U.S. patent application number 09/828403 was filed with the patent office on 2001-10-25 for fluoropolymeric composition.
Invention is credited to Comeaux, Christopher M., David, Lawrence D., Effenberger, John A., Pollock, Timothy P., Sahlin, Katherine M., Socha, Laura A., Stone, Richard L., Verbicky, John W..
Application Number | 20010034414 09/828403 |
Document ID | / |
Family ID | 26737178 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010034414 |
Kind Code |
A1 |
Effenberger, John A. ; et
al. |
October 25, 2001 |
Fluoropolymeric composition
Abstract
A blended solid composition is provided containing a
fibrillatable microparticulate PTFE polymer in an unfibrillated
state and at least one elastomeric and/or fluoroplastic component.
The composition is useful in making microfiber-reinforced solid
compositions and articles produced therefrom.
Inventors: |
Effenberger, John A.;
(Bedford, NH) ; Comeaux, Christopher M.;
(Merrimack, NH) ; David, Lawrence D.; (Dover,
NH) ; Pollock, Timothy P.; (Manchester, NH) ;
Sahlin, Katherine M.; (Somerville, MA) ; Socha, Laura
A.; (Westford, MA) ; Stone, Richard L.;
(Manchester, NH) ; Verbicky, John W.; (York Beach,
ME) |
Correspondence
Address: |
WHITE & CASE LLP
PATENT DEPARTMENT
1155 AVENUE OF THE AMERICAS
NEW YORK
NY
10036
US
|
Family ID: |
26737178 |
Appl. No.: |
09/828403 |
Filed: |
April 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09828403 |
Apr 6, 2001 |
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09096700 |
Jun 12, 1998 |
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6239223 |
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60058054 |
Sep 5, 1997 |
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Current U.S.
Class: |
525/199 ;
525/200 |
Current CPC
Class: |
D01F 6/12 20130101; C08L
27/18 20130101; D01D 5/40 20130101; D06M 15/256 20130101; C08L
2205/16 20130101; C08L 27/12 20130101; C08L 2205/02 20130101; C08L
27/12 20130101; C08L 2666/04 20130101 |
Class at
Publication: |
525/199 ;
525/200 |
International
Class: |
C08L 027/12; C08L
001/00 |
Claims
What is claimed is:
1. A blended solid composition of polymeric components comprising:
one or more microparticulate fluoroplastic components distributed
homogeneously throughout the composition, wherein at least one of
said fluoroplastic components is a fibrillatable PTFE polymer in an
essentially unfibrillated state; and at least one elastomeric
component.
2. A blended solid composition according to claim 1, wherein the
PTFE has a high molecular weight.
3. A blended solid composition according to claim 2, wherein the
PTFE has a melt viscosity at 380.degree. C. of greater than
10.sup.9 poise and is fibrillatable.
4. A blended solid composition according to claim 2, wherein the
PTFE has been prepared in an aqueous dispersion.
5. A blended solid composition according to claim 2, wherein the
microparticulate components are composed of particles having a size
of 1.0.mu. or less.
6. A blended solid composition according to any of claims 2 or 5,
wherein the elastomeric component is a fluoroelastomer or a
perfluoroelastomer.
7. A blended solid composition according to claim 6, wherein the
elastomer has been prepared in an aqueous dispersion.
8. A blended solid composition according to claim 6 wherein at
least one elastomeric component is uncured, the composition further
comprising a curative capable of curing said uncured
elastomer(s).
9. A blended solid composition according to claim 8, wherein the
curative is selected from the group consisting of amines, acid
acceptors, bisphenols, quaternary onium salts, peroxides,
persulfates, triallyl imidazole, triallyl isocyanurate, and
photoexcitable ketones.
10. A blended solid composition according to claim 6, wherein at
least one elastomeric component has undergone curing.
11. A blended solid composition composition according to claim 6,
further comprising metallic, mineral, ceramic, or carbonaceous
additives for modifying the physicochemical properties of the
composition.
12. A blended solid composition according to claim 6, wherein the
elastomer is selected from polymers and copolymers of TFE,
VF.sub.2, HFP, fluorovinyl ethers including perfluorovinyl ethers,
CTFE, ethylene, and propylene.
13. A blended solid composition of polymeric components comprising:
first a microparticulate fluoroplastic component distributed
homogeneously throughout the composition, said component including
a fibrillatable PTFE polymer in an essentially unfibrillated state;
and one or more additional microparticulate fluoroplastic
components, each of which has a melting or softening point below
the melting point of the fibrillatable PTFE.
14. A blended solid composition according to claim 13, wherein the
PTFE has a high molecular weight.
15. A blended solid composition according to claim 14, wherein the
PTFE has a melt viscosity at 380.degree. C. of greater than
10.sup.9 poise and is fibrillatable.
16. A blended solid composition according to claim 13, wherein the
fluoroplastic component other than PTFE includes at least one
amorphous fluoropolymer.
17. A blended composition according to claim 16, wherein the
amorphous fluoropolymer is a CTFE-containing copolymer.
18. A blended solid composition according to claim 14, wherein the
PTFE is polymerized as an aqueous dispersion.
19. A blended solid composition according to claim 13, wherein the
microparticulate components are composed of particles having a size
of 1.0.mu. or less.
20. A blended solid composition according to any of claims 15 or
19, further comprising metallic, mineral, ceramic, or carbonaceous
additives for modifying the physicochemical properties of the
composition.
21. A microfiber-reinforced solid composition produced by a process
comprising: subjecting the blended solid composition according to
any one of claims 1 or 13 to a shear force that induces the PTFE
component to fibrillate and creates a microfibrous reinforcement
within the blended composition.
22. A microfiber-reinforced composition according to claim 21,
wherein the shearing process is conducted at a temperature below
the melting point of any of the components of the composition.
23. A microfiber-reinforced composition according to claim 21,
wherein the shearing process is conducted at a temperature chosen
to selectively melt or soften one or more of the components of the
composition other than the PTFE.
24. A microfiber-reinforced solid composition according to claim
21, wherein the composition is free of curatives during the process
of subjecting the composition to a shear force.
25. A microfiber-reinforced solid composition according to claim
21, wherein the composition contains curatives during the process
of subjecting the composition to a shear force.
26. A microfiber-reinforced solid composition according to claim
21, wherein the microfibrous PTFE reinforcement is aligned with and
extended parallel to the direction of the applied shear force.
27. A microfiber-reinforced solid composition according to claim
21, wherein the microfibrous PTFE reinforcement is oriented
uniaxially in the composition.
28. A microfiber-reinforced solid composition according to claim
21, wherein the microfibrous PTFE reinforcement is oriented
multiaxially in the composition.
29. A microfiber-reinforced solid composition comprising a
plurality of uniaxially oriented microfibrous PTFE reinforcements
according to claim 27 layered as lamallae in varying orientations
to create multiaxial reinforced lamellae.
30. A microfiber-reinforced solid composition according to claim
29, wherein the individual lamellae are co-cured during thermal
consolidation.
31. A reinforced composite derived from the composition according
to claim 21, wherein the microfiber PTFE reinforcement serves as a
matrix system for a textile-based reinforcement.
32. A reinforced composite according to claim 31, wherein the
textile-based reinforcement is a yarn, monofilament, or a system of
yarns such as a woven, knitted, or nonwoven fabric.
33. A reinforced composite according to claim 31, wherein the
textile is based on fiberglass, carbonaceous, graphitic, polyester,
polyamide, or polyolefinic materials.
34. A reinforced composite prepared from the composition according
to any one of claims 1 or 13, wherein the PTFE reinforcement serves
as a matrix system for a textile-based reinforcement.
35. A reinforced composite according to claim 34, wherein the
textile based reinforcement is a yarn, monofilament or a system of
yarns such as a woven, knitted, or nonwoven fabric.
36. A reinforced composite according to claim 34, wherein the
textile is based on fiberglass, carbonaceous, graphitic, polyester,
polyamide, or polyolefinic materials.
37. A method of making a blended solid composition composed of
unfibrillated, yet fibrillatable PTFE, said method comprising:
isolating the blended composition of any of claims 1 or 13 as a
solid from an aqueous system by a low shear process that does not
induce fibrillation of the PTFE.
38. A method according to claim 37, wherein the low shear process
is a co-coagulation method.
39. A method according to claim 37, wherein the low shear process
is a film-casting process.
40. A method according to claim 37, wherein the low shear process
is a freeze-drying process.
41. A method according to claim 37, wherein the low shear process
is a dessication process.
42. A method according to claim 37, wherein the composition is
sheared at a temperature chosen to selectively melt or soften one
or more of the components in the composition other than the
PTFE.
43. A method according to claim 37, wherein curatives are
incorporated into the composition after isolation of the blended
polymers from the aqueous dispersion.
44. An article produced from the blended solid compositions
according to any of claims 1 or 13.
45. An article produced from the microfiber-reinforced composition
according to claim 21.
46. An article produced from the reinforced composite according to
claim 31.
47. An article produced from the reinforced composite according to
claim 34.
48. A blended, homogeneous, solid composition, comprising a low
molecular weight, nonfibrillatable PTFE and an elastomeric polymer,
wherein the PTFE is present at greater than 35% by weight, based on
total polymer solids of the composition.
49. A blended composition according to claim 48, wherein the
elastomeric polymer is a fluoroelastomer or perfluoroelastomer.
50. A film, comprising an unsintered and unfibrillated, yet
fibrillatable high molecular weight PTFE and an elastomeric
polymer, wherein the PTFE is present at a loading level up to 80%
by weight PTFE, based on total polymer solids.
51. A film according to claim 50, wherein the elastomeric polymer
is a fluoroelastomer or a perfluoroelastomer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit, under 35 U.S.C.
.sctn.119(e), of U.S. Provisional Patent Application No. 60/058,054
filed on Sep. 5, 1997.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally concerns fluoropolymeric
compositions. In particular, the invention concerns an improved
composition that is isolated from an aqueous blend of
fluoroelastomer and microparticulate fluoroplastic materials. The
improved composition is particularly useful as a manageable
intermediate in the development of microfiber-reinforced
fluoropolymeric components.
[0004] 2. Description of the Prior Art
[0005] Polytetrafluoroethylene (PTFE) is in many respects an
unusual polymer. It is exceptional in its chemical inertness as a
result of the strength of its carbon-fluorine bonds and shielding
of its carbon-carbon bonds by the bulky fluorine atom. PTFE is
exceptionally useful for high temperature applications because it
has a high melting point and remains chemically inert at high
temperatures. In addition, PTFE's unusually low frictional
coefficient, surface free energy, and dielectric constant all
testify to its unusual morphological structure. While these
extremely attractive properties cause PTFE to be useful in a broad
array of end use applications, they also lead to an unusual set of
problems in characterizing some properties of PTFE as well as to
difficulties in processing compositions based on PTFE.
[0006] The inertness and insolubility of PTFE make it virtually
impossible to characterize the molecular weight of a PTFE component
by direct, conventional means such as osmometry. The prior art
typically resorts to indirect means, such as the determination of
specific gravity after recrystallization from a melt at a
controlled rate of cooling, as an indicator of molecular weight.
The higher the molecular weight of the PTFE, the longer its chain
length and the more difficult it is to recrystallize to a highly
ordered (crystalline) and, therefore, dense structure.
Consequently, the specific gravity of PTFE at any given
crystallinity level is an indirect measure of molecular weight.
Crystallinity may be independently assessed by X-ray
crystallography or calorimetry, and the specific gravity obtained
upon cooling (recrystallizing) a PTFE melt at a prescribed rate
(referred to as the standard specific gravity (SSG)) is a commonly
employed measure of molecular weight.
[0007] It is well established that certain physical behavior of
PTFE is a strong function of molecular weight and crystallinity
(Blair, John A., Fluorocarbons, Polymers, "Encyclopedia of
Industrial Chemical Analysis," vol. 13, pps. 73-93). For example,
most commercial molding powders of PTFE have a very high molecular
weight corresponding to an SSG of between about 2.16 and 2.25,
depending on crystallinity. High molecular weight is needed to
develop adequate tensile strength and the elongation required for
typical end uses of an essentially waxy polymer.
[0008] At lower molecular weight, PTFE becomes very weak and
brittle while retaining its low coefficient of friction. Low
molecular weight PTFE is typically a friable powder, which can be
very highly crystalline, and enjoys use as a dry lubricant.
[0009] An important distinction in behavior between low molecular
weight and high molecular weight PTFE lies in the propensity of the
high molecular weight PTFE to fibrillate when in its highly
crystalline, as-polymerized condition upon being subjected to
mechanical shear stresses. Low molecular weight PTFE, on the other
hand, simply reaches its ultimate elongation at low stress and
disintegrates into a lubricating (low coefficient of friction)
powder while highly crystalline high molecular weight PTFE
substantially transforms its morphological character under shear
and forms an extensive network of fibers. This is most obvious in
the case of aqueous, dispersion-polymerized, high molecular weight
PTFE in which the growing polymer chains are highly organized into
dense, tightly packed spheres or rods with a very high degree of
crystallinity. The rod-shaped particles, when present, typically
have a length to diameter ratio (L/D) of 2-3:1 and the diameter is
typically on the order of 0.1 micron (.mu.). The spherical
particles typically have a diameter of approximately 0.2-0.3.mu.,
as measured by light scattering. Because of their very high
crystallinity and high molecular weight, it is possible for these
particles to fibrillate into rod-like structures when subjected to
a relatively low mechanical shear force, forming fibers having a
very high L/D ratio. These PTFE fibers have the ability to form
aggregated structures in which the rod-like aggregates of high
molecular weight PTFE serve as a microfiber reinforcement within
the polymer mass of fibrillated and unfibrillated PTFE. The
presence of such structures results in an increase in the tensile
modulus and strength of the polymer matrix in which they are
present and for this reason may be referred to as a microfiber
reinforcement. The ease with which such fiber formation occurs is
such that one must take great care to control the level and
direction of applied shear forces to avoid uncontrolled
entanglement of propagating fibers which can result in physical
unmanageability during subsequent processing.
[0010] Melt viscosity is another commonly measured surrogate for
molecular weight of polymers such as PTFE. Commonly measured at
380.degree. C., the melt viscosity of high molecular weight PTFE is
typically about 10.sup.10 to 10.sup.12 poise. High molecular weight
PTFE readily forms fibers of the type discussed above when at a
high level of crystallinity. As the melt viscosity at 380.degree.
C. decreases (indicating lower molecular weight), however, PTFE's
ability to fibrillate falls off markedly. Below about 10.sup.9
poise, PTFE becomes a much more brittle, friable material.
[0011] Many attempts have been made in the prior art to combine
PTFE with other polymeric compounds, such as elastomers, to form
multicomponent systems. Fairly sophisticated processes have been
developed to control the properties of such multicomponent systems,
and skilled artisans have been able to enhance various desirable
properties of final products created from such multicomponent
systems. "Rubber-toughened" plastics are a good example of such an
enhancement.
[0012] In creating these multicomponent systems, skilled artisans
have used with some success "microparticulate polymers", i.e.,
emulsions and dispersions of elastomeric and plastic polymers, as
coating and casting fluids. Various processes have been developed
for applying such fluids, blended in the microparticulate state,
enabling skilled artisans to thermally consolidate thin films in an
extremely short duration of time and at surprisingly high
temperatures, if necessary. Short interval thermal processing
yields surprisingly compatible blends of microparticulates, even
those with greatly disparate melt flows. For example, these
processes have been used to combine polymers perceived to be
non-extrudable due to their high viscosity or molecular weights or
to combine non-melt-processible polymers, such as PTFE, with other
more flowable polymeric components. Short interval thermal
processing has also been used to combine materials with vastly
different melting points (ranging from 150.degree. C. to
335.degree. C.). The absence of substantial mechanical shear during
the high temperature phase of the thermal consolidation avoids
mechanically-induced thermal deterioration of molecular weight in
the materials, such as might occur during a melt-extrusion
process.
[0013] Despite all of these efforts, the prior art has not been
able to develop blended solid compositions containing
fluoroplastics at particularly high useful levels into
fluoroplastic/fluoroelastomer blends, while maintaining facile
processibility of the blends.
[0014] Polymeric intermediates (for example, gum rubbers) must
first be isolated before they can be compounded into a processible
composition that can incorporate fillers, such as carbon or talc.
