %, and a zero MFR.
Polymer E (PFA)
[0046] 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
had been 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 PPVE/TFE
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 Pa.circle-solid.s.
Polymer F (Non-Melt Flowable PTFE)
[0047] 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 solution 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 polymer is non-melt flowable PTFE
because it has a measurable SSG. The PTFE also exhibited a melt
creep viscosity greater than 10.sup.8 Pa.circle-solid.s at
380.degree. C. and a zero MFR.
Polymer G (Non-Melt Flowable PTFE)
[0048] 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 10.sup.8
Pa.circle-solid.s at 380.degree. C. and a zero MFR.
Polymer H (PFA)
[0049] 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.
[0050] 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 Pa.circle-solid.s. The copolymer also exhibited a
tensile strength of 3027 psi (21.2 MPa) and elongation at break of
349%.
Comparative Example A (Blend of FEP with PTFE Micropowder)
[0051] The aqueous dispersions of Polymer A and Polymer B were
mixed together by rolling at a rate of 5-10 rpm for 10 minutes,
resulting in an aqueous blend that was 90 wt % Polymer A and 10 wt
% Polymer B 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 56.4 g/10 min. For this
blend, the viscosity change (reduction), .DELTA..eta., was 108
Pa.circle-solid.s. The blend exhibited a tensile strength and
elongation at break of 2661 psi (20.8 MPa) and 272%, respectively.
The addition of 10 wt % of a low molecular weight, melt-flowable
PTFE does not result in either the desired thixotropic behavior or
any improvement in physical properties.
Comparative Example B (Blend of FEP with PTFE Micropowder)
[0052] The aqueous dispersions of Polymers A and B were mixed
together by rolling at a rate of 5-10 rpm for 10 minutes, resulting
in a blend that was 60 wt % Polymer A and 40 wt % Polymer B 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 109.2 g/10 min. The blend exhibited a tensile
strength and elongation at break of 2211 psi (15.24 MPa) and 12%,
respectively. The addition of 40 wt % of a low molecular weight,
melt flowable PTFE greatly embrittled the blend.
Example 1 (Blend of FEP with Non-Melt Flowable PTFE)
[0053] This Example describes the blending of FEP dispersion and
ultra-high molecular weight PTFE (Polymer C). The aqueous
dispersions of Polymers A and C were mixed together by rolling in a
container at a rate of 5-10 rpm for 10 minutes, resulting in a
blend that was 7 wt % Polymer C and 93 wt % Polymer A 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 Pa.circle-solid.s. The blend also exhibited a
tensile strength and elongation at break of 2676 psi (18.45 MPa)
and 219%, respectively.
Example 2 (Blend of FEP and Non-Melt Flowable PTFE)
[0054] This Example describes the blending of FEP dispersion and
high molecular weight PTFE (modified with HFP). Aqueous dispersions
of Polymers A and D were mixed together by rolling at a rate of
5-10 rpm for 10 minutes, resulting in a blend that was 7 wt %
Polymer D and 93 wt % Polymer A 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 7.5 g/10 min
and exhibited a viscosity change (reduction), Ail, of 1404 Pas. The
blend exhibited a tensile strength and elongation at break of 2844
psi (19.61 MPa) and 278%, respectively.
Example 3 (Blend of FEP and Non-Melt Flowable PTFE)
[0055] This Example describes the blending of FEP dispersion and
small particle high molecular weight PTFE. Aqueous dispersions of
Polymers A and F were mixed by rolling at a rate of 5-10 rpm for 10
minutes, resulting in a blend that was 7 wt % Polymer F and 93 wt %
Polymer A 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 Pa-s. The blend
exhibited a tensile strength and elongation at break of 2997 psi
(20.66 MPa) and 306%, respectively.
Example 4 (Blend of FEP and Non-Melt Flowable PTFE)
[0056] This Example describes the blending of FEP dispersion and
very small particle size high molecular weight PTFE. Aqueous
dispersions of Polymers A and G were mixed together by rolling at a
rate of 5-10 rpm for 10 minutes, resulting in a blend that was 7 wt
% Polymer G and 93 wt % Polymer A 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 499 Pa.circle-solid.s. The blend exhibited a
tensile strength and elongation at break of 2999 psi (20.68 MPa)
and 321%, respectively.
Example 5 (Blend of PFA and Non-Melt Flowable PTFE)
[0057] This Example describes the blending of PFA dispersion and
ultra-high molecular weight PTFE. Aqueous dispersions of Polymers E
and C were mixed together by rolling at a rate of 5-10 rpm for 10
minutes, resulting in a blend that was 20 wt % Polymer C and 80 wt
% Polymer E 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) of at
least 2 times that for the PFA by itself. The blend exhibited a
tensile strength and elongation at break of 3672 psi (25.31 MPa)
and 404%, respectively.
Example 6 (Blend of PFA with Non-Melt Flowable PTFE)
[0058] This Example describes the blending of PFA dispersion and
high molecular weight PTFE, which contains HFP modifier. Aqueous
dispersions of Polymers E and D were mixed together by rolling at a
rate of 5-10 rpm for 10 minutes, resulting in a blend that was 20
wt % Polymer D and 80 wt % Polymer E 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 Pa.circle-solid.s. The blend
exhibited a tensile strength and elongation at break of 3242 psi
(22.35 MPa) and 311%, respectively.
