U.S. patent application number 10/009422 was filed with the patent office on 2003-06-05 for fep with increased flexural fatigue strength and a low level of die deposits.
Invention is credited to Kaulbach, Ralph, Kloos, Friedrich.
Application Number | 20030102593 10/009422 |
Document ID | / |
Family ID | 21737543 |
Filed Date | 2003-06-05 |
United States Patent
Application |
20030102593 |
Kind Code |
A1 |
Kaulbach, Ralph ; et
al. |
June 5, 2003 |
FEP WITH INCREASED FLEXURAL FATIGUE STRENGTH AND A LOW LEVEL OF DIE
DEPOSITS
Abstract
Provided is a melt-processible perfluorinated polymer
composition comprising (a) a melt-processible perfluoropolymer
comprising (i) from 80 to 98% by weight of repeating units derived
from tetrafluoroethylene, (ii) from 2 to 20% by weight of repeating
units derived from hexafluoropropylene, and (iii) from 0 to 5% by
weight of repeating units derived from further comonomers other
than tetrafluoroethylene and hexafluoropropylene, and wherein the
proportion by weight of the repeating units derived from
hexafluoropropylene units is greater than that of the repeating
units of said further comonomers, and (b) from 0.01 to 5% by
weight, based on perfluoropolymer (a), of a high-molecular-weight
perfluorinated polymer with a melting point at least 20.degree. C.
above that of the fluoropolymer (a). Also provided are processes
for making and using such melt-processable compositions.
Inventors: |
Kaulbach, Ralph; (Emmerting,
DE) ; Kloos, Friedrich; (Kastl, DE) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Family ID: |
21737543 |
Appl. No.: |
10/009422 |
Filed: |
December 10, 2001 |
PCT Filed: |
June 28, 2001 |
PCT NO: |
PCT/US01/20620 |
Current U.S.
Class: |
264/211 |
Current CPC
Class: |
C08L 2205/02 20130101;
C08L 27/18 20130101; C08L 27/18 20130101; C08L 2666/04 20130101;
H01B 3/445 20130101 |
Class at
Publication: |
264/211 |
International
Class: |
D01F 001/02 |
Claims
1. A melt-processible perfluorinated polymer composition comprising
a) a melt-processible perfluoropolymer comprising (i) from 80 to
98% by weight of repeating units derived from tetrafluoroethylene,
(ii) from 2 to 20% by weight of repeating units derived from
hexafluoropropylene, and (iii) from 0 to 5% by weight of repeating
units derived from further comonomers other than
tetrafluoroethylene and hexafluoropropylene, and wherein the
proportion by weight of the repeating units derived from
hexafluoropropylene units is greater than that of the repeating
units of said further comonomers, and b) from 0.01 to 5% by weight,
based on perfluoropolymer a), of a high-molecular-weight
perfluorinated polymer with a melting point at least 20.degree. C.
above that of the fluoropolymer a).
2. A melt-processable composition as claimed in claim 1, wherein
the melting point of the perfluorinated polymer b) is at least
30.degree. C., preferably more than 40.degree. C., above the
melting point of the perfluoropolymer a).
3. A melt-processable composition as claimed in claim 1 or 2,
wherein the perfluorinated polymer b) has a melting point above
270.degree. C., preferably above 290.degree. C.
4. A melt-processable composition as claimed in one or more of
claims 1 to 3, wherein the perfluorinated polymer b) has an MFI
(372/5)<0.5.
5. A melt-processable composition as claimed in one or more of
claims 1 to 4, wherein the perfluoropolymers a) and b) have fewer
than 70 thermally unstable end groups per 10.sup.6 carbon
atoms.
6. A melt-processable composition as claimed in one or more of
claims 1 to 5, wherein the proportion of hexafluoropropylene in the
perfluoropolymer a) is from 7 to 16% by weight.