The initial isolation of the polymeric intermediate generally
involves the steps of coagulating the polymer from a polymerization
medium, followed by washing the polymer, drying the polymer, and
compacting the polymer into a slab. This polymeric slab is then
mixed with desired curing additives and fillers on a high shear
mill, such as a Banbury mill, keeping the mixture below
temperatures that would initiate localized or premature cure of the
rubber compound. Such premature cure is known in the art as
"scorch." Other additives may be incorporated in a similar manner.
Fillers are generally incorporated before the curatives because the
incorporation process generates heat, which would cause scorch if
the curative were present. The amount of such additives is limited
because additional space in the compound is needed to accommodate
the presence of the curative. Additives and curatives are added
sequentially, which overall affects the workability of the gum.
Once the additives and curatives are added, the compound is stored
at a cool temperature until used. The compounded formulation is
then typically "freshened" on a mill or calender and extruded at
non-scorching conditions to yield a form that may be
compression-molded, transfer-molded, or injection-molded to produce
a shaped, cured part such as an "o"-ring or seal. Cured parts may
then be demolded and post-cured at elevated temperatures to develop
maximal mechanical properties or chemical resistance.
[0015] The prior art evidences substantial effort to identify
desirable fillers and curing additives that improve end properties.
PTFE has been identified as an excellent filler because of its
desirable tribological properties. The prior art processes
incorporating milled PTFE, however, have been limited to filler
loadings below 25 parts per hundred rubber by weight to avoid
problems both during milling and in subsequent processing.
Furthermore, the prior art has also been constrained to employ low
molecular weight PTFE micropowder, such as DuPont's MP 1000, as a
filler. For this reason, users of such blends have not been able to
exploit the benefits of high molecular weight PTFE related to its
tendency to fibrillate under applied mechanical shear. Thus, the
prior art processes have not generally been able to achieve a
processible composition as a blended solid containing a high filler
loading of PTFE, and particularly with high molecular weight PTFE.
The potential benefits of a homogeneously distributed, high
molecular weight PTFE at a high filler loading in the compositions
have been, consequently, forgone.
[0016] The present invention achieves the desirable benefits
discussed above through the use of an essentially non-fibrillating,
low-shear isolation process that incorporates in excess of 25%, and
typically 40-50%, and as high as 80% by weight of a plastic
(particularly high molecular weight PTFE, a perfluoroplastic) into
the rubber (based on total polymer weight). The process yields
compositions containing the plastic PTFE in an unfibrillated form,
yet the PTFE is fibrillatable and may be fibrillated by subsequent
processing. The range of the substantial benefits obtained are
described as follows.
SUMMARY OF THE INVENTION
[0017] It is a prime objective of the invention to produce a
curable, solid composition containing (i) an elastomer, preferably
a fluoroelastomer, and (ii) a fluoroplastic at greater than 25% by
weight based on total polymer, wherein the fluoroplastic is in an
unfibrillated yet fibrillatable state. Fluoroplastics useful in the
invention include: perfluoroplastic(s), such as PTFE or copolymers
of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP);
perfluorovinyl ethers; or, other halogenated ethylenic monomers,
such as vinylidene fluoride (VF.sub.2), chlorotrifluoroethylene
(CTFE), or vinyl fluoride (VF). Ethylene and propylene copolymers
of TFE and HFP are also envisioned as applicable plastics in this
invention. The particulate fluoroplastic and elastomers have a
particle size in the range of 0.001.mu. to 1.0.mu., i.e.,
"micron-sized," thus their designation or definition as
"microparticulate."
[0018] A further objective of this invention is to obtain curable
compositions isolated directly from aqueous blends of the elastomer
and plastic microparticulates. Such isolation may be as a thin film
or as a powder that may be extruded well below the melting point or
softening point of the plastic component and above the glass
transition temperature of the elastomeric component. The curable
composition may also take the form of a gel. The solid forms, which
can contain curatives, may be directly molded by, for example,
compression molding methods to directly produce cured
elasto-plastic parts such as diaphragms or "o"-rings.
[0019] It is yet a further objective of this invention to
demonstrate the superior performance properties of cured compounds
or parts created from compositions of the invention in regard to
mechanical behavior (notably, retention of high elongation, reduced
coefficient of friction, reduced wear rate), as well as very good
resistance to swell in solvents or fuels. It is also an objective
to demonstrate good capability for high temperature end use for
such parts.
[0020] It is also an objective to show that the useful properties
of parts processed from these blends may be tailored through
selective choice of process temperatures to achieve selectively
crosslinked or melted morphological domains specific to the melting
points or crosslinking propensities of the various plastic or
elastomeric components.
[0021] Lastly, it is an objective to demonstrate that such solids
isolated from these aqueous blended polymers may be subsequently
subjected to mechanical forces to induce fiber formation by the
particulates, particularly of the high molecular weight PTFE
component, to obtain microfiber-reinforced composites with unique
mechanical behavior.
[0022] Importantly, the process described to achieve these
excellent or superior properties employs the least costly form of
the selected polymers. Polymerized emulsions and dispersions, in
principle, represent a preferred lowest cost position for the
resin. Particularly for materials-intensive components, such as
those based on fluoropolymers, this may be a decisive advantage.
The compounds and processes of the invention provide for the use of
PTFE as a filler, which can be more cost-effective than
conventional fillers because the PTFE can be filled to a higher
loading than is normally practiced. The PTFE filler of the
invention is a polymer with its own strength potential and
elongation potential through fibrillation--as well as temperature
and chemical resistance properties that can exceed those of a
hydrogen-containing fluoroelastomer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows a scanning electron micrograph illustrating
fiber formation of a PTFE laminate.
[0024] FIGS. 2 and 3 show scanning electron micrographs
illustrating fiber formation of a T30B hand-cranked sample
discussed in the specification.
[0025] FIGS. 4A through 4L are scanning electron micrographs taken
at 10,000.times. magnification, except for 4B (2,600.times.), of a
milled-film sample containing a high molecular weight PTFE
composition of the invention isolated in a matrix by a low shear
process of the invention.
[0026] FIG. 5 is a graph demonstrating the log shear stress versus
the log shear rate of low and high molecular weight PTFE-containing
compositions prepared by casting or milling processes.
[0027] FIGS. 6A through 6D are scanning electron micrographs of a
high molecular weight PTFE-containing composition of the
invention.
[0028] FIGS. 7A through 7D are scanning electron micrographs of a
low molecular weight PTFE-containing composition.
[0029] FIG. 8 is a bar graph demonstrating tensile measurements of
compositions containing PTFE at various concentrations (percent by
weight).
[0030] FIG. 9 is a bar graph demonstrating elongation and 100%
modulus of compositions containing PTFE at various concentrations
(percent by weight).
[0031] FIG. 10 is a bar graph showing the change in peak strain of
cast and milled compositions.
[0032] FIG. 11 is a bar graph showing the change in 100% modulus in
cast and milled compositions.
[0033] FIG. 12 is a bar graph showing the change in tensile
strength in cast and milled compositions.
[0034] FIG. 13 is a bar graph showing the tear strength at room
temperature of cast and milled compositions.
[0035] FIG. 14 is a bar graph showing the tensile set at room
temperature of cast and milled compositions.
[0036] FIG. 15 is a bar graph showing the tensile strength and
modulus at room temperature of various elastomers and PTFE at
increasing loadings.
[0037] FIG. 16 is a bar graph of the tear strength at room
temperature of PTFE/FKM compositions.
[0038] FIG. 17 is a scanning electron micrograph at 10,000.times.
magnification of a PTFE-containing composition of the invention
showing fibrillated PTFE.
[0039] FIG. 18 is a scanning electron micrograph at 10,000.times.
magnification of a cross-section of a PTFE-containing composition
of the invention.
[0040] FIGS. 19 and 20 are photographs of wide-angle x-ray (WAXS)
analysis of the present compositions containing PTFE.
[0041] FIGS. 21A through 21I are scanning electron micrographs
taken at 2,500 through 10,000.times. magnification of extruded
compositions containing PTFE.
[0042] FIG. 22 is a scanning electron micrograph of a
PTFE-containing composition which has been extruded, and which then
had the non-PTFE component extracted.
[0043] FIG. 23 is a photograph of a WAXS analysis of a
PTFE-containing composition which has been extruded.
[0044] FIG. 24 is a scanning electron micrograph of a coagulated
50/50 (wt %) PTFE/FKM composition of the present invention.
[0045] FIG. 25 is a scanning electron micrograph of a 50/50 (wt %)
PTFE/FKM coagulate which was hand-pressed, inducing fiber formation
of the PTFE.
[0046] FIG. 26 is a scanning electron micrograph at 10,000.times.
magnification of a coagulated PTFE-containing composition of the
invention.
[0047] FIG. 27 is a photograph of a WAXS analysis of an extruded
PTFE-containing composition.
[0048] FIG. 28 is a scanning electron micrograph of a cross-section
through a cast PTFE-containing composition of the invention, which
was prepared by a freeze-fracturing technique.
[0049] FIG. 29 is a scanning electron micrograph of a longitudinal
section through a PTFE-containing composition which was extruded,
then extracted with MEK.
[0050] FIG. 30 is a photograph of a WAXS analysis of a
PTFE-containing composition shown in FIG. 29.
[0051] FIG. 31 is a scanning electron micrograph at 10,000.times.
magnification of an extruded low molecular weight PTFE-containing
composition which was subjected to MEK extraction.
DETAILED DESCRIPTION OF THE INVENTION
[0052] The invention relates to a blended solid composition of
polymeric components, comprising one or more microparticulate
fluoroplastic components distributed homogeneously throughout the
composition and at least one elastomeric component. At least one of
the fluoroplastic components is a fibrillatable PTFE polymer in an
essentially unfibrillated state. The PTFE has a high molecular
weight (a melt viscosity at 380.degree. C. of greater than 10.sup.9
poise, and preferably greater than 10.sup.11 poise) and has been
prepared using an aqueous dispersion process. The microparticulate
components of the compositions are composed of particles having a
size of 1.0.mu. or less in diameter. The elastomeric component of
the composition is preferably selected from fluoroelastomers or
perfluoroelastomers, which have been preferably prepared in an
aqueous dispersion process. The preferred elastomers may be
selected from the group consisting of polymers and copolymers of
TFE, VF.sub.2, HFP, fluorovinyl ethers including perfluorovinyl
ethers, CTFE, ethylene, and propylene, and are preferably of high
molecular weight.
[0053] The compositions may further comprise at least one
elastomeric component which is uncured. In this aspect of the
invention, the composition may also comprise a curative capable of
curing said uncured elastomer(s). A curative is a substance which
can induce any degree of crosslinking of the polymers in the
compositions. Curatives are known in the art of polymer chemistry
and include: amines, acid acceptors, bisphenols, quaternary onium
salts, peroxides, persulfates, triallyl imidazole, triallyl
isocyanurate, and photoexcitable ketones. In one aspect of the
invention, the composition may be provided wherein at least one
elastomeric component has undergone curing, that is, some degree of
crosslinking.
[0054] The compositions may further comprise one or more additives,
such as metallic, mineral, ceramic, and carbonaceous materials for
modifying the physicochemical properties of the compositions.
[0055] The invention also relates to a blended solid composition of
polymeric components, comprising first a microparticulate
fluoroplastic component distributed homogeneously throughout the
composition, and one or more additional microparticulate
fluoroplastic components, each of which has a melting or softening
point below the melting point of the fibrillatable PTFE. In this
aspect of the invention, the fluoroplastic (including the
fibrillatable PTFE polymer) is in an essentially unfibrillated
state, the PTFE is highly crystalline and preferably of high
molecular weight, polymerized as an aqueous dispersion, and the
fluoroplastic component other than PTFE may include at least one
amorphous fluoropolymer. In a preferred embodiment of this aspect
of the invention, the amorphous fluoropolymer is a CTFE-containing
copolymer with VF.sub.2.
[0056] The invention further relates to a microfiber-reinforced
solid composition produced by a process comprising subjecting a
blended solid composition of the invention to a shear force that
induces the PTFE component to fibrillatablete and create a
microfibrous reinforcement within the blended composition. In this
aspect of the invention, the shearing process is conducted at a
temperature chosen to selectively melt or soften one or more of the
components of the composition other than the PTFE, and the
composition is free of, or may contain curatives. Alternatively,
the microfiber-reinforced composition is produced by subjecting the
blended solid composition to a shearing process at a temperature
below the melting point of any of the components of the
composition.
[0057] The compositions may also be provided wherein the
microfibrous PTFE reinforcement is aligned with and extended
parallel to the direction of the applied shear force or may be
oriented uniaxially or multiaxially. In this aspect of the
invention, the microfiber-reinforced solid composition comprises a
plurality of uniaxially-oriented microfibrous PTFE reinforcements
which are layered as lamellae in varying orientations to create
multiaxially-reinforced lamellae. In this embodiment, the
individual lamellae may be co-cured during thermal consolidation,
and the resulting microfiber PTFE-reinforced polymeric system can
then serve as the polymeric matrix system for conventional
textile-based reinforced composites. Compositions containing
unfibrillated PTFE, however, may also serve as a matrix system for
textile-based reinforced composite. The textile-based reinforcement
may be a yarn, monofilament, or a system of yarns such as a woven,
knitted, or nonwoven fabric, such as fiberglass, carbonaceous,
graphitic, polyester, polyamide, or polyolefinic materials.
[0058] The invention also relates to a method of making the blended
solid compositions composed of unfibrillated, yet fibrillatable
PTFE. The method comprises isolating the blended composition of the
invention from an aqueous system by a low shear process that does
not induce fibrillation or fiber propagation in the PTFE. The low
shear process may comprise a co-coagulation method comprising a
freeze-drying method or a desiccation method. In this aspect of the
invention, the composition is sheared at a temperature chosen to
selectively melt or soften one or more of the components in the
composition other than the PTFE, and curatives may be incorporated
into the composition after isolation of the blended polymers from
the aqueous dispersion. Many types of articles of manufacture may
be produced by this process.
[0059] The invention also relates to a blended, homogeneous, solid
composition, comprising a low molecular weight, nonfibrillatable
PTFE and an elastomeric polymer, wherein the PTFE is present at
greater than 35% by weight based on total polymer solids of the
composition.
[0060] The invention also relates to a film made by the method of
the invention, comprising an unsintered and unfibrillated, yet
fibrillatable high molecular weight PTFE and an elastomeric
polymer, wherein the PTFE is present at a loading level up to 80%
by weight PTFE based on total polymer solids. The elastomeric
polymer of the film is preferably a fluoroelastomer or a
perfluoroelastomer.
[0061] Following the methodologies of U.S. Pat. No. 4,555,543 and
U.S. Pat. No. 5,194,335 (coating and casting fluids based on blends
of fluoroplastic and fluoroelastomeric dispersions to produce cast
films, which disclosures are incorporated herein by reference), it
has been surprisingly found that a cohesive film or "leaf stock"
based on dispersion-blended polymer based on PTFE and
TFE/VF.sub.2/HFP elastomer, or other elastomer, such as
VF.sub.2/HFP, can be obtained at very modest temperatures. In fact,
the process can be viewed as simply drying and eliminating the
water at temperatures near its boiling point (100.degree. C.) while
maintaining a homogeneous mixture. This temperature is low enough
that curatives may actually be incorporated into the aqueous blend
so that the dried "leaf stock" may be considered to be a direct
feedstock for compression molding. It has been found by scanning
electron microscopy (SEM) examination that the PTFE in such a
processed blend was, in fact, essentially free of fibrous PTFE.
[0062] The as-cast leaf stock, typically about 5 mils in gauge, can
be easily handled to prepare a multilayer sheet of about 20-30 mils
by simple compression molding, taking care to allow venting of
gaseous byproducts. A cured elastomer is readily developed at a
pressure of 200 psi at 350.degree. F. after 30 minutes in a press
when Diak #3 (N,N' dicinnamylidene-1,6-hexanediamine, purchased
from DuPont Dow Elastomers, Elkton, Md.) is employed as a curative
and zinc oxide as an acid acceptor, and when both are included in
the casting fluid.
[0063] This process may also be conducted with non-fiber-forming
fluoroplastics, such as FEP, and their blends with PTFE.
Additionally, other non-polymeric additives such as carbon black,
or other finely divided minerals and the like, may also be blended
directly into the casting fluids.
[0064] Thus, the degree of uniformity and homogeneity of additives
and fillers obtainable in the finished part is exceptionally great.