Example 7 (Blend of PFA and Non-Melt Flowable PTFE)
[0059] This Example describes the blending of PFA dispersion and
as-polymerized small particle PTFE. Aqueous dispersions of Polymers
E and F were mixed together by rolling at a rate of 5-10 rpm for 10
minutes, resulting in a blend that was 20 wt % Polymer F and 80 wt
% Polymer E 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
4.1 g/10 min and exhibited a viscosity change (reduction),
.DELTA..eta., of 5664 Pas. The blend exhibited a tensile strength
and elongation at break of 3311 psi (22.83 MPa) and 376%,
respectively.
Example 8 (Blend of PFA and Non-Melt Flowable PTFE)
[0060] This Example describes the blending of PFA dispersion and
as-polymerized very small particle PTFE. Aqueous dispersions of
Polymers E and G were mixed together by rolling at a rate of 5-10
rpm for 10 minutes, resulting in a blend that was 20 wt % Polymer G
and 80 wt % Polymer E 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
2.0 g/10 min and exhibited a viscosity change (reduction),
.DELTA..eta., of 4797 Pa.circle-solid.s. The blend exhibited a
tensile strength and elongation at break of 3693 psi (25.46 MPa)
and 419%, respectively.
Examples 9-14 (Core/Shell Polymer wherein the Shell is FEP)
[0061] Core/shell polymer when the shell polymer is FEP made in
accordance with the process of Comparative Example A, 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 initiator
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 polymerization began, 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 (90.87g)/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 lbs
(907 g) more 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 9 in Table 1 Examples 10 through 14 were prepared in a
manner similar to Example 9, 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 Core,
Content, Content, Content in Content in MFR, Strength, Elongation
.DELTA..eta., Ex. No. wt % wt % wt % Shell, wt % Shell, wt % g/10
min MPa at Break, % Pa s 9 7.6% 6.84 1.37 7.41 1.48 0 26.7 357
12936 10 11.5% 6.42 1.43 7.25 1.62 0.4 23.8 393 6273 11 15.4% 6.41
1.47 7.57 1.74 0.7 21.3 358 6495 12 19.2% 6.18 1.69 7.65 2.09 0
24.9 394 9000 13 26.9% 5.83 1.81 7.98 2.48 0 20.9 338 9113 14 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 perfluoropolymer 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.
Examples 15-19 (Core/Shell Polymer wherein the Shell is PFA)
[0062] This Example shows the preparation of core shell polymer in
which the shell is essentially the same PFA as Comparative Example
E.
[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 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
Pa.circle-solid.s. These results are included in Table 2 as Example
15. According to these results, the core/shell polymer exhibits a
melt viscosity reduction of almost 20.times. of that of the PFA by
itself and improved tensile strength and elongation at break.
Examples 16 through 19 were prepared as described in Example 15.
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 15-19 are given in
Table 2 below. TABLE-US-00002 TABLE 2 PTFE PEVE PEVE Tensile Core,
Content, Content in MFR, Strength, Elongation MIT Flex Life,
.DELTA..eta. Ex. No. wt % wt % Shell, wt % g/10 min MPa at Break, %
cycles Pa s 15 4.9% 2.99 3.15 8.6 23.9 392 5384 2051 16 9.7% 2.70
3.00 2.3 25.8 392 22884 6772 17 19.5% 2.41 2.99 0.0 27.0 397 48748
15834 18 29.4% 1.86 2.64 0.0 26.6 411 14209 41128 19 39.6% 1.12
1.85 0.0 28.3 396 3752 3500
The polymerizations shown in Table 2 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 2 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. 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.
Example 20
[0064] This Example shows the preparation of core/shell polymer
containing a very small amount of non-melt flowable PTFE.
[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, 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 Pa.circle-solid.s,
which is much greater than for the FEP of Comparative Example A.
The viscosity change at about 0.1 wt % core is estimated to be
about 2 times greater than that of the FEP by itself.
Examples 22-29
[0066] This Example shows the preparation of core/shell polymer in
which the core is non-melt flowable PTFE and the shell is the melt
processible PFA of Polymer H, wherein the core content extends to
high amounts.
[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 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 9) 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 Pa.circle-solid.s. These results are included as Example
22 in Table 3 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-00003 TABLE 3
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 22 4.85 2.93 3.08 4.12 28.1 353 851 7780 29.2 519 23 9.71
2.85 3.16 3.34 28.3 353 1016 8667 27.2 487 24 19.51 2.45 3.04 0
28.4 372 1796 9533 30.1 334 25 29.4 2 2.83 0 28.7 384 2560 5180
16.7 72 26 49.33 1.35 2.66 0 22 275 2070 19283 31.3 559 27 59.27
1.21 2.98 0 20.4 298 460 (1) (1) (1) 28 74.42 0.77 3 0 16.6 219 233
16500 18.5 153 29 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 3, 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 30
[0068] This Example is directed to improved injection molding and
resulting from the use of the polymer mixture used in the present
invention, wherein the mixture is supplied by core/shell
polymer.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
Test Procedures for this Example
[0073] 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.
[0074] 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.
[0075] The equipment used was a Nissei Injection Molding Machine,
Model FN-4000. 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
[0076] 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 4 and 5 summarize the
results for injection pressures of 80 MPa and 120 MPa respectively.
TABLE-US-00004 TABLE 4 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
[0077] TABLE-US-00005 TABLE 5 Injection Pressure 120 MPa Polymer
PFA 2 State 2 MFR 12.6 4 Shot Wt. (g) 13.1 12.7
[0078] The results show that the core/shell polymers 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.
[0079] 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 6. TABLE-US-00006 TABLE 6 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.
[0080] The flex life test results show that States 1 and 2
core/shell polymer, 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.
* * * * *