7. A process for preparing a melt-processable composition as
claimed in one or more of claims 1 to 6, which comprises forming a
mixture of said perfluorinated polymer b) with said
perfluoropolymer a) in such proportion that said perfluorinated
polymer b) is contained in said mixture in an amount of 0.01 to 5%
by weight, based on perfluoropolymer a).
8. The process as claimed in claim 7, wherein said mixture is
formed by mixing a dispersion of said perfluorinated polymer b)
with a dispersion of said perfluoropolymer a), by seed
polymerization or by core-shell polymerization.
9. A process for producing an electrical cable, which comprises
extruding, around a wire, a melt-processible composition as claimed
in one or more of claims 1 to 6.
10. The use of a melt-processable composition as claimed in any of
claims 1 to 6 for processing by extrusion.
11. The use of a melt-processable composition as claimed in any of
claims 1 to 6 for producing insulation for electrical cables.
12. Electrical cable comprising as an insulation, the composition
of any of claims 1 to 6.
13. Electrical cable according to claim 12 wherein said cable is a
plenum wire cable.
Description
[0001] The invention relates to melt processible fluoropolymer
compositions, in particular compositions that comprise a melt
processible fluoropolymer comprising repeating units derived from
tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). Such
copolymers are called "FEP".
[0002] FEP fluoropolymers have been known for a long time (U.S.
Pat. No. 2,946,763), and are commercially available. FEP
fluoropolymers are perfluorinated thermoplastic fluoropolymers that
have excellent heat resistance and chemical resistance. FEP
fluoropolymers also have a low dissipation factor (EP-A-423 995).
Due to all of these properties FEP polymers are of interest for use
as an insulating material for cable-wire insulation, in particular
for what are known as plenum wire cables, used for example in LANs
(local area networks). The processing speeds for producing
insulating plenum cables are very high. FEP polymers that can be
used in producing such plenum cables are therefore generally those
which permit processing at high shear rates without loss of the
necessary mechanical properties.
[0003] FEP polymers which have a broad molar mass distribution
ensure relatively fast processing at relatively high shear rates
(DE-A-26 13 795, DE-A-26 13 642, EP-A-88 414, EP-A-362 868).
Modification with another comonomer (DE-A-27 10 501, EP-A-75 312),
such as perfluoro vinyl ethers, yields retention of the necessary
mechanical properties. To generate high extrusion speeds while
retaining a smooth melt surface, nucleating agents are often added
to the polymeric materials to suppress, and/or shift the occurrence
of "shark skin" (melt surface instability, giving a rough surface)
to higher shear rates (U.S. Pat. No. 5,688,457).
[0004] Besides the formation of "shark skin" at high shear rate,
the tendency of perfluorinated thermoplastics to form die deposits
has to be considered. These die deposits are processing condition
dependent, and take effect in different ways. In fast extrusion
procedures, such as cable-wire insulation, large accumulations of
die deposits separate from the die and cause break-off of the melt
cone and thus interruption of the production, and also interruption
of the continuous cable. High processing temperatures promote die
deposits, and at these temperatures the FEP products decompose more
rapidly, as becomes apparent through discoloration and molecular
degradation. This thermal instability is attributable to unstable
end groups, HFP diads in the main polymer chain (EP-A-150 953) and
metal contamination. The decomposition reaction of the thermally
unstable end groups has been described in "Modern Fluoropolymers",
Ed. John Scheirs, Wiley & Sons 1997, page 228. For this reason
thermally unstable end groups, including COOH, CONH.sub.2 and COF
groups, are preferably converted into thermally stable end groups
by fluorination (GB-A-1 210 794, EP-A-150 953, EP-A-222 945) or by
a stabilization process in the presence of water vapor (DE-A-26 13
795, DE-A-26 13 642). The amounts of die deposits can be minimized
by preparing FEP materials with stable end groups, combined with
high purity with respect to metal ions and narrow molecular weight
distribution (German Patent Application 199 03 657.8 of Jan. 29,
1999, corresponding to PCT/EP00/00528 of Jan. 24, 2000). However,
this high purity is accompanied with increased purification costs,
and a narrow molecular weight distribution, which further promotes
the onset of "shark skin".