It is believed that the exceptional properties related to
coefficient of friction, wear resistance, and the retention of
outstanding extensibility after high temperature exposure derives
from the choice of filler (high molecular weight PTFE) and,
particularly, its small particle size (less than 1 micron) and its
ability to remain unfibrillated in the isolation process, and
importantly, to the extremely high loading levels attainable in a
molded part. While other fluoroplastics do not commonly have an
equivalent tendency to form fibers but may be as readily processed
by this methodology, PTFE is a preferred filler from the viewpoint
of the development of resistance to chemically induced
stress-cracking and the utmost in thermal capability for end use
and cost.
[0065] In a preferred embodiment, the high molecular weight PTFE
suitable for making the microfiber reinforcement of the present
invention is a PTFE having a melt viscosity at 380.degree. C. of
greater than 10.sup.9 poise and preferably greater than 10.sup.11
poise. High molecular weight PTFE can be obtained from various
companies and is sold under various trademark names, for example,
TEFLON-T30B (E.I. Dupont de Nemours and Company, Inc., Wilmington,
Del.); ALGOFLON-D6027 (Ausimont USA, Thorofore, N.J.) and FLUON
AD1LN and FLUON CD 123 (ICI Americas, Bayonne, N.J.).
[0066] The following examples are not intended to be limiting to
the basic concept of this invention. The examples show that the
blended aqueous microparticulates of the invention may be used as a
feedstock for isolation through casting or other non-fibrillating
coagulation techniques to a moldable solid form (e.g., agglomerated
powder). The examples also show that rubber curatives may be
incorporated into the blended microparticulate casting fluid, prior
to isolation, and show the use of such feedstocks as molding
intermediates.
EXAMPLE 1
[0067] Raw fluoroelastomer and perfluoroelastomer latexes used in
the following examples are detailed in Table 1. One particularly
novel aspect of this approach to blending materials is that the
components are mixed at a microparticulate level, as contrasted
with the aggregate mixing accomplished with conventional rubber
mills and mixers. Average particle size for these aqueous latexes
varies from 0.01.mu. to almost 1.0.mu. as seen in Table 1. The
particle size for PTFE as well as other fluoroplastic latexes used
in these experiments averaged around 0.2.mu.. Particle size was
determined in the aqueous state using a Leeds & Northrup
Microtrac Ultrafine Particle Size Analyzer, which operates via
dynamic light scattering. Measurements were made on the material
directly or on a dilution which did not exceed 3 parts water to 1
part latex.
[0068] Fluorine content was disclosed to us by the manufacturer.
Those skilled in the art of fluoroelastomer formulation realize the
improvement in chemical resistance provided by increasing fluorine
content as well as changes in physical properties which also
result. Applications currently served by fluoroelastomers can be
envisioned using materials prepared as detailed here, as well as
applications not currently served due to limitations in
conventional rubber formulation, or applications which can be
served well through a microparticulate-reinforced matrix.
Therefore, a range of materials was evaluated in order to make
comparisons to the range which is commercially available.
[0069] Solids content was disclosed by the manufacturer in the case
of Tecnoflon TN1 latex. In all other cases the solids content was
determined by differential weight loss after drying off water at
110.degree. C. to a constant weight. A hand-held digital meter was
used to determine pH. Standard hydrometers were used to determine
specific gravity. Viscosity was measured with a Brookfield Model LV
viscometer with the indicated spindle at the indicated speed.
[0070] Glass transition temperatures were determined using a Perkin
Elmer DSC7 differential scanning calorimeter at a rate of
20.degree. C. per minute. The glass transition temperature, like
fluorine content, will have some bearing on potential applications
of these materials.
[0071] Creaming of a latex to higher solids was performed in order
to generate a latex with the convenient potential for higher build
during casting (as described in Example 4). Latex at lower solids
(as polymerized) can also be used for casting and has been used in
examples of other isolation procedures, such as co-coagulation.
Creaming was generally accomplished by addition of Rhodofac RE-610
surfactant (Rhone-Poulenc, Cranbury, N.J.) at 4% by weight of
polymer solids, then addition of aqueous NaOH to a pH of 6.4-6.5.
Aqueous ammonium alginate was then added with thorough mixing in
the range of 0.5% to 1.0% by weight of polymer solids, then the
mixture was allowed to settle at room temperature for 3-7 days
until the creamed latex attained a stable separation. Separated
supernatant water was then siphoned from the top and the creamed
latex was used in various blends as described in the following
examples.
1TABLE 1 Fluoro and Perfluoroelastomer latex materials Dyneon
Tecnoflon Dyneon Dyneon Dyneon Dyneon Dyneon Dyneon Ausimont DuPont
Elastomer L10180 TN-1 latex E14674 E14674-2 E14673 E14734 E6582
E14897 PFR94 VTX-5307 Description terpolymer terpolymer terpolymer
terpolymer terpolymer TFE/P HFP/VF2 Dynamine FFKM terpolymer
TFE/P/VF2 % Fluorine 68 68 68 68 70.2 54 65.9 57 ? NK pH 6.8 5.1
6.6 4.3 4.7 3.8 5.2 3.8 6.5 5.5-6.5 % solids as received 20.5 66.5
20.2 28.3 26.4 24.5 31.6 23.7 about 28% 60-65% viscosity (cp) - 3.4
48.8 3.1 4.3 3.5 <10 4 3.4 4.3 200-400 spindle 1, 60 rpm
specific gravity 1.10 1.42 1.10 1.15 1.15 1.09 1.16 1.106 1.186
1.41 particle size (.mu.) 0.31 irregular 0.34 0.10 0.10 0.11 0.06
0.1 0.05+ about 0.01 0.87 Tg (.degree. C.) -11 -13.2 -15.0 -12.0
-5.2 5.5 -18 -1 -1 NK Creamed solids (%) 69.9 NA 56.0 59.3 51.1 51
54.1 44.8 NA NA NA denotes not applicable NK denotes not known
*Terpolymer of VF.sub.2, HFP, and VF.sub.2 (FKM) P = propylene
EXAMPLE 2
[0072] In preparation for fluoroplastic/fluoroelastomer
dispersions, two ball mill mixtures were prepared. What will be
referred to as the Diak mix was prepared with 100 parts (by weight)
of Diak #3 mixed with 30 parts of a 10% aqueous solution of Daxad
11 (Hampshire Chemical Corp., Deer Park, Tex.), 30 parts of a 10%
aqueous solution of ammonium caseinate (obtained from Technical
Industries, Inc., Peace Dale, R.I.) and 140 parts deionized water.
This was charged to a ball mill with 3/8" ceramic media and was
ground to a Hegman value of 2.5 to 3.0. The Diak is the elastomer
curative and other components are present to effect aqueous
dispersion of the curative.
EXAMPLE 3
[0073] What will be referred to as the Black mix was prepared by
mixing 100 parts (by weight) Thermax Stainless Medium Thermal
Carbon Black ("MT Black") (obtained from R.T. Vanderbilt Co., Inc.,
Norwalk, Conn.), 50 parts of a 10% aqueous solution of Marasperse
N-22 (obtained from Ligno Tech USA, Rothschild, Wis.), 5 parts of a
10% aqueous solution of sodium hydroxide, and 145 parts deionized
water. This was charged to a ball mill with 3/8" ceramic media and
was ground to a Hegman value of 2.5 to 3.0. MT Black is an
elastomer filler and other components are dispersants for the
filler.
EXAMPLE 4
[0074] The first fluoroplastic/fluoroelastomer dispersion was
prepared starting with L-10180 (see Example 1), which was
concentrated to 69.9% solids. With gentle stirring, a 25% aqueous
solution of surfactant Triton X-100 (Union Carbide, Danbury, Conn.,
2 parts dry weight on rubber) and a zinc oxide paste (60% solids,
obtained from Technical Industries, Inc., Peace Dale, R.I., 10
parts dry weight on rubber were mixed with the concentrated
elastomer, along with the Diak mix (Example 2, 5 parts dry weight
on rubber and high molecular weight PTFE (T30B, 60% by weight
polymer solids, from E.I. DuPont de Nemours and Company, Inc.,
Wilmington, Del., 67 parts by dry weight on rubber). The mixture
was vacuum-deaerated for one hour at a rating of 20 inches of
mercury. The resulting mixture had a specific gravity of 1.46 and a
viscosity of 85 centipoise measured at 72.degree. F., using a
Brookfield Model LV viscometer with the number 2 spindle at 60
rpm.
[0075] The composition was vertically cast on a polyester carrier
film (5 mils, ICI Films, Wilmington, Del.) by dipping the carrier
in the fluid at a rate of 2 feet per minute and drying at
195.degree. to 210.degree. F. for a total of 8 minutes per pass in
accordance with the method described in coassigned U.S. Pat. No.
4,883,716, which disclosure is incorporated herein by reference.
The coating thickness on each face of the carrier was 1 mil per
pass. The coated carrier was interleaved with butcher paper in the
take-up roll to accommodate the blocky nature of the dried film on
the carrier. The casting was repeated for a total of five passes
and a total thickness of 5 mils per face. It was necessary to
remove and replace the interleaf with each pass. The film was
flexible and cohesive such that it could be readily stripped from
the carrier in a continuous manner.
EXAMPLES 5-8
[0076] Compositions described in Table 2 below were generated in
similar fashion to that set forth in Example 4. The data for
Example 4 are also shown in the table. The table describes the
formulations in relative dry parts, using the convention of the
elastomer portion fixed at 100 parts. The fluoroelastomer used in
these examples was L10180, the plastic used was high molecular
weight PTFE (T30B).
2TABLE 2 Dispersion Compositions phr - per hundred rubber
(elastomer) Component Example 4 Example 5 Example 6 Example 7
Example 8 Fluoro- 100 100 100 100 100 elastomer PTFE 67 100 0 67 0
Diak #3 5 5 5 5 5 MT Black 0 0 0 17 67 Zinc Oxide 10 10 10 10 10
Triton X-100 2 2 2 2 2 Plastic/ 40/60 50/50 0/100 40/60 0/100
Elastomer Ratio (based on weight)
[0077] In formulations where the Black mix (Example 3) was
incorporated, this was added after Triton and before zinc oxide.
Specific gravity of finished formulations ranged from 1.325 to
1.460. Viscosity of finished formulations ranged from 27 to 90
centipoise, as measured in Example 4. Films were cast according to
the methods of Example 4 at rates ranging from 2 to 5 feet per
minute with a temperature range of 195.degree. F. to 215.degree. F.
Interleaving was required in all examples due to the blocky nature
of the film. All examples were run in multiple passes to a final
film thickness of 5 mil.+-.0.5 mil. All examples stripped from the
carrier maintained integrity as continuous free films.
[0078] Scanning electron microscopy (SEM) of Examples 5, 6 and 7 at
a range of 500 to 10,000.times. magnification showed well-dispersed
zinc oxide as well as dispersed particles of PTFE at a size of 0.2
microns. It should be noted that a composition as high as 100 phr
PTFE (using a standard low molecular weight additive such as DuPont
MP 1000) in fluoroelastomer, such as Example 5, could not be
accomplished with conventional rubber mixing.
EXAMPLE 9
[0079] Compositions in Examples 4-8 were cured by laying up
sufficient plies of the composition to produce a total thickness of
25-40 mils. The dimension of the article was small enough to fit in
one of two Carver presses (Fred S. Carver Inc., Wabash, Ind. Model
M:25 tons, dimensions 9".times.9"; Model CMV50H-13-C:50 tons,
dimensions 18".times.18") used to cure the sheet. Curing conditions
for these five examples were 30 minutes at 300.degree. F. and
100-300 psi. Some later samples were cured for 30 minutes at
350.degree. F. and up to 600 psi, or were cured in a shorter 15
minute cycle at 400.degree. F. Post-curing was conducted in an oven
at 400-450.degree. F. for 24 hours.
EXAMPLE 10
[0080] In this example, tensile characteristics, including tensile
properties at break, tensile set, and stress at elongation, were
measured for each of Examples 4-8. Tensile properties at break are
shown in Table 3, tensile set is shown in Table 4, and stress at
elongation is shown in Table 5. A comparative analysis of the
characteristics is set forth below.
[0081] Die C was used to cut out ultimate tensile and elongation
samples (Table 3) and these pulls were also used to generate stress
at elongation data which did not involve liquid immersion (Table
5). Tensile set tests used 1/2 inch wide rectangular samples which
were at least 6 inches in length. All tensile samples in this group
were cured for 30 minutes at 300.degree. F. and 100 psi, then
post-cured for 24 hours at 400.degree. F.
[0082] Tensile properties (Table 3) of post-cured materials were
measured according to ASTM D412 and were monitored after air aging
at 400.degree. F. as well as after immersion in two chemical
agents, methanol and Reference Fuel C. In general air aging did not
cause drastic loss in tensile properties in any of the five
compositions.
[0083] The most dramatic property loss was exhibited in stress at
elongation (Table 5), resulting from methanol exposure, as expected
from published data of commercial fluoroelastomers exposed to
methanol. Example 8 in Table 5 shows a complete loss of sample
integrity which would not permit tensile testing of the samples of
this composition which had been immersed in methanol. The remaining
four compositions exhibited the most dramatic tensile loss after
methanol exposure with the PTFE-filled compositions (4, 5, 7)
retaining slightly more of their original strength than the
unfilled sample (Example 6).
[0084] Tensile properties at break (Table 3) show that a heavily
carbon black-filled composition (Example 8, Table 3) has a much
higher tensile strength and lower elongation than the other filled
compositions and more closely resembles a plastic. Example 6 in
Table 3 shows unfilled fluoroelastomer to have the most elastomeric
character in terms of ultimate elongation. The examples containing
PTFE (4, 5, and 7), however, all have retained elastomeric
elongation that is much closer to that of the unfilled
fluoroelastomer than Example 8 (a combination elastomer/MT Black
mixture). In fact, Examples 4 and 8 have the same loading (67 parts
per hundred rubber) of their respective fillers, PTFE and carbon
black, yet the first is still elastomeric and the second is quite
stiff and plastic.
3TABLE 3 Tensile Properties at Break Property Units Example 4
Example 5 Example 6 Example 7 Example 8 Plastic/Elastomer polymer
wt. % 40/60 50/50 0/100 40/60 0/100 Plastic filler phr 67 100 0 67
0 MT Black filler phr 0 0 0 17 67 Ultimate Tensile Strength psi
1227 827 2828 1073 1970 Change after 48 hrs. @ 400.degree. F. % of
original 105 103 112 97 86 Change after 96 hrs. @ 400.degree. F. %
of original 91 86 100 72 100 Change after 192 hrs. @ 400.degree. F.
% of original 104 79 95 77 87 Ultimate Elongation % 381 317 440 251
108 Change after 48 hrs. @ 400.degree. F. % of original 99 94 103
91 86 Change after 96 hrs. @ 400.degree. F. % of original 90 103
105 50 109 Change after 192 hrs. @ 400.degree. F. % of original 105
99 99 62 98
[0085] Tensile set (Table 4) is a measure of elastic recovery. At
50% elongation, all compositions exhibit a tensile set of less than
10% and therefore good elastic recovery. At 100% elongation all
filled compositions (Examples 4, 5, 7, 8) exhibit a tensile set
from 14 to 16%, while the unfilled elastomer still exhibits a value
less than 10%. At an elongation of 200% the heavily carbon
black-filled composition (Example 8) will not survive such an
elongation and breaks, while compositions filled with plastic to
comparable levels and higher (Examples 4, 5, 7) retain elastomeric
recovery. It is expected that an unfilled elastomer will exhibit
greater recovery than a companion sample with filler, as seen in
Table 4 (Example 6). Air aging did cause increases in tensile set
(Table 4), indicating loss of elastomeric recovery upon air aging,
again for all five compositions. Increase in tensile set upon air
aging represents continued curing in these samples. This is a
courtology trend based on the state of elastomer and cure. The air
aging data for the tensile set of these samples do not show clear
trends based on filler levels.
4TABLE 4 Tensile Set Property Units Example 4 Example 5 Example 6
Example 7 Example 8 Plastic/Elastomer polymer wt. % 40/60 50/50
0/100 40/60 0/100 Plastic filler phr 67 100 0 67 0 MT Black filler
phr 0 0 0 17 67 Tensile Set @ 50% elongation % 7 8 3 8 5 Change
after 48 hrs. @ 400.degree. F. % of original 115 94 167 ND 107
Change after 96 hrs. @ 400.degree. F. % of original 99 87 162 172
153 Change after 192 hrs. @ 400.degree. F. % of original 110 125
187 95 141 Tensile Set @ 100% elongation % 15 16 8 15 14 Change
after 48 hrs. @ 400.degree. F. % of original 136 125 121 ND ND
Change after 96 hrs. @ 400.degree. F. % of original 109 103 126 116
ND Change after 192 hrs. @ 400.degree. F. % of original 140 125 145
124 101 Tensile Set @ 200% elongation % 35 44 15 37 NE Change after
48 hrs. @ 400.degree. F. % of original 112 94 112 ND NE Change
after 96 hrs. @ 400.degree. F. % of original 88 82 116 101 NE
Change after 192 hrs. @ 400.degree. F. % of original 122 113 162
111 NE ND denotes no data available NE denotes that the material
will not elongate to that percent
[0086]
5TABLE 5 Stress at Elongation Property Units Example 4 Example 5
Example 6 Example 7 Example 8 Plastic/Elastomer polymer wt. % 40/60
50/50 0/100 40/60 0/100 Plastic filler phr 67 100 0 67 0 MT Black
filler phr 0 0 0 17 67 Stress @ 50% elongation psi 454 507 331 645
1750 Change after 48 hrs. @ 400.degree. F. % of original 92 95 92
90 100 Change after 96 hrs. @ 400.degree. F. % of original 101 73
94 90 99 Change after 192 hrs. @ 400.degree. F. % of original 95 69
85 93 89 Stress @ 100% elongation psi 472 499 451 705 1959 Change
after 48 hrs. @ 400.degree. F. % of original 94 98 93 95 ND Change
after 96 hrs. @ 400.degree. F. % of original 102 73 95 95 98 Change
after 192 hrs. @ 400.degree. F. % of original 96 71 96 97 84 After
70 hrs MeOH @ RT % of original 51 48 55 56 NE After 70 hrs MeOH @
50.degree. C. % of original 56 48 54 54 NE After 70 hrs Ref Fuel C
@ RT % of original 70 60 84 78 58 After 70 hrs Ref Fuel C @ % of
original 60 54 64 56 43 50.degree. C. Stress @ 200% elongation psi
611 578 806 926 NE Change after 48 hrs. @ 400.degree. F. % of
original 99 105 97 103 NE Change after 96 hrs. @ 400.degree. F. %
of original 103 78 96 99 NE Change after 192 hrs. @ 400.degree. F.
% of original 96 75 106 102 NE ND denotes no data available NE
denotes that the material will not elongate to that percent
EXAMPLE 11
[0087] In this example, the swelling effects of two chemical agents
were determined according to ASTM D471 using methanol and Reference
Fuel C at room temperature and at 50.degree. C. Compositions were
tested in triplicate for 70 hours using a sample size of
1".times.2" by 20-35 mils thick. All swell samples were cured for
30 minutes at 300.degree. F. and 100 psi, then post-cured for 24
hours at 400.degree. F. Data are expressed in volume percent
increase over the original dimensions. Samples air-aged at
400.degree. F. for 48, 96, and 192 hours were also subsequently
subjected to these immersion tests, as shown in Table 6.
Improvements in volume swell after air aging have occurred due to
completion of cure in these samples. Later samples subjected to
these tests were cured at 350.degree. F. to improve cure and test
results. In general, the unfilled 100% fluoroelastomer sample
(Example 6) was the most dramatically swollen in these tests, as
expected. Samples filled with PTFE (Example 4, 5, and 7) swelled
less than the unfilled elastomer and to a similar degree. The
sample filled with carbon black only (Example 8) swelled the least.
While low swelling is desirable, it is important that the
composition also retain elastomeric properties, which has been
discussed with reference to Example 8 in Tables 3, 4, and 5.
6TABLE 6 Swelling Due to Liquid Immersion Property Units Example 4
Example 5 Example 6 Example 7 Example 8 Plastic/Elastomer polymer
40/60 50/50 0/100 40/60 0/100 wt. % Plastic filler phr 67 100 0 67
0 MT Black filler phr 0 0 0 17 67 Volume swell in MeOH @ % 12.95
11.38 18.12 12.82 3.14 Room Temperature after 48 hrs. @ 400.degree.
F. % 10.70 10.27 15.30 10.88 8.01 after 96 hrs. @ 400.degree. F. %
11.23 9.53 19.93 11.15 7.48 after 192 hrs. @ 400.degree. F. % 10.69
10.08 20.77 11.35 7.08 Volume swell in MeOH @ 50.degree. C. % 12.52
11.63 16.91 12.76 12.69 after 48 hrs. @ 400.degree. F. % 11.45
10.45 16.73 11.31 9.44 after 96 hrs. @ 400.degree. F. % 11.79 10.28
19.71 11.83 8.81 after 192 hrs. @ 400.degree. F. % 12.01 10.70
20.28 11.20 8.71 Volume swell in Fuel C @ % 6.84 10.12 4.9 6.03
10.52 Room Temperature after 48 hrs. @ 400.degree. F. % 2.78 3.31
3.26 2.50 2.45 after 96 hrs. @ 400.degree. F. % 2.96 2.52 4.71 2.85
2.25 after 192 hrs. @ 400.degree. F. % 2.75 2.94 4.80 2.88 1.71
Volume swell in Fuel C @ % 12.08 11.44 14.74 10.16 8.33 50.degree.
C. after 48 hrs. @ 400.degree. F. % 9.74 8.49 14.65 9.04 6.46 after
96 hrs. @ 400.degree. F. % 9.59 8.24 15.33 8.77 6.14 after 192 hrs.
@ 400.degree. F. % 9.28 8.02 14.98 8.88 5.41
EXAMPLE 12
[0088] For the purpose of comparing the present compositions to a
conventionally prepared PTFE-filled fluoroelastomer, a sample of
conventionally compounded Viton B terpolymer (HFP/VF.sub.2/TFE,
from E.I. DuPont de Nemours and Company, Inc., Wilmington, Del.)
from Rainbow Master Mixing (Akron, Ohio) was obtained. The
conventional sample was formulated to contain 72% by weight Viton
B, 21.6% by weight MP1000 (PTFE micropowder from E.I. DuPont de
Nemours and Company, Inc., Wilmington, Del.), 2.2% by weight zinc
oxide, 1.8% by weight Diak #7 (polyfunctional triazine coagent
useful in peroxide cure, manufactured by E.I. DuPont de Nemours),
1.8% Varox DBPH 50 (peroxide curative, R.T. Vanderbilt Co.,
Norwalk, Conn.), and 0.007% by weight carnauba wax (a processing
aid). The material was received premixed and was freshened on a two
roll rubber mill before curing in a 6".times.6".times.75 mil mold
for 15 minutes at 350.degree. F. at 555 psi. The sample was
post-cured at 450.degree. F. for 23 hours.
EXAMPLE 13
[0089] In this example, wear and abrasion testing was performed on
a model LRI-1a tribometer (Lewis Research, Inc., Lewes, Del.),
using the elastomer sample as the rotating specimen and a
stationary stainless steel thrust bearing as the wear surface. The
instrument monitors thickness changes, dynamic coefficient of
friction, and temperature at 4-7 minute intervals based on test
duration. The velocity was fixed at 10 ft/min throughout all tests,
while the pressure range was from 30 to 100 psi for different
tests. The increasing pressure is reflected in the data found in
Tables 7 through 10 (where the pressure used was 30, 50, 75, and
100 psi, respectively). With all other test conditions constant,
raising the pressure applied to the sample rendered the unfilled
composition (Example 6) incapable of enduring the test. The sample
became totally deformed at higher pressure. Filled samples were run
successfully up to 100 psi, with the two Chemfab compositions
testing similarly through 75 psi. At 100 psi the PTFE/Black filled
composition (Example 7) exhibited a lower wear rate as well as a
much lower coefficient of friction than the more conventionally
formulated composition (Example 12).
7TABLE 7 Wear and Abrasion Testing At 30 psi (PV = 300 psi-ft/min)
Property Units Example 4 Example 6 Example 7 Example 12
Plastic/Elastomer polymer wt. % 40/60 0/100 40/60 23/77 Plastic
filler phr 67 0 67 30 MT Black filler phr 0 0 17 0 coefficient of
friction 0.409 0.871 0.342 ND Wear Rate .times.10.sup.-7 in/min
0.79 50.3 0.44 ND Wear Factor 158 10,100 88 ND Total Wear mils 1.9
22.8 1.0 ND Duration hours 167 96 167 ND Temperature .degree. F.
83.6 83.9 75.5 ND ND denotes no data
[0090]
8TABLE 8 Wear and Abrasion Testing at 50 psi (PV = 500 psi-ft/min)
Property Units Example 4 Example 6 Example 7 Example 12
Plastic/Elastomer polymer wt. % 40/60 0/100 40/60 23/77 Plastic
filler phr 67 0 67 30 MT Black filler phr 0 0 17 0 coefficient of
friction 0.479 FT 0.418 ND Wear Rate 10.sup.-7 in/min 1.4 FT 0.54
ND Wear Factor 170 FT 64 ND Total Wear mils 0.5 FT 0.2 ND Duration
hours 50 FT 50 ND Temperature .degree. F. 88.5 FT 81.1 ND FT
denotes sample failure at these test conditions ND denotes no
data
[0091]
9TABLE 9 Wear and Abrasion Testing at 75 psi (PV = 750 ps-ft/min)
Property Units Example 4 Example 6 Example 7 Example 12
Plastic/Elastomer polymer wt. % 40/60 0/100 40/60 23/77 Plastic
filler phr 67 0 67 30 MT Black filler phr 0 0 17 0 coefficient of
friction 0.589 FT 0.504 ND Wear Rate 10.sup.-7 in/min 3.9 FT 1.8 ND
Wear Factor 314 FT 144 ND Total Wear mils 3.2 FT 0.9 ND Duration
hours 120 FT 94 ND Temperature .degree. F. 95.8 FT 87.0 ND FT
denotes sample failure at these test conditions ND denotes no
data
[0092]
10TABLE 10 Wear and Abrasion Testing at 100 psi (PV = 1,000
psi-ft/min) Property Units Example 4 Example 6 Example 7 Example 12
Plastic/Elastomer polymer wt. % 40/60 0/100 40/60 23/77 Plastic
filler phr 67 0 67 30 MT Black filler phr 0 0 17 0 coefficient of
friction 0.47 FT 0.547 1.094 Wear Rate 10.sup.-7 in/min 37.1 FT 7.2
9.8 Wear Factor 2,230 FT 433 590 Total Wear mils 3.3 FT 7.8 5.8
Duration hours 12 FT 287 68 Temperature .degree. F. 97.0 FT 101.2
121.4 FT denotes sample failure at these test conditions
[0093] Additionally, a group of samples was prepared in order to
make direct comparisons of high molecular weight PTFE versus low
molecular weight PTFE at comparable levels in similar elastomers
prepared via casting or conventional milling. The data for these
samples are detailed in Table 10A. Control samples of FKM elastomer
alone (E14674 for cast samples and FT2481 gum for milled samples,
as described in Example 17) as well as samples with a 10 weight
percent loading of PTFE were prepared. The high molecular weight
PTFE used was FLUON AD1LN (ICI Americas, Bayonne, N.J.) and the low
molecular weight PTFE used was DuPont MP1000. Cast samples were
prepared as detailed in Example 4, while milled samples were
prepared on a two roll mill at Akron Rubber Development Laboratory.
All samples were cured with the same relative loading of DIAK #3
and zinc oxide as the acid acceptor. All samples were cured for
one-half hour at 350.degree. and 500 psi and post-cured at
400.degree. F. for 23 hours.
[0094] The two control samples lacking PTFE (Sample 1 and Sample 2)
exhibited similar wear rates and derived wear factors. The
coefficient of friction for the cast sample, however, was slightly
lower than that of the milled sample. Upon addition of PTFE in
either high molecular weight (Sample 3) or low molecular weight
(Sample 4) form, the coefficient of friction dropped, with the cast
sample lower again than the milled sample, 0.761 vs. 0.921,
respectively. Despite the addition of the same level of PTFE,
however, the wear rates of the two were dramatically different.
While addition of low molecular weight PTFE (Sample 4) improved the
wear rate by a factor of 5 relative to the 100% elastomer control,
the addition of high molecular weight PTFE (Sample 3) yielded a
better than 6-fold improvement in wear rate over the low molecular
weight PTFE, or about a thirty-fold improvement relative to the
unfilled elastomer.
11TABLE 10A Wear and Abrasion Testing at 100 psi Sample 3 Sample 4
(high molecular (low molecular Property Units Sample 1 Sample 2
weight PTFE) weight PTFE) Plastic/Elastomer wt. % 0/100 0/100 10/90
10/90 Sample Isolation cast milled gum cast milled gum coefficient
of friction 1.111 1.296 0.761 0.921 Wear Rate 10.sup.-7 in/min 815
877 25 169 Wear Factor 48,900 52,600 1,500 10,100 Total Wear mils
20 27.8 16.9 27.4 Duration hours 3.2 1.9 96 6.3 Temperature
.degree. F. 113.1 122.4 107.0 112.5
EXAMPLE 14
[0095] SEM was performed on various examples set forth above.
Post-cured samples of Examples 5, 6, and 7 were freeze-fractured
using liquid nitrogen in order to expose a fresh surface for
analysis, then coated prior to imaging in the SEM. Example 6 was
included for the purpose of comparison to the two PTFE-containing
samples. Magnification to 10,000.times. showed individual PTFE
particles (typically around 0.2 microns) and showed no evidence of
widespread fibrillation of the PTFE. These samples have been cast,
layered, cured under pressure, and post-cured without causing
fibrillation of the PTFE particles within the fluoroelastomer
matrix. This is important when comparing the state of high
molecular weight PTFE in these examples to the state of such PTFE
in a milled form, such as in Example 17.
EXAMPLE 15
[0096] FIG. 1 shows a scanning electron micrograph of a Chemfab
PTFE laminate. This figure has been included for the purpose of
visually illustrating PTFE fibrillation, which appears as the
winding rooting fibers in the figure. The SEM sample was prepared
in the same manner noted in Example 14. The laminate was prepared
by a process with induced shear as described in U.S. Pat. No.
5,141,800.
EXAMPLE 16
[0097] Table 11 describes samples that were evaluated using a
Tinius Olsen melt indexer, which is designed to measure the flow
rate of visco-elastic fluids (such as molten resins) in accordance
with standard methods as defined by the American Society for
Testing and Materials (ASTM) D 1238. This evaluation was performed
in order to gauge better the subsequent processibility of these
compositions. All experimental runs, with the exception of runs 7
and 17, employed films cast from aqueous dispersions. Run 17 used a
sample of PTFE and THV that was isolated by co-coagulation and air
drying (see Example 22B below). Run 7 used a sample of conventional
Viton composition with peroxide cure, which was freshened on a two
roll mill. Runs 16, 18-20, 25-27, 29, and 30 used films which were
cast by hand. Such films were created by dipping a small piece of
Melinex (generally about 3".times.4") in the casting fluid and
drying in an oven at 200.degree. F. for 1 minute. This dipping and
drying was repeated until a final thickness of 5 mm was reached.
Such hand samples were exposed to this drying temperature for a
shorter time than films derived from the process detailed in
Example 4. This is important when interpreting the data in this
table, since hand samples are less cured than samples prepared on
larger equipment such as that described in Example 4. A less cured
elastomer will exhibit different flow characteristics than one
which is further cured (Table 12). All remaining melt indexer runs
used film generated on pilot equipment as was detailed in Example
4.
[0098] The shear stress calculated for the device set up as
indicated in Table 11 was 23.8 psi, a very low pressure when
compared to typical elastomer or plastic processing. The shear rate
was under 10/sec, a value comparable to that employed in
compression molding operations.
[0099] Table 11 indicates the weight percent of plastic and
elastomer as well as the sources of these components: T30B (PTFE)
and T121A (FEP) are products of E.I. DuPont de Nemours &
Company, Inc., Wilmington, Del., D6027 (PTFE) is a product of
Ausimont USA, Thorofare, N.J. Samples that did not contain added
curative are indicated by underlining the run number and sample
composition values. Samples were charged to the melt indexer
chamber and preheated at the indicated temperature for the
indicated length of time. At the end of the preheat time the
indicated weight was applied to induce flow of the sample. The
times at which flow then began and ended (where applicable) are
indicated in the table, as well as the flow rate as determined by
the weight of sample collected over even time intervals.