[0005] When FEP polymers are melt extruded, high extrusion speeds
increase deposits on the die. These deposits, known as die
deposits, accumulate as time passes and break away from the die
when they reach a particular size. This results in damage to the
final product or, in the case of break-off of the melt, to
interruption of production with other serious consequences. After a
break-off production has to be interrupted until a new cable has
been threaded into the die. The break-off also limits the length of
the cable, thus producing unnecessary waste material in the
twisting of a number of cables of different length. Die deposits of
this type are therefore regularly removed from the die during
processing, but this removal is almost impossible during high-speed
processing, such as cable-wire insulation, and particularly in this
application it had to be accepted that frequent break-offs of the
melt cone would occur.
[0006] The present inventors have found that it would thus be
desirable to reduce the number of times a melt cone breaks off in
the extrusion of FEP polymers, in particular at high speed.
Preferably, this problem is solved without sacrificing the
resulting mechanical properties. Desirably, the mechanical
properties of the FEP polymers are further improved.
[0007] In accordance with the present invention there is provided a
melt-processible perfluorinated polymer composition comprising
[0008] a) a melt-processible perfluoropolymer comprising
[0009] (i) from 80 to 98% by weight of repeating units derived from
tetrafluoroethylene,
[0010] (ii) from 2 to 20% by weight of repeating units derived from
hexafluoropropylene, and
[0011] (iii) from 0 to 5% by weight of repeating units derived from
further comonomers other than tetrafluoroethylene and
hexafluoropropylene,
[0012] and wherein the proportion by weight of the repeating units
derived from hexafluoropropylene units is greater than that of the
repeating units of said further comonomers, and
[0013] b) from 0.01 to 5% by weight, based on perfluoropolymer a),
of a high-molecular-weight perfluorinated polymer with a melting
point at least 20.degree. C. above that of the fluoropolymer
a).
[0014] The invention further provides a method of producing the
above melt-processible composition, the use thereof in
melt-extrusion, in particular to extrude insulation around a wire
to produce an electrical cable. The invention also relates to an
electrical cable having the melt-processible composition as an
insulation.
[0015] The inventors have recognized that the actual problem is not
the die deposit itself but excessive accumulation of the same and
the release of relatively large accumulations, which finally leads
to break-off of the melt cone during wire coating.
[0016] A mixture of the aforementioned melt-processible
perfluoropolymer with a small proportion of a high-melting, i.e.
having a higher melting point than the melt-processible
perfluoropolymer, and high-molecular-weight fluoropolymer, i.e.
having a higher molecular weight than the melt-processible
perfluoropolymer, performs quite differently than known FEP
melt-processible perfluoropolymers. Under the same conditions,
there is very little accumulation of the die deposits which form,
since they regularly break away from the die at very short
intervals. Without intending to be bound by any theory, it is
believed that a possible reason for this is the presence of minor
non-uniformities in the melt, which may be brought about by the
high-molecular-weight, higher-melting fluoropolymer which entrains
the die deposits. The use of the mixture of the melt-processible
perfluoropolymer with the higher-melting, higher molecular weight
perfluoropolymer for high-speed cable-wire sheathing, for example
plenum wire production, may reduce by a factor of 5 the number of
break-offs observed of the melt cone. This ensures continuous
production with fewer interruptions to production and longer
cables.
[0017] The melt-processible composition of the invention generally
also permits an improvement in mechanical properties in comparison
with prior art FEP polymers.
[0018] For example the flexural fatigue strength ("flex life") of
the melt-processible composition of this invention is often many
times greater than that of known FEP copolymer products with the
same melt flow index (MFI). In the art, high flexural fatigue
strengths of FEP products have hitherto been achieved by
modification with perfluoro alkyl vinyl ethers (PAVEs). However,
the dipole moment of PAVEs and the high dissipation factor
associated with this makes these materials disadvantageous,
particularly for high-frequency cable applications.