[0100] The three fluoroelastomer resins run through the melt
indexer as cast film, without curative, all exhibited different
flow rates, with TN-1 latex the fastest (run 29), E14673
intermediate (run 30), and L10180 the slowest (runs 18, 19). Rates
for PTFE isolated as a cast film without curative components
differed depending upon source. Neither T30B (run 20) nor D6027
(run 27) flowed upon application of the 18 kg weight. The T30B,
however, could be hand-cranked through the melt indexer (at an
unknown pressure and stress), while the D6027 could not be forced
through. SEM of the T30B hand-cranked sample along the axis of
extrusion shows the dramatic fibrillation which is possible with
the higher molecular weight PTFE which has been used in these
experiments (FIGS. 2 and 3). This fibrillation does not occur with
the very low molecular weight PTFE micropowders, such as MP1000,
which are generally added to currently available commercial
PTFE-containing FKM (as defined by the ASTM-D1418) compounds
(Example 12).
[0101] Curing is occurring in samples with Diak #3, as compared to
those without, even at the modest pressure and temperature of these
melt indexer runs. A 30% PTFE/10% FEP/60% FKM composition with Diak
#3 (run 8) ceased to flow at 14 minutes into the test, whereas the
comparable composition without Diak #3 (run 9) flowed sooner, and
beyond the 14 minute mark. Similarly a 40% PTFE/60% FKM composition
containing Diak #3 (run 23) exhibited a lower flow rate than the
comparable composition without Diak #3 (run 16).
[0102] FEP appears to behave as a process aid and appears to have
an optimum amount with regard to maximizing flow. A composition of
40% FEP/60% FKM with or without curative (runs 14 and 21) did not
exhibit flow under the test conditions. A 10% PTFE/30% FEP/60% FKM
under the same conditions (run 13) had a very modest flow of 0.01
g/min, while the 20% PTFE/20% FEP/60% FKM analog (run 10) had a
flow of 0.06 g/min. Comparing the behavior of two hand samples, the
composition of 5% FEP/35% PTFE/60% FKM (run 26) exhibited a flow
rate three times that of a composition of 1% FEP/39% PTFE/60% FKM
(run 25).
[0103] Table 12 illustrates how variable processing heat history
contributed to varying degree of cure as measured by melt flow in
the melt indexer. Hand samples were exposed to less cumulative heat
than pilot samples and even when comparing such samples in the
"green" state (before press curing), they are different in terms of
degree of cure. The table is grouped by composition and in all
cases when comparing a hand versus a pilot sample, the flow rate
was higher for the hand sample, or was evident in the hand sample,
and non-existent in the pilot sample (runs 35 vs. 8, 36 vs. 10, 37
vs. 14, 38 vs. 23, 39 vs. 12).
12TABLE 11 Evaluation of Compositions via Melt Indexer Compositions
are by Weight Percent ZAK Applied Preheat Flow Free Flow Run Hand
vs. PTFE PTFE FEP FKM FKM FKM 5050 Rainbow Temp Weight Time Rate
Window (3) # Pilot T30B D6027 T121A L10180 TN-1 E14673 (1) (2)
(.degree. F.) (kg) (min) (g/min) (min) 1 P 100 275 4.9 7 NA NA 2 P
40 60 311 9.8 7 NA NA 3 P 40 60 350 9.8 5 NA NA 4 P 40 60 350 18 7
NA NA 5 P 40 60 350 18 7 NA NA 6 P 40 60 220 18 7 NA NA 7 Other 100
220 18 7 0.07 11-13 8 P 30 10 60 220 18 7 0.07 8-14 (4)9 P 30 10 60
220 18 7 0.07 7.5-14+(5) 10 P 20 20 60 220 18 5 0.06/(6) 6-11.5 11
P 40 60 220 18 5 NA NA 12 P 40 60 220 18 5 0.06 6-18.5+ 13 P 10 30
60 220 18 5 0.01 9-11.5 14 P 40 60 220 18 5 NA NA 15 P 40 60 220 18
5 0.03 7-11 16 H 40 60 220 18 5 0.15 6-25+ 17 Other 100 220 18 5 NA
NA (7)18 H 100 220 18 5 0.01 7.5-19.5+ 19 H 100 220 18 5 0.01
5.5-20+ 20 H 100 220 18 5 NA NA Applied Preheat Flow Free Flow Run
PTFE PTFE FEP FKM FKM FKM MT Carbon Temp Weight Time Rate Window #
T30B D6027 T121A CF6200 TN-1 E14673 Black (.degree. F.) (kg) (min)
(g/min) (min) 21 P 40 60 220 18 5 NA NA 22 P 40 60 10 220 18 5 NA
NA 23 P 40 60 220 18 5 0.04 5.5-15.5 24 P 70 30 220 18 5 NA NA 25 H
39 1 60 220 18 5 0.25 5.0-30.0 (8)26 H 35 5 60 220 18 5 0.77 5-18
27 H 100 220 18 5 NA NA 28 P 60 40 220 18 5 NA NA (9)29 H 100 220
18 5 .21 5.0-25.0+ 30 H 100 220 18 5 0.04 7.0-25.0+ NA denotes no
flow observed (1)Co-precipitate of THV and PTFE, both raw
dispersions (Example 22B) (2)Peroxide cured terpolymer containing
30 phr of MP1000 PTFE (Example 12) (3)Free flow after indicated
preheat, once weight was applied (4)All underlined compositions
indicate curative was not added (5)+ indicates sample was still
flowing when weights were removed (6)Forced flow at variable rate
(7)Zinc oxide was deleted from this formulation as well, harder to
remove from chamber than sample with ZnO, #19. (8)Flowed extremely
well, only material which completely emptied out of core chamber
(9)Flowed as soon as weights were applied, and rate increased with
time
[0104]
13TABLE 12 Comparison of Hand and Pilot Samples - Grouped by
Composition Melt Index Experiments - 18 kg applied weight,
220.degree. F., 5 minute preheat Compositions are by Weight Percent
Run # THV THV Flow Rate Free Flow RLS4-1 Process T30B D6027 T121A
350C 530R(1) CF6200 TN-1 (g/min) Window (min) Comments 31 H 20 20
60 0.15 5-20+ 32 H 40 60 2.64 5-8(3) 33 H 35 5 60 0.63 5-17(3) 34 H
40 60 0.07 5-19 35 H 30 10 60 0.11 5-20+ 8 P 30 10 60 0.07 8-14 7
min preheat 9 P 30 10 60 0.07 7.5-14+ 7 min preheat 36 H 20 20 60
0.07 5-16 10 P 20 20 60 0.06 6-11.5 37 H 40 60 0.03 5-14 14 P 40 60
NA NA 21 P 40 60 NA NA 38 H 40 60 0.15 5-20 23 P 40 60 0.04
5.5-15.5 16 H 40 60 0.15 6-25+ 39 H 40 60 0.86 5-14(3) 12 P 40 60
0.06 6-18.5+ (1)THV 530R is raw 30% solids dispersion with melt
temperature range 302-356.degree. F. (2)+ indicates sample was
still flowing when weights were removed (3)All material flowed,
chamber emptied
EXAMPLE 17
[0105] In order to highlight the potential difference in these
systems between high and low molecular weight PTFE, that is,
fibrillatable and non-fibrillatable PTFE, as well as conventional
methods versus the present methods of processing, the following set
of experiments, represented as Examples A-D in Table 13 below, were
performed using these two styles of PTFE and holding the other
ingredients as constant as possible. Because conventional milling
was used for half of these samples, a relatively modest loading of
30 phr PTFE was chosen. The low molecular weight PTFE was MP1000.
The high molecular weight PTFE was AD1LN (61% solids aqueous
dispersion, ICI Americas, Exton, Pa.) or Fluon CD123 (PTFE molding
powder, ICI Americas, Exton, Pa.). The FKM fluroelastomer used was
E14674 for film casting (Example 1) and the gum version of that
dispersion, FT2481 (Dyneon), was used for milling. Process aids
were specific to either film casting or conventional milling.
Triton X-100 was used in casting (25% aqueous solution made from
concentrate, Union Carbide, Danbury, Conn.), while standard
carnauba wax was used in milling. Milling was performed (Rainbow
Master Mixing, Inc., Akron, Ohio) on a two roll mill starting at
room temperature. The carnauba wax was incorporated first, followed
by the PTFE. Curatives were not used in this set of examples.
[0106] MP1000 low molecular weight PTFE incorporated easily into
the FT2481 in Example C. When CD123 high molecular weight PTFE was
added to the elastomer, for Example D on the mill, it caused the
banded elastomer to break up and interfered with normal sheeting on
the rollers. The mixture became warm to the touch and became
stiffer the longer the matrix was processed. Upon completion of
milling, Example C was a normal, smooth, flat slab of well-mixed
elastomer, while Example D containing high molecular weight PTFE
was not smooth, but rather dramatically irregular, with a knotted,
striated texture. This was expected behavior from a PTFE of
fibrillatable molecular weight and was a dramatic example of why
fibrillatable PTFE is not a practical additive in a conventional
dry mix process which introduces substantial shear. (Compare this
to isolation of high molecular weight PTFE in FKM discussed in
Example 14.)
[0107] Formulation and film casting of Example B were similar to
that described in Example 4 above. Formulation of Example A was
complicated by the fact that MP1000 is a powder, not an aqueous
dispersion. A castable fluid was prepared from 43% by weight
MP1000, 55% by weight deionized water, 0.2% by weight active
Fluorad FC118 surfactant (20% aqueous solution, 3M, St. Paul,
Minn.) and 2.3% by weight active Triton X-100 (25% aqueous
solution). These were charged to a vessel sitting in a constant
temperature water bath and mixed with an overhead paddle stirrer
while bringing the mixture up to 75.degree. C. (above the
64.degree. C. cloud point of Triton X-100). This slurry was then
cooled back down to room temperature while continuing stirring. The
cooled slurry was then passed through a Microfluidizer.RTM. (Model
M110-F, Microfluidics Corporation, Newton, Mass.) at 8,000-12,000
psi resulting in a stable aqueous suspension of MP1000 (42.9%
solids). The Microfluidizer.RTM. was held in an ice-water bath in
order to maintain the low temperature of the slurry while
processing. At this point, Example A was then formulated similar to
Example B in combining aqueous ingredients and the film was also
cast in the fashion described in Example 4 above.
14TABLE 13 Example A B C D Processing cast cast milled milled
Fluoroelastomer E14674 E14674 FT2481 FT2481 PTFE MP1000 30 phr
AD1LN 30 phr MP1000 30 phr AD1LN 30 phr Process aid Triton X-100
Triton X-100 camauba wax carnauba wax 2 phr 2 phr 1 phr 1 phr
Condition of isolate smooth film smooth film smooth slab textured
slab
[0108] Important again is the condition of the PTFE in the
isolation process. While low molecular weight PTFE can be used as
an additive in conventional milling as well as in the present
methods of processing, without a change in the physical state of
the PTFE, such is not the case with high molecular weight PTFE. The
disclosed methods of isolation enable the isolation of PTFE in an
elastomer matrix in an unfibrillated, but fibrillatable state.
EXAMPLE 18
[0109] Uncured fluoroelastomer of the type used in Example 17 is
soluble in solvents such as methylethylketone (MEK). Solvation of
uncured elastomer has been used in order to ascertain the condition
of PTFE in these mixtures since PTFE is not soluble in such
solvents.
[0110] Cast films (Examples 17A and 17B) containing low and high
molecular weight PTFE, respectively, were milled in order to
illustrate the transition from fibrillatable, but unfibrillated, to
fibrillated PTFE. Milling was performed (Akron Rubber Development
Laboratories, Inc., Akron, Ohio) on a two roll mill preheated to
150.degree. F. Once again, the composition containing MP1000 low
molecular weight PTFE easily knitted together and banded on the
rolls to produce a smooth-textured slab, similar to the sample
which was originally mixed on the mill (Example 17C). Sample 17B,
containing high molecular weight PTFE, toughened and heated up
while processing, and was in general hard to sheet out due to its
rough texture. Once completed, the milled version of 17B appeared
the same as sample 17D, although 17B was originally isolated as a
smooth 5 mil film, containing unfibrillated PTFE.
[0111] Samples 17A and 17B, as well as their milled counterparts,
were then suspended in stirring, warm MEK (60.degree. C.) for 4
hours in order to dissolve FKM elastomer, which was not cured in
these examples. Three samples disintegrated totally due to this
treatment: 17A, and its milled counterpart, as well as 17B. The
fluoroelastomer dissolved in the MEK and the PTFE, thus liberated
from the matrix, settled out as a fine precipitate, whether it was
low molecular weight PTFE (17A and 17A milled), or high molecular
weight PTFE (fibrillatable) isolated in the matrix in a low shear
process (but not fibrillated, 17B). The milled version of 17B,
which exhibited a very rough texture, retained some of its original
weight, while also retaining the original textured
three-dimensional shape. A later extraction of the milled version
of 17B in room temperature MEK after 6 days resulted in loss of two
thirds of the original weight, but retention of the original
textured three-dimensional structure. Thermal gravimetric analysis
(TGA) of the undissolved third revealed a sample comprised of 77%
PTFE and 23% FKM by weight, a reversal of the original 23% PTFE/77%
FKM. Unlike the low molecular weight PTFE examples, or the
unfibrillated high molecular weight PTFE example, this high
molecular weight PTFE example has retained a definite
three-dimensional structure, despite salvation of 90% of the
surrounding elastomer, due to PTFE fibrillation. SEM photographs of
this milled film (17B) extracted in MEK, (FIGS. 4A-4L) show a
tangled and complex network of PTFE fibrils, along with some of the
remaining elastomer, consistent with what was observed with the
naked eye.
EXAMPLE 19
[0112] Samples generated in Example 17 were processed through an
Instron Model 3213 capillary rheometer at modest temperature
(80-120.degree. C.). Capillaries used ranged from 50 to 60 mils
with a length to diameter ratio range of 10/1 to 40/1. FIG. 5 shows
the log shear rate versus log shear stress for samples A-D at
100.degree. C. through a 60 mil 10/1 L/D capillary. Crosshead speed
ranged from 0.2 to 20 inches per minute.
[0113] As seen in FIG. 5, cast films containing both low and high
molecular weight PTFE exhibited the same rheological behavior under
these conditions. The high molecular weight PTFE has been isolated
in a form which is fibrillatable, but not yet fibrillated (this
sample was later fibrillated by milling in Example 18). Samples of
the cast films which were run at a crosshead speed of 20"/min. were
subjected to a shear stress of 154 psi and shear rate of about
2600/sec (mild conditions compared to PTFE paste extrusion). These
two extrudates were extracted in MEK to remove fluoroelastomer and
analyzed by SEM. FIGS. 6A-6D are scanning electron micrographs of a
high molecular weight PTFE-containing sample showing the beginnings
of fibrillation along the axis of extrusion, as well as overall
particle elongation, due to the shear of the rheometer. FIGS. 7A-7D
are scanning electron micrographs of low molecular weight
PTFE-containing sample showing round particles, as expected for
this control of non-fibrillatable PTFE.
[0114] The milled samples in FIG. 5 exhibit very different
rheological behavior based on the condition of PTFE in these
samples. As seen in the figure, the high molecular weight PTFE
sample generated through milling experienced shear, sufficient to
result in fibrillated PTFE in the matrix. Passing such a sample
through the rheometer resulted in a higher shear stress for any
given shear rate, holding all other conditions constant. In fact,
this milled sample which contained fibrillated PTFE could not be
passed through the rheometer at the higher crosshead speeds due to
the pressure generated by the sample and the pressure limit of the
system. FIG. 5 also shows that the milled sample containing low
molecular weight PTFE, by contrast, could be passed through the
capillary rheometer even at the highest crosshead speed.
EXAMPLE 20
[0115] A cast film sample (3.8 pounds) containing high molecular
weight PTFE (40 weight %), fluoroelastomer (60 weight %), zinc
oxide, and Diak #3 curative similar to that described in Example 4
was sent to Akron Rubber Development Laboratory for milling.
Milling was performed for a half hour at a beginning roll
temperature of 90.degree. F., ending roll temperature of
162.degree. F. and a material temperature of 187.degree. F. The
material was consistently fed to the rolls in the same direction,
resulting in a long thin strip. Normally, milled samples are folded
and rotated to promote good mixing of components. The purpose of
this milling, however, was not to mix (the cast film already
contained well-dispersed ingredients prior to milling), but rather
to induce shear in the matrix in a constant uniaxial direction.
This was done to controllably fibrillate the PTFE dispersed in the
sample in the direction of the milling. Wide angle X-ray scattering
(described in Example 32) of this uniaxially-milled sample clearly
showed orientation of the crystalline (PTFE) portion of the sample.
By contrast, the milled sample described in Example 18 (17B milled)
did not show orientation by wide angle X-ray scattering. This
sample was fibrillated, as clearly seen with the unaided eye as
well as by SEM, but the fibrillation was random not oriented. In
this instance (Example 20), the orientation of the PTFE
fibrillation has been controlled by controlling the milling
direction, as verified by wide angle X-ray scattering. Controlling
the fibrillation of PTFE dispersed in elastomer would be of benefit
in some applications of such a blend.
EXAMPLE 21
[0116] A series of compositions which ranged from 100%
fluoroelastomer to 80% PTFE/20% fluoroelastomer in 10% increments
was generated in the manner of Example 4. The high molecular weight
PTFE used was AD1LN (61% solids, ICI, Bayonne, N.J.) and the
fluoroelastomer used was E14674, described in Example 1. All
compositions were isolated as a free, manageable film and
surprisingly, samples containing as much as 80% by weight unfused
and unoriented PTFE could be handled as free film. A set of control
samples using a comparable gum elastomer and low molecular weight
PTFE (MP1000) was also prepared via milling. These controls ranged
from 100% elastomer to 30% PTFE/70% FKM. The cast samples, which
averaged 5 mils in gauge, were plied to approximately 30 mils. Both
cast and milled samples were cured in a mold at 350.degree. F. for
1/2 hour at 555 psi, then post-cured for 22 hours at 400.degree.
F.
[0117] These materials were tensile-tested according to ASTM D412
using die C and the results are shown in FIGS. 8 and 9. In FIG. 8
the room temperature tensile strength data is displayed for all
thirteen compositions. Overall there is a trend towards lower
tensile strength with increasing PTFE loading. In the case of the
four pairs of samples which compare cast and milled compositions
and, therefore, high and low molecular weight PTFE, the cast
compositions exhibit a higher tensile strength in all but the last
of the pairs (30/70). The room temperature tensile pulls of samples
from 100% FKM through 50 PTFE/50 FKM exhibit a classic elastomeric
curve; that is, increasing tensile strength at a relative constant
slope until break, with the maximum strength at break. Tensile
pulls of 60/40, 70/30, and 80/20 PTFE/FKM exhibit a classic plastic
curve with a maximum strength, a drop in strength to some plateau,
then eventual break at the end of the plateau, with the maximum
strength occurring not at the break, but early in the pull. For
this reason it is more accurate to describe the tensile strength of
such a mixture of samples as peak strength, rather than strength at
break. It would be expected that this transition from elastic to
plastic behavior would be present in other material combinations,
but as the starting materials are varied it could occur at a
different relative composition.
[0118] In FIG. 9 room temperature peak strain and modulus at 100%
elongation are displayed for all thirteen samples. The data show
that in cast compositions which contain high molecular weight PTFE,
the elongation as compared to that of a 100% elastomer composition,
does not drop dramatically until the PTFE filler level has exceeded
50% by weight. The milled compositions exhibit a reduction in
elongation upon addition of the low molecular weight PTFE, which
increase somewhat in the 30/70 composition. It does not, however,
return to its original value. In the case of modulus at 100%
elongation, clearly the milled samples show higher modulus for all
four pairs of compositions, with the filled compositions being
lower than the 100% elastomer composition. The cast compositions,
while lower in modulus, exhibit a consistent modulus value above
the 50% by weight filler level.
[0119] Materials were subjected to tensile testing at elevated
temperature (350.degree. F.) as well as subsequent to air aging at
500.degree. F. for 70 hours. The tensile values were then
contrasted with the original values seen in FIGS. 8 and 9. In FIG.
10 the hot peak strain (elongation) of twelve of the original
thirteen compositions can be seen. Throughout the 50% filler level,
hot peak strain was roughly 50%-80% lower than the strain at room
temperature. The 100% cast elastomer sample was slightly less
reduced than the 100% milled sample. This is reversed for the
filled samples which are somewhat less reduced than the cast
samples. All exhibit dramatic loss of elongation, however. The
60/40 and 70/30 specimens exhibit plastic behavior and a relatively
modest elongation at room temperature, accounting for the unusual
data seen for these two samples. Slight increases in elongation
after air aging are likely due to cleavage of crosslinks during the
aging process. This can be seen in cast as well as in milled
samples.
[0120] In FIG. 11 change in modulus at 100% elongation for twelve
of the original thirteen samples is displayed. There is a dramatic
difference in retention of tensile strength at high temperature or
after aging in the case of a cast vs. milled composition of pure
elastomer, with the cast composition being clearly superior. Once
these samples are filled with PTFE, however, the cast and milled
samples are comparable in their loss of properties, whether due to
hot tensile testing or the results of air aging. In the case of hot
tensile testing there is a trend towards greater property loss
corresponding to increasing filler PTFE content, without regard to
PTFE molecular weight. In the case of property loss after air
aging, there appears to be no such trend.
[0121] In FIG. 12, change in tensile strength for twelve of the
original thirteen samples is displayed. Hot tensile strength is
universally poor at these conditions, without regard for filler
level or type, with losses of 75%-85% through the 60/40 filler
level. In the case of air-aged samples, however, low molecular
weight PTFE-filled gum samples experience lower losses in tensile
strength than their counterparts filled with higher molecular
weight PTFE. The tensile strengths of the high molecular weight
PTFE filled compositions, however, were higher at room temperature
at the outset.
[0122] Test of strength (ASTMD624, die B) at room temperature were
performed on these compositions, as well as the companion tests at
elevated temperature and after air aging. In FIG. 13 tear strength
through the 50% PTFE loading level ranges from about 100 to 150
lb.ft./in. The cast compositions containing 60% or more high
molecular weight PTFE exhibit tear strengths at the high end of
this range or higher, but are much more rigid and plastic-like in
character. While the milled 100% FKM exhibits a higher tear
strength than its cast companion, the high molecular weight
PTFE-filled samples are slightly higher in tear strength than their
low molecular weight PTFE-filled companions. The high molecular
weight PTFE in the cast, filled compositions is fibrillatable, but
purposefully not fibrillated, in the samples in this series. The
formation of a network of PTFE fibrils would be expected to result
in a significant improvement in tear strength, as demonstrated in
Example 25.
[0123] Tensile set is an important measure of elastic behavior, as
it measures elastic recovery from a tensile deformation. A lower
value represents a recovery close to the original dimensions of the
challenged part. In FIG. 14 tensile set is displayed for the
thirteen original samples at three different sample elongations,
50%, 100%, and 200%. In the compositions with a more plastic
character, tensile set data is poor or non-existent. This is not
surprising, given that we have subjected a plastic sample to a test
which measures elastic behavior. As filler level increases, the
tensile set values also increase, regardless of PTFE type. This
would be the expected behavior due to other fillers as well, such
as carbon black, clay, or silica. In the paired samples, the
tensile set data for high molecular weight PTFE-filled samples are
higher or equivalent to those of the low molecular weight
PTFE-filled samples, with one very important exception. There are
no 200% elongation tensile set data for the three gum samples
loaded with low molecular weight PTFE. As was seen in FIG. 9, these
low molecular weight PTFE-filled gum compositions exhibit low
elongation relative to their cast counterparts, and they cannot
withstand elongation to 200%, especially when followed by the 10
minute holding period at elongation prescribed by the tensile set
test.
[0124] The cast samples detailed in this example have been compared
to cast samples prepared in the course of this work as described in
Example 4. FIG. 15 compares the tensile strength and modulus at
room temperature of Example 21 cast samples (therefore containing
high molecular weight PTFE) with cast samples derived from other
high molecular weight PTFE latexes and other elastomer latexes as
described in Example 1. The x-axis of this figure is arranged by
increasing PTFE content. As was seen in FIGS. 8 and 9, there is a
general trend of decreasing tensile strength with increasing filler
loading, as well as relatively constant modulus values despite
increasing PTFE content. This is an important distinction since the
tensile strength is a value at failure for elastomeric samples,
while modulus is a value at a given stress, but not at failure. The
data in FIG. 15 show that samples exhibiting superior tensile
strength and modulus in this group all contain the same elastomer,
CF6200. This is the creamed version of L10180, an elastomer
described in Example 1 and notable for its relatively large
particle size, 0.35.mu.. This elastomer is also known to have a
higher molecular weight than the others used in the samples
presented in FIG. 15. Those skilled in the art of polymer
manufacture and application realize the improvement in physical
properties which follow from increases in molecular weight. They
also realize the challenges presented by processing such higher
molecular weight materials in conventional processes such as
milling and extrusion. For many polymers, there is a cut-off in
molecular weight beyond which such a polymer cannot be efficiently
processed by such conventional means. Our isolation processes have
no such limitations with regard to the molecular weight of the
polymeric substituents, and we, therefore, may obtain the benefits
in physical strength of such higher molecular weight materials
without the limitations of mixing ingredients with conventional
equipment. This is displayed again through tear strength data in
FIG. 16. Within the group of blends exhibiting elastomeric-type
tensile behavior, that is, those no higher than 50% by weight of
high molecular weight PTFE, the two samples with higher tear
strengths are those containing the higher molecular weight
elastomer present in CF6200.
EXAMPLE 22
[0125] The first blend of co-coagulated fluoroplastics was prepared
starting with an unstabilized, high molecular weight PTFE
dispersion, AD058 (ICI Americas, Bayonne, N.J., 1.165 s.g., 24.8%
solids, pH 3.0, surface tension 72 dynes/cm) and unstabilized THV
330R (TFE/HFP/VF.sub.2 terpolymer, Dyneon, Oakdale, Minn., 1.19
specific gravity (s.g.), 33.7% solids, pH 6.0, surface tension 68
dynes/cm). The term "unstablized" refers to a lack of hydrocarbon
surfactants which are normally added after polymerization. Both
dispersions were diluted to 18% solids with deionized water, then
equal volumes (1.5 liters) of each were charged to a 5-liter,
3-necked flask. Glass rods (1/2" diameter) were inserted through
both outer necks to act as baffles. A folding blade paddle was
inserted in the center neck and turned via an overhead stirrer at a
rate of approximately 200 rpm. The mixture coagulated after
approximately eight minutes, yielding white particulate solids in a
slightly milky liquid. The solids were filtered off using
cheesecloth and spread evenly into a metal pan to dry in a
convection oven for 15 hours at 110.degree. C. The dried coagulate
was gently broken (so as not to fibrillate the PTFE) into small
hard lumps using a screen with a 1/8" open mesh.
EXAMPLE 23
[0126] Co-coagulation was accomplished in a 10 gallon stainless
steel pot which was 18" tall and 15" in diameter. Baffles were used
to modify the pot by welding four pieces of 1.5" stainless steel
angle iron vertically and evenly spaced along the interior. The
stirrer used was made from a 3/8" aluminum rod with three
1".times.7" flat blades welded perpendicular to the rod and spaced
equidistant around the rod. The blades were displaced along the
length of the rod at 0, 7, and 12 inches from the bottom of the
rod.
[0127] A 50/50 by weight mixture of high molecular weight PTFE and
THV was charged to the vessel using material as described in
Example 22, in the proportions and order detailed in Table 14.
15TABLE 14 Component Volume (ml) Description PTFE 5988 AD057, 26.4%
solids, 1.178 s.g., pH 3.6 THV 4640 330R, 33.7% solids, 1.19 s.g.,
pH 6 NaCL solution 4252 0.1 M deionized water 8508 sulfuric acid 4
96.7% reagent grade
[0128] After mixing for four minutes at approximately 215 rpm
another 4 ml of sulfuric acid was added. After 23 minutes the
mixture resembled a thin gel and after 105 minutes mixing was
stopped as a fine white powder had coagulated out of the
dispersion. The powder was dry to the touch in air and the
remaining liquid was very clear. The powder was filtered out of the
liquid with cheesecloth and air-dried overnight at ambient
temperature. The composition of the isolated powder was determined
by TGA to be 42% PTFE/58% THV by weight.
EXAMPLE 24
[0129] The 42/58 PTFE/THV isolated powder generated in Example 23
was extruded in a 1" 3HP single-screw extruder. The extruder was
equipped with a grooved feed throat, a 1:1.4 screw, and a 1/4" rod
die. The screen and breaker plate were not used. The temperatures
used were 200, 300, and 400.degree. F. along the barrel and
400.degree. F. at the die. The screw speed was 110 rpm and material
extruded at 1100 psig to produce a rod which was subsequently
pelletized by feeding through a chopper. The pelletized material
was re-extruded with temperatures of 150, 250, and 350.degree. F.
along the barrel and a die temperature of350.degree. F. The melt
temperature of THV 330R is about 300.degree. F. The rod extrudate
of this second extrusion was left intact.
EXAMPLE 25
[0130] A film was made from the rod extrudate of Example 24 by
pressing a short length of the rod in a Carver press at 350.degree.
F. with forces of 0 tons gauge for 1 minute, to 50 tons gauge for 1
minute, then 90 tons for 2 minutes. This produced a clear film
(TPP5-47B) approximately 12 mils thick. The composition of the film
was determined by TGA to be 40% by weight PTFE/60% THV. Films used
to compare properties of this PTFE/THV construction to more
standard materials were made by laminating thinner film stock
together. A 9 mil THV comparator was made from two cast films made
from THV 330R, which were then laminated together in the Carver
press. An 11 mil high molecular weight PTFE/FEP film was made by
laminating two DF1700 films (Chemfab Corp., Merrimack, N.H.)
together at the FEP faces.
[0131] Tear Strength Initiation tests (Table 15) were performed on
an Instron model 4208 with a 100 pound load cell, 2"/minute
crosshead speed, and 2 inch gauge length on an ASTM D624 Die C
specimen. The PTFE/THV film containing fibrillated PTFE as a result
of the extrusion process exhibits superior resistance to tear in
these samples as compared to standard cast THV or PTFE/FEP.
16TABLE 15 Tear Initiation Tear Initiation Film Thickness (mil)
Strength (lb.) PTFE/THV TPP5-47B 12 19.9, 14.0 PTFE/FEP 11 5.5,
5.8, 5.5, 6.2 THV 9 4.7, 3.8
[0132] Tensile Strength tests (Table 16) were performed using an
Instron Model 4208 with a 100 pound load cell, 2"/minute crosshead
speed, 2" gauge length, on 1/2" wide specimens. While breaking
strength was comparable in these samples, elongation (expressed as
deformation at break) is lower in the case of a fibrillated PTFE
dispersed in THV as compared to the standard cast film.
17TABLE 16 Tensile Strength Test Thickness Breaking Deformation at
Film (mils) Strength (lb.) Break (in.) TPP5-47B 12 20.5, 27.6, 37.5
0.8, 1.0, 0.9 PTFE/FEP 11 25.9, 28.9, 27.5 12.7, 13.2, 12.8
[0133] Hydrostatic burst tests were performed on a Mullen Diaphragm
Burst Tester. The 11 mil PTFE/FEP film burst at 65 pounds pressure,
while the 12 mil PTFE/THV (with PTFE fibrillated via extrusion)
film sustained 265 pounds of pressure before a small hole was
initiated along an edge of the test zone. THV alone would not be
expected to exhibit such an improvement in burst pressure; the
improvement is attributable to PTFE fibrillation. The fibrillated
PTFE in a THV matrix provides an obvious improvement in tear
strength.
[0134] Tear Strength propagation tests (trouser tear) (Table 17)
were performed on an Instron Model 4208 with a 100 pound load cell,
10"/minute crosshead speed, and a 2" gauge length. The fibrillated
PTFE in a THV matrix provides an obvious improvement in tear
strength.