[0019] The melt-processible perfluorinated polymer composition
according to the invention, may achieve comparable flexural fatigue
properties without the need for any modification with PAVEs. Of
course, if desired modification with PAVEs may be utilized to
further improve flexural fatigue strength and elongation at break
at high temperatures.
[0020] In accordance with the present invention, the
melt-processible perfluorinated polymer composition comprises a
melt-processible perfluoropolymer comprising from 80 to 98% by
weight of repeating units derived from TFE, between 2 and 20% by
weight, preferably between 7 and 16% by weight of repeating units
derived from HFP and between 0 and 5% by weight of further
comonomers other than TFE and HFP and wherein the proportion by
weight of the HFP derived repeating units is larger than that of
the repeating units derived from the further comonomers. Suitable
further comonomers include PAVEs. Examples of suitable PAVE
monomers include those corresponding to the formula:
CF.sub.2.dbd.CF--O--R.sub.f (I)
[0021] wherein R.sub.f represents a perfluorinated aliphatic group
that may contain one or more oxygen atoms. Preferably, the
perfluorovinyl ethers correspond to the general formula:
CF.sub.2.dbd.CFO(R.sub.fO).sub.n(R'.sub.fO).sub.mR".sub.f (II)
[0022] wherein R.sub.f and R'.sub.f are different linear or
branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are
independently 0-10, and R".sub.f is a perfluoroalkyl group of 1-6
carbon atoms. Examples of perfluorovinyl ethers according to the
above formulas include perfluoro-2-propoxypropylvinyl ether,
perfluoro-3-methoxy-n-propylvinyl ether,
perfluoro-2-methoxy-ethylvinyl ether, perfluoromethylvinyl ether
(PMVE), perfluoro-n-propylvinyl ether and
CF.sub.3--(CF.sub.2).sub.2--O---
CF(CF.sub.3)--CF.sub.2--O--CF(CF.sub.3)--CF.sub.2--O--CF.dbd.CF.sub.2.
[0023] Preferably, the melt-processible perfluoropolymer of the
melt-processible perfluorinated polymer composition has a melt flow
index (MFI) in grams per 10 minutes (g/10 min) of more than 0.5,
preferably at least 2, more preferably at least 5 when measured at
372.degree. C. at a load of 5 kg. The melting point of the
melt-processible perfluoropolymer is generally 230 to 280.degree.
C., preferably 240 to 270.degree. C.
[0024] The melt-processible perfluorinated polymer composition
further contains from 0.01% by weight to 5% by weight, preferably
0.05 to 0.5% based on the weight of the melt-processible
perfluoropolymer of a high-molecular weight, higher melting
perfluoropolymer. The higher melting perfluoropolymer typically has
a melting point of at least 20.degree. C. above, preferably at
least 30.degree. C., and more preferably at least 40.degree. C.
above the melting point of the melt-processible perfluoropolymer.
The higher melting perfluoropolymer that is contained in the
composition as a minor component, typically will have a MFI
measured at 372.degree. C. and with a load of 5 kg of not more than
0.5. Preferably, the higher melting perfluoropolymer will have a
melting point of at least 270.degree. C., preferably at least
290.degree. C. Examples of suitable higher melting
perfluoropolymers include copolymers of TFE and PAVEs, e.g. as
mentioned above.
[0025] In a preferred embodiment, the perfluoropolymers of the
melt-processible composition of the invention will have fewer than
70, in particular fewer than 5, thermally unstable end groups per
10.sup.6 carbon atoms. Thermally unstable end groups include COOH,
CONH.sub.2 and COF groups. These can be readily converted into more
stable end groups through fluorination as disclosed in GB-A-I 210
794, EP-A-150 953 or EP-A-222 945 or by a stabilization process in
the presence of water vapor as disclosed in DE-A-26 13 795 or
DE-A-26 13 642.