18TABLE 17 Tear Strength Propagation Test Film Thickness (mils)
Tear Strength Propagation (lb.) TPPS-47B 12 10.4, 12.6 PTFE/FEP 11
1.5, 1.5
[0135] Samples of a similarly extruded rod of high molecular weight
PTFE/THV were analyzed by SEM and wide-angle x-ray scattering
(WAXS) in order to elucidate the oriented nature of the PTFE within
the matrix. SEM was performed at Analytical Answers (Woburn,
Mass.). A sample of extruded rod was prepared by microtoming
directly along the axis of extrusion as well as across the face of
the rod. SEM at 10,000.times. magnification clearly showed
fibrillated, oriented PTFE along the axis of extrusion (FIG. 17),
with the ends of the PTFE fibrils clearly viewed on end (FIG. 18).
WAXS was performed at Virginia Polytechnic Institute (Blacksburg,
Va.). Nonuniformity of the intensity of the scattered X-rays is
indicative of orientation in a crystalline PTFE domain in the
specimen. (FIGS. 20 and 21)
EXAMPLE 26
[0136] The aqueous formulation used in this example was 955 ml of
high molecular weight PTFE (DuPont T30B, 60% solids, 1.506 s.g.)
and 1121 ml THV (Dyneon 350C, 51% solids). The formulation was cast
at 12 feet per minute onto a dimensionally stable carrier with a
PTFE surface, then dried in a vertical oven. The drying zones were
245.degree. F., then 480.degree. F., proceeding vertically. After
eight layers were cast onto the carrier the material was calendered
twice at 1500 psi gauge, 10 feet per minute, and 180.degree. F. Two
pieces of the coated and calendered carrier were sealed to each
other at 350.degree. F., 80 psig, for 30 seconds in a Carver press.
A two-ply film was stripped from between the two carriers, then
fused in a convection oven at 660.degree. F. for 30 seconds. Two
pieces of the two-ply film were then heat sealed together in a
Carver press at 350.degree. F. for 1 minute, yielding a
well-consolidated 7.5 mil film.
[0137] Tensile tests were performed on the 7.5 mil cast and pressed
film on an Instron Model 4208 with a 100 pound load cell and
crosshead speed of 0.5"/minute. Gauge length was 0.5 inches.
Tensile data for these samples are presented in Table 18 and are
compared to a similar composition which was extruded, then pressed
(Example 25), resulting in fibrillation of PTFE and reinforcement
of the matrix. The improvement in break strength seen in the table
for the extruded sample is due to this reinforcement.
19TABLE 18 Tensile Strength of PTFE/THV Compositions Thickness Test
Speed Break Strength Deformation at Film (mils) (in./min.) (lb.)
Break (in.) Example 25 extruded & 12 2.0 20.5, 27.6, 37.5 0.8,
1.0, 0.9 pressed Example 26 cast & pressed 7.5 0.5 4.2, 5.0
0.09, 0.145
EXAMPLE 27
[0138] PTFE and THV were co-coagulated from unstabilized
dispersions using the same equipment described in Example 23.
Materials were added as shown in Table 19.
20TABLE 19 Component Volume (ml) Description PTFE 5189 AD057, high
molecular weight THV 4640 330R NaCl solution 4252 0.1 M deionized
water 8508
[0139] After approximately four minutes of mixing, 4 ml of
H.sub.2SO.sub.4 was added. After mixing for a total of 70 minutes,
a fine, air dry, white powder had coagulated. The powder was
filtered and dried in a manner similar to that in Example 23. TGA
of the powder indicated a final mixture of 52% by weight PTFE and
48% by weight THV.
[0140] The coagulated powder was lubricated by mixing 140 grams of
the powder with 125 g deionized water and 70 g of a 6% by weight
Triton X-100 in water solution. The mixture was charged to a ball
mill and mixed for 26 hours. The mixture was then air-dried for 24
hours, followed by drying in a vacuum oven for seven hours at
110.degree. C.
[0141] The lubricated powder was processed through an Instron model
3213 capillary rheometer. The capillary used was 0.8853 inches long
and 0.0595 inches in diameter. The rheometer was run at 90.degree.
C., well below the melt temperature of both polymers, and with a
plunger rate of 2 inches per minute. The applied shear stress was
123 pounds per square inch, and the shear rate was 178 sec.sup.-1.
TGA of the extrudate indicated a final mixture of 51% by weight
PTFE and 49% by weight THV, verifying that the mixture was
successfully processed without substantially altering the ratio of
the two polymer components.
[0142] SEM of the extrudate clearly showed fibrillation within the
matrix and even in fissures seen in photographs of the exterior of
the extrudate.
EXAMPLE 28
[0143] This example demonstrates solvent-lubricated extrusion of
PTFE/THV, below the melting points of both polymers. A
co-coagulation of PTFE and THV was performed as described in
Example 22 with the relative proportions of the polymers at 70% by
weight PTFE and 30% by weight THV. The dried powder was mixed with
about 17% by weight Isopar H as a lubricant and mixed on a drum
roller for 20 minutes, which is standard for PTFE processing. The
lubricated crumb was then preformed at 210 psig (also standard for
PTFE processing) into a tube shape with an inner radius of 0.5",
outer radius of 1.75", and overall length of 4". The preform was
quite smooth and easy to feed into the ram extruder. Extrusion was
at 3750 psgi with a 1.75" ram through a die at 115.degree. F. SEM
of the extrudate (FIGS. 22A-22I, 2500 to 10,000.times.) shows the
beginning of fibrillation, which is preferentially oriented as a
result of the extrusion.
EXAMPLE 29
[0144] A blended composition of a 40/60 wt % PTFE/FKM was
coagulated with curative from dispersion, washed, dried, and
isolated. The high molecular weight PTFE used was AD310 (ICI,
Bayonne, N.J.), 29.8 wt. % solids aqueous dispersion; the FKM used
was E14674 terpolymer (Dyneon Corp.) (Example 1), 20.2 wt. % solids
aqueous dispersion. The curatives used were Mg(OH).sub.2 as the
acid acceptor (Phillips milk of magnesia, Bayer Corp., 7.8 wt. %
solids aqueous dispersion), and Viton Curative 50 (VC50, DuPont,
Wilmington, Del.). VC50 is a pelletized mixture of bisphenol AF and
benzyltriphenylphosphonium chloride/bisphenol AF salt. The Viton
Curative 50 was dispersed by grinding it in a mortar and pestle to
a fine powder, and then sonicating it in isopropanol, 4.8 g VC50
per 100 ml isopropanol (VC50 dispersion). The Triton X-100 was used
as a 25 wt % in water solution. Other curing additives which could
be incorporated include Ca(OH).sub.2.
[0145] The co-coagulation vessel was a 5 l stainless steel vessel.
The pneumatic mixer used was fitted with a 23/4" inch propeller
type blade, with a speed of about 600 rpm. 2000 g of 1 M KCl was
charged to the vessel, and stirring was started and was continuous
during all subsequent additions. Mg(OH).sub.2 dispersion (72.5 g)
was charged to the vessel, followed by the AD310 dispersion (45 ml)
and the VC50 dispersion (120 ml). A blend of 1200 g E14674
dispersion and 400 g AD310 dispersion was made, then poured into
the co-coagulation vessel slowly, taking 2 minutes for complete
addition. More AD310 dispersion (60 g) was then added. Triton X-100
solution (5.4 g) was added and the mixture was allowed to stir for
4 minutes to redisperse some of the solids which had appeared
floating on the surface. The coagulated material was filtered off
using multiple layers of cheesecloth, and then washed twice with
deionized (DI) H.sub.2O by adding water, mixing, and refiltering.
The sample was then dried in a vacuum oven at 105.degree. C. for 12
hours. Co-coagulation of these materials can be done without the
Mg(OH).sub.2, without the VC50, or without either curing component.
This co-coagulation provided a mixture with the formulation shown
in Table 20, and was crumb-like in consistency.
21TABLE 20 Formulation for Curative Containing Coagulated
Dispersion phr E14674 100 AD310 708 Mg(OH).sub.2 2.4 VC50 1.9
[0146] Oscillating disk rheometry (ODR) was performed on the above
sample, (Monsanto Rheometer 100, 350.degree. F., 1.degree. arc) and
showed a t.sub.s1 (scorch time) of 21 minutes, a t(90) (time to 90%
of maximum torque) of 108 minutes, and a total increase in torque
of 16 lb-in (per ASTM D-2084). For samples analyzed with no VC50
and/or no Mg (OH).sub.2, there was no increase in torque. The
sample from Table 20 (12 g) removed from the ODR, which contained a
complete cure system, retained 92% of its weight after being
subjected to methylethylketone (MEK, 100 ml, with stirring) for 15
hours at room temperature. A sample removed from the ODR of
material similar to that in Table 20, but which only contained VC50
and lacked Mg(OH).sub.2 retained only 39.2% of its weight after
similar MEK exposure, indicating a lower degree of cure.
EXAMPLE 30
[0147] A coagulation was done using the same materials as in
Example 29. The VC50 and the milk of magnesia were premixed before
adding to the coagulation mix, and will be referred to as the "VC50
curative blend". The VC50 curative blend was produced by sonicating
the VC50 (4.6 g, ground) and the milk of magnesia (30 g) in
isopropanol (100 g) until there were no large (>1 mm) visible
chunks of VC50. The VC50 curative blend was more stable towards
settling than the VC50 dispersion made in Example 29, and did not
show signs of settling after two minutes.
[0148] The coagulation vessel used was a Waring blender. A salt
solution was charged to the blender vessel (650 g, 1 M KCl), and
the blender set to high power for the duration of the coagulation.
Some of the AD310 dispersion was added (15 ml), interspersed with
additions of the VC50 curative blend (105 ml). Order of addition
was 5 ml AD310, 45 ml VC50 curative blend, 5 ml AD310, 60 ml VC50
curative blend, 5 ml AD310. The premixed AD310 (82 g) and E14674
(248 g) was then poured steadily into the blender. The total
addition time was 1 minute. Another addition of AD310 (13 g) was
made, followed by Triton X-100 (1 g of 25 wt % X100 in water).
[0149] The coagulate was filtered off using a milk filter (Agway),
washed three times with DI water (300 ml) with reblending and
filtering each time, and then dried. The composition of the
coagulate is shown in Table 21.
22TABLE 21 Formulation for Curative Containing Coagulated
Dispersion for Example 30 phr E14674 100 AD310 66 Mg(OH).sub.2 3
VC50 9.2
[0150] Oscillating disk rheometry (ODR) was performed on the above
sample, (Monsanto Rheometer 100, 350.degree. F., 1.degree. arc),
and showed a t.sub.s1 of 12.2 minutes, a t(90) of 38 minutes, and a
total increase in torque of 29.8 lb.-in., an improvement in cure
speed and final torque as compared to Example 29. For samples
analyzed with no VC50 and/or no Mg (OH).sub.2, there was no
increase in torque. The sample (12 g) removed from the ODR which
contained both VC50 and Mg (OH).sub.2 retained 98% of its weight
after being subjected to methylethylketone (MEK, 100 ml, with
stirring) for 28 hours at room temperature. A sample (12 g) removed
from the ODR of material which only contained VC50 and lacked
Mg(OH).sub.2 retained only 31.2% of its weight after similar MEK
exposure, indicating a lower degree of cure.
EXAMPLE 31
[0151] A 40/60 wt. % blend of AD310/E14674 was isolated similarly
to Example 29. It consisted of the formula in Table 22, which
contained no acid acceptor, so was therefore incapable of
curing.
23TABLE 22 Formula for Coagulated Dispersion for Example 31 phr
E14674 100 AD310 70.8 VC50 (Viton Curative 50) 1.9
[0152] This material was extruded through a 0.03" capillary
(L/D=66) at 150.degree. C. using a capillary rheometer (Instron
Corp., Model 3213, Canton, Mass.) at an extrudate speed of 23
inches/min.
[0153] Some of this extrudate was subjected to MEK extraction in a
beaker with stirring for 12 hours at room temperature. The sample
retained 38% of its initial weight, and was determined via TGA to
be 95 wt. % PTFE. The sample was prepared for SEM analysis by
hardening with osmium tetroxide, embedding in epoxy, and cutting in
a microtome to expose a fresh surface just inside the edge of the
extracted extrudate. SEM photographs at 10,000.times. magnification
clearly show individual PTFE particles, as well as particles which
have extended into fibers (FIG. 22). These fibers were oriented in
the machine direction of the extrudate.
EXAMPLE 32
[0154] The isolated and processed material from Example 31 was
analyzed by x-ray diffraction. Wide-angle X-ray scattering (WAXS)
was performed on the above sample, and based upon the azimuthal
dependence of the 4.9 .ANG. reflection, it was determined that
there was some orientation of the PTFE in the axial direction of
the extrudate (FIG. 23). WAXS was performed on a Phillips model
PW1720 generator with a Warhus camera. Nickel-filtered CuK.alpha.
radiation was used with a wavelength of 1.542 .ANG. and a pinhole
collimator with a 0.020 inch diameter.
EXAMPLE 33
[0155] A co-coagulation of a 50/50 wt. % blend of AD310 aqueous
dispersion (29.8% solids) and E6582 aqueous dispersion (31.6%
solids, copolymer, Dyneon Corp.) was produced using a 5 gallon pail
as the coagulation vessel. The mixer was an electric (0.5 Hp)
Laboratory Dispersatore (Series 2000, model 84, Premier Mill Corp,
USA). A salt solution (9 liters, 1.12 M NaCl) was charged to the 5
gallon pail, and stirring was started and was maintained throughout
the co-coagulation. A premixed blend of AD310 aqueous dispersion
(2346 g) and E6582 aqueous dispersion (2373 g) was poured steadily
into the pail. The coagulated material was filtered through
multiple layers of cheesecloth, and returned to the pail for
repeated blending (3 times with tap water, then twice with
deionized water, filtering through cheesecloth after each step),
then dried. Final yield was 89%.
[0156] To verify that isolation had occurred without fibrillation
of the high molecular weight PTFE, a sample of this coagulated
50/50 AD310/E6582 material was sputter-coated with gold and
examined by SEM. The SEM photographs showed that the sample
contained essentially no evidence of fibrillation (FIG. 24).
[0157] A sample of this coagulated 50/50 AD310/E6582 material was
then heated to 100.degree. C. and hand-pressed between two steel
plates (one of which was the SEM sample holder) in order to produce
a smooth sample for analysis. The sample was then sputter-coated
with gold and examined by SEM. The photographs showed that the
sample contained evidence of fibrillation of the PTFE particles,
which had occurred due to the applied hand pressure (FIG. 25).
EXAMPLE 34
[0158] In this example, Aclon PCTFE (polychlorotrifluorethylene
modified with sufficient VF.sub.2 and other comonomer to render the
polymer fully amorphous; approximately greater than 18 wt. %
VF.sub.2) has been substituted for FKM. A co-coagulation of a 50/50
wt. % blend of PTFE and PCTFE was performed using a Waring blender
as the co-coagulation vessel. A salt solution (395 g of 1.3 M NaCl)
was charged to the blender, and the blender was set to high power
and remained on high throughout the coagulation. The PTFE aqueous
dispersion (167 g, AD310, ICI, Bayonne, N.J., 28.7 wt. % solids)
was mixed with the PCTFE aqueous dispersion (105 g Aclon, 400
series, Allied Signal, Specialty Films, Morristown, N.J., 47.5 wt %
solids) and diluted with deionized water (122 g). This blended
aqueous dispersion was poured steadily into the blender, until
addition was complete. The coagulated sample was vacuum-assist
filtered with wet-strengthened filter paper (Whatman no. 114). The
coagulated material was reblended and filtered three times with tap
water, then twice with deionized water, and then dried in a vacuum
oven at 90.degree. C. for 24 hours.
[0159] A sample of this coagulated AD310/Aclon material was
sputter-coated with gold and examined by SEM at 10,000.times.
magnification. The micrographs show that the sample contained
essentially no evidence of PTFE fibrillation (FIG. 26).
[0160] This material was extruded through a 0.03" capillary
(L/D=66) at 150.degree. C. using a capillary rheometer at an
extrudate speed of 38 in/min. As the material was extruded, it was
drawn at 180 in./min. WAXS was performed on this extruded sample
and indicated orientation of the PTFE crystalline regions, based
upon the azimuthal dependence of the 4.9 .ANG. reflection (FIG.
27).
EXAMPLE 35
[0161] A 40/60 wt. % blend of T30B (60 wt. % solids high molecular
weight PTFE) and FKM (L10180, Example 1) was mixed with diamine
curatives according to Example 4. The material was cast onto a
carrier similar to Example 4, but after leaving the dispersion
bath, the carrier passed between two rotating metering bars
(rotating with web, #36, rotating 3.times. speed of web). The
rotating metering bars were employed to gauge the sensitivity of
the PTFE to fibrillation by shearing during the casting process
outlined in Example 4. Seven layers were cast onto the carrier,
with a total pickup of 4 mil, and a web speed of 2-4 feet per
minute. The material was stored on a roll, with no interleaving
paper.
[0162] In order to check for signs of fibrillation of the PTFE in
this sample, some of the FKM matrix was dissolved away, and SEM
photos were taken. A sample of the cast film (consisting of 5-7
consolidated layers) was subjected to refluxing MEK for 24 hours,
with stirring. The sample was removed and dried. Thermal
gravimetric analysis (TGA) showed the sample to be a 56/44 wt. %
blend (PTFE/FKM), at 67.5% of its initial weight. SEM photographs
taken of the surface and of cross sections produced by
freeze-fracturing showed that the sample contained no evidence of
PTFE fibrillation (FIG. 28).
EXAMPLE 36
[0163] A 50/50 wt. % blend of AD310/E14674 was isolated similarly
to Example 31. It consisted of the formula in Table 23 and
contained no acid acceptor, so was therefore incapable of
curing.
24TABLE 23 Formula for Coagulated Dispersion for Example 36 phr
E14674 100 AD310 100 VC50 3.7
[0164] This material was extruded through a 0.03" capillary
(L/D=66) at 150.degree. C. using a capillary rheometer at an
extrudate speed of 16 ft/min. The extrudate was drawn at 50 ft/min
as it was extruded.
[0165] Some of this extruded sample was subjected to MEK extraction
in a Soxhlet extractor for 20 hours. The sample retained 49% of its
initial weight. The sample was prepared for SEM by
freeze-fracturing it to expose surfaces from the interior of the
sample. SEM photographs at 10,000.times. magnification clearly
showed individual PTFE particles, as well as particles which had
extended into fibers (FIG. 29). These fibers were oriented mostly
in the machine direction of the extrudate.
[0166] Wide angle X-ray scattering (WAXS) was performed on the
above sample. Based upon the azimuthal dependence of the 4.9 .ANG.
reflection, it was determined that there was some orientation of
the PTFE in the axial direction of the extrudate (FIG. 30).
[0167] A control for the above 50/50 wt. % PTFE/FKM sample was
prepared, using a low molecular weight PTFE micropowder (MP1000
micropowder, DuPont). The coagulation vessel used was a Waring
blender. A salt solution was charged to the vessel (400 g, 1.14 M
NaCl), and the blender set to high power for the duration of the
coagulation. MP1000 micropowder was added to the vessel (50 g),
followed by a wetting agent, Triton X-100 (1 ml of a 25 wt %
solution in water) which allowed the MP1000 micropowder to be
dispersed in the salt solution. A fluoroelastomer copolymer (E6582,
Dyneon, 158 g, 30.2% solids) was charged to the vessel, producing a
coagulated mixture. The coagulate was vacuum-filtered through
Whatman 114 filter paper, and was then washed three times with tap
water and twice with deionized water, blending and filtering
between each step. The material was dried in a vacuum oven at
90.degree. C. for 3 days.
[0168] This material was extruded through a 0.03" capillary
(L/D=66) at 150.degree. C. using a capillary rheometer at an
extrudate speed of 16 ft/min. The extruded bead could not be drawn
to greater than 20% elongation without breaking, a marked change
from the similar sample containing high molecular weight PTFE.
[0169] Some of this extruded sample above was subjected to MEK
extraction in a Soxhlet extractor for 20 hours. The sample retained
50% of its initial weight. The sample was prepared for SEM by
freeze-fracturing it to expose fresh surfaces from the interior of
the sample. SEM photographs at 10,000.times. magnification clearly
showed that the sample was formed of individual PTFE particles, and
contained no signs of fibrillation (FIG. 31).
[0170] Wide angle X-ray scattering (WAXS) was performed on the
above sample. Based upon the azimuthal dependence of the 4.9 .ANG.
reflection, it was determined that there was no orientation of the
PTFE in the axial direction of the extrudate, an expected result in
the case of a blend containing PTFE which will not fibrillate.
EXAMPLE 37
[0171] Co-coagulation has been demonstrated which results in
material other than crumbs or powders. Co-coagulation can also
produce a gelled coagulate, which can be washed, dried, and formed
in further processes. In a 400 ml beaker, AD310 dispersion (28.7%
solids, 80 g) was combined with E14674 terpolymer dispersion (28.3%
solids, 81.5 g) and gently mixed by swirling the combined
dispersions in the beaker. A salt solution (140 g of 1 M NaCl) was
poured into the beaker with manual agitation. After 30 seconds, the
mixture had become a gelled mass, taking the shape of the interior
of the beaker. When removed from the beaker, clear NaCl solution
ran out from the sample, which then shrank to about 30% of its
initial size as it air-dried.
EXAMPLE 38
[0172] This example demonstrates a low-shear, non-fibrillating
isolation process for PTFE/FKM blends under freezing conditions. A
blend of aqueous dispersions of PTFE (AD310, ICI, Bayonne, N.J.)
and FKM (E14674 terpolymer, Dyneon) was made (45 g of each on a dry
basis) in a plastic vessel, and gently stirred to ensure good
mixing. The vessel was placed in a chest freezer at -15.degree. C.,
and allowed to freeze solid. When removed from the freezer, the
frozen mixture was taken out of the vessel and placed on a piece of
filter paper at room temperature. As the frozen sample melted, the
polymers maintained their position in a sponge-like state, as some
of the water drained from the material. The gelled form was dried
in an oven at 110.degree. C. for 3 hours. The final weight of the
sample was 84 g, and the sample had shrunk upon drying.
[0173] This method was repeated for blends of PTFE (AD310) and
PCTFE (Aclon 400 series aqueous dispersion, AlliedSignal) following
a similar procedure. When thawed, the material was not gel-like,
but was a flaky powder, from which clear water ran off. For a
sample of PTFE (AD310) and FFKM (PFR94, Ausimont perfluoroelastmer
aqueous dispersion, Example 1) a sponge-like material was created,
from which clear water could be squeezed out.
[0174] This method was repeated using liquid nitrogen instead of
the freezer. The combinations used were AD310/E14674 and
AD310/PCTFE. The material was showered into the liquid N.sub.2,
which produced frozen drops of material. When thawed, both
combinations of these materials were similar in consistency to the
companion samples produced in the -15.degree. C. freezer.
EXAMPLES 39-43
(Differential Young's Modulus Examples)
[0175] 40/60 PTFE/FKM (percentage by weight) composites, when
pressed in a Carver press at pressures of at least 250 psi, exhibit
differential radial modulus (Young's Modulus, referred to as "E" in
the tables below) effects when tensile-tested with an Instron.
Pancake-shaped samples are produced by the flow of the composite
under the pressure, and the shearing and resulting fibrillation of
the PTFE induces these effects. Sections from the pancakes cut
parallel to a radius will exhibit a modulus as much as ten times
those cut perpendicular to a radius (Examples 39, 42, and 43). The
data show that the measured modulus increases on material further
from the center of the pancake (Examples 39, 40, and 43). These
effects have been demonstrated to be independent of the cure state
of the elastomer (Examples 40 and 42). A control sample utilizing
low molecular weight PTFE shows no increase in modulus along a
radius as would be expected from a non-fibrillatable
PTFE-containing blend (Example 41).
[0176] Tensile samples were cut by finding the center of the
pancake, slicing a 1/2" wide strip which includes that center spot
across the entire pancake and cutting two other 1/2" wide strips,
of which the radii include the center point, perpendicular to the
first strip. Such cuts form the pattern of a simple cross. These
pancakes were typically 15 to 20 cm in diameter and 5 to 10 mils
thick; the thickness in the center was as much as 50% more than
that measured at the edge.
[0177] The following are examples illustrating these effects.
EXAMPLE 39
[0178] A 40/60 (percent by weight ) composite of T30B PTFE/PFR94
FFKM was prepared by drying an aqueous latex blend of the two,
which also included 2 phr succinic acid peroxide as curing agent,
2.5 phr DIAK #7 as coagent, and 2 phr magnesium hydroxide as acid
acceptor. The dried rubbery compound was pressed in a Carver press
at 140.degree. F. at pressures ranging from 400 to 600 psi, and the
resulting pancakes were re-folded into squares and re-pressed under
identical conditions twice more, yielding white,
homogeneous-looking pancake samples.
[0179] One of these pancakes was then pressed at about 825 psi for
45 minutes at 275.degree. C. This pancake was cured, as it could
not be refolded and re-flowed again at any temperature without
yielding a crumbed mass. The pancake was sampled per the cross
pattern, as described above, and tensile pulls were performed on
sections near the center of the pancake as well as sections out as
near to the rim of the pancake as practicable. The modulus (E)
measured near the rim averaged 18,863 psi. The modulus of sections
measured near the center averaged 6,605 psi.
[0180] The modulus of one section cut perpendicular to a radius,
near the pancake rim, was 1,652 psi, about 9% that of the sections
parallel to radii.
[0181] These trends are typical for composites processed at similar
temperatures and pressures. Though the absolute numbers and ratios
may vary with differing conditions, the trends persist.
EXAMPLE 40
[0182] A 40/60 (by weight percent) T30B PTFE/TFE-propylene
copolymer elastomer composite, which is not crosslinkable by
conventional elastomer curatives, was formulated in a similar
manner to Example 39, with 2.5 phr succinic acid peroxide, 2.5 phr
DIAK #7 and 2 phr magnesium hydroxide. A pancake of this composite
was compressed at nearly 450 psi at 250.degree. C. for 45 minutes.
Sections near the rim averaged modulus of 17,486 psi. Sections
tested nearer the center averaged 2,272 psi modulus. This data
demonstrates that the effect of increasing modulus from the center
is not caused by the ability of the elastomer to cure.
EXAMPLE 41
[0183] A 40/60 (percent by weight) MP1000 (low molecular weight
PTFE)/TFE-propylene copolymer elastomer composite was formulated in
a similar manner to Example 40. Two pancakes of this composite were
pressed in a Carver press at 484 and 497 psi at 250.degree. C. for
45 minutes. The first pancake was then subjected to tensile pulls
from 18 various locations, from the center to the rim. The second
was sampled in 19 locations. The tensile modulus of the first
pancake averaged 878 psi, with a standard deviation of only 171
psi. The second yielded statistically identical results--tensile
modulus averaging 878 psi with standard deviation of only 164 psi.
Not only was this a small fraction of what was achieved with high
molecular weight PTFE in Example 40, but also location of the
tested sample was immaterial, as evidenced by the tight
distribution of data. The results demonstrate that the high
molecular weight of the PTFE confers the radial modulus effects and
enables the achievement of high modulus when a PTFE/elastomer
composite is repeatedly compressed at pressures .gtoreq.250
psi.
EXAMPLE 42
[0184] A 40/60 (percent by weight) T30B PTFE/TFE-propylene
copolymer elastomer composite similar to that described in Example
40 was formed into a pancake and pressed in a Carver press at
200.degree. C. for one hour at 500 psi. Even at these milder
temperature and pressure conditions, compared especially to
Examples 39 and 40, the composites exhibit a differential radial
modulus effect. The five samples tensile-tested which were radially
sampled averaged 4,206 psi modulus. The four samples tested which
were on sections perpendicular to a radius averaged only 627 psi
modulus.
EXAMPLE 43
[0185] One differential radial modulus effect, that of sections
being parallel to radii exhibiting superior modulus to those
perpendicular, can be obliterated by repeatedly reworking the
composite below curing temperature or by repeatedly reworking a
composite which does not cure. A 40/60 (percent by weight)
T30B/PFR94 composite analogous to that described in Example 39 was
prepared. The composite pancake was worked by folding it into a
square and pressing it at 25000 pounds gauge (.about.600 psi) at
200.degree. C. for 20 minutes for a given number of iterations,
then folding into a square, subjecting to the same pressure, and
curing at 250.degree. C. for 45 minutes. Sections were then
tensile-tested in the Instron apparatus, sampling both parallel and
perpendicular to radii, as well as interior and rim samples. A
tabulation of the results can be seen in Table 24, with n=the
number of times the composite was pressed at 200.degree. C. and
modulus (E) reported in psi.
25TABLE 24 E parallel E perpendicular E interior E rim n to radii
to radii sections sections 1 25,045 4,202 14,950 25,045* 2 27,740
7,799 7,681 27,740* 3 26,945 23,446 15,538 25,305 5 17,042 28,538
12,432 22,700 7 35,590 42,208 20,749 40,979 *samples on simple
cross only.
[0186] As seen in Table 24, as the composite is worked more and
more prior to final temperature treatment (or cure), the edge areas
greatly increase in modulus in the final cured product, and the
modulus then becomes a function only of distance from the center of
the composite. This effect was observed to materialize with three
iterations. Prior to that, moduli of sections sampled perpendicular
to a radius and near the rim were low.
[0187] The data show that the percent strain at break is reasonably
well correlated (inversely) with the modulus. Thus, the
MP1000/elastomer composites, with their moduli under 1000 psi,
featured percent strains at break typically around 1300%, while the
higher modulus samples usually displayed percent strains at break
of 300% or less. This is consistent with the expected reinforcing
effect of fibrillated PTFE.
EXAMPLE 44
[0188] This example describes the inducement of PTFE fibrillation,
including identification of the fibrils via micro-FTIR. A 44/56
(percent by weight) composite of AD310 PTFE/PFR94 was fabricated by
coagulating and drying an aqueous latex mixture, including curing
agent, coagent, and acid acceptor as disclosed in Example 39. About
45 grams of the pancakes prepared in a similar manner as disclosed
in Example 39 were pressed together into one mass at 140.degree. F.
for 15 minutes at .about.900 psi through three press and refold
cycles. The sample was then loaded into a 6".times.6" mold, heated
to 200.degree. C. at 1100 psi, bumped by releasing the platens,
compressed again at 1100 psi for 5 minutes at 200.degree. C.,
bumped again by releasing the platens, and then compressed at 1225
psi at 200.degree. C. for 30 minutes. The temperature of the press
was then increased sequentially to 225.degree. C. for 35 minutes,
then 250.degree. C. for 30 minutes.
[0189] The resulting flesh-colored pad removed from the mold was
well-consolidated, and consisted of two dense plies, between which
were discovered fibrils resembling cotton candy. These fibrils were
approximately 10 .mu.m in diameter and visible to the naked eye.
Micro-FTIR analysis identified these as pure PTFE, confirming that
the formation of fibrils of PTFE had been induced by the pressing
described above.
EXAMPLE 45
[0190] This example describes the use of photoinitiators to effect
crosslinking in the composites. A 40/60 (percent by weight)
composite of T30B PTFE/PFR 94 FFKM was prepared by drying an
aqueous latex blend which also included 4 phr of a photocuring
agent (Darocur 1173; 2-hydroxy-2-methyl-1-phenyl-propan-1-one), a
coagent (DIAK #7), and an acid acceptor (magnesium hydroxide). The
dried latex was formed into pancakes via the method described in
Example 39. These pancakes were exposed to various dosages of
ultraviolet radiation using a UV Process Supply irradiation
apparatus equipped with a mercury lamp which could be adjusted from
125 to 300 watts/inch power. Dosages ranged from zero up to an
estimated 18 joules/cm.sup.2.
[0191] Cure was proven by attempting to re-fold the pancakes into
squares and re-pressing them at 140.degree. F. at 500 psi or more.
A cured pancake yielded a crumbed, incoherent mass. It was observed
that the higher the total dose of UV radiation a pancake received,
the greater its tendency to crumb on subsequent pressing at
elevated temperatures.
[0192] In contrast, a 40/60 (percent by weight) composition of T30B
PTFE/E14734 Dyneon TFE-propylene copolymer with photoinitiators
could be re-pressed into coherent pancakes, no matter how high a
photodose it received. This copolymer sample is not curable.
[0193] Also in contrast, a 40/60 (percent by weight) T30B/PFR 94
composite with no photoinitiator underwent a modulus decline of up
to 50% upon similar irradiation and was reflowable with subsequent
pressing at elevated temperatures.
* * * * *