[0026] The fluoropolymers constituting the melt-processible
composition of the invention may be prepared by any of the known
polymerization methods including aqueous or nonaqueous
polymerization.
[0027] The melt processible compositions may be prepared by mixing
the melt processible perfluoropolymer and the high molecular
weight, higher melting perfluoropolymer. In particular, the
composition can be prepared by mixing the dispersions of the
respective perfluoropolymers or alternatively, the composition can
be prepared through seed polymerization or core-shell
polymerization. For example, in a seed polymerization, the higher
melting, high molecular weight perfluoropolymer may be used as a
seed in an aqueous emulsion polymerization for making the melt
processible perfluoropolymer. As a result of such a seed
polymerization, a melt-processible composition according to the
invention can be directly obtained. Similarly, in a core shell
polymerization, the high molecular weight perfluoropolymer may be
polymerized in the first stage of the polymerization and in a
subsequent stage of the polymerization, the composition of the
polymerization system may be changed to produce the
melt-processible perfluoropolymer. Again, a melt-processible
composition according to the invention may thus be directly
obtained.
[0028] The melt-processible composition is particularly suitable
for producing electrical cables wherein the composition of the
invention serves as an insulator. Cables with a low dissipation
factor may be produced, and such cables are thus particularly
suitable for high-frequency applications (e.g. 100 MHz to 10 GHz)
as for example with plenum wire cables, coaxial cables for
transmitting for example a television signal and "twisted pair"
cables. To produce an electrical cable, the melt-processible
composition of the invention is typically extruded around a central
conductor. To produce a coaxial cable, an outer conductive element,
for example, a metallic foil, a woven or braided composite wire or
a drawn aluminum, copper or other metallic tube may be provided
around the insulated cable. Typically, this outer conductive
element will be encased in further protective insulation. Twisted
pair cables are similar to coaxial cables in that a central
conductor is surrounded by a low-loss insulation, except that a
plurality, typically two, of such conductors are twisted
together.
[0029] Analytical Methods:
[0030] The content of perfluorinated comonomers (U.S. Pat. Nos.
4,029,868, 4,552,925) and the number of end groups (EP-A-226 668,
U.S. Pat. No. 3,085,083) are determined by IR spectroscopy. For
this, a Nicolet Magna 560 FTIR is utilized. The total number of
unstable end groups is calculated from the number of isolated and
bonded COOH groups, CONH.sub.2 groups and COF groups. The total
number of these end groups is in all cases given below.
[0031] The MFI gives the amount of a melt in grams per 10 min which
is extruded from a holding cylinder through a die by the action of
a piston loaded with weights. The dimensions of die, piston,
holding cylinder and weights are standardized (DIN 53735, ASTM
D-1238). All of the MFIs mentioned here have been measured with a
die measuring 2.1 mm in diameter and 8 mm in length, using a
superimposed weight of 5 kg and a temperature of 372.degree. C.
[0032] The flexural fatigue strength ("flex life") tests were
carried out using a model 956, no. 102 device from Frank, built in
1967. Strips of film having a width 15 mm, a thickness of 0.3 mm,
and a length of at least 100 mm were tested. Adhesive strips were
used to hold a film sample of about DIN A5 size to the drum of a
film cutter, a draw-knife system was put in place, and the cutting
drum was rotated to produce strips at the preset knife separation.
The strips of film were clamped into the screw clamps of the
flexural fatigue (Frank) device and loaded with a suspended weight
of 1529.6 g. The strips of film were flexed in the apparatus
through an angle of 90.degree. in both directions at a folding
frequency of 250 double flexures per minute until fracture
occurred. A counter on the device recorded the number of double
flexures until fracture. The flexural fatigue strength, or flex
life, of a material was the average number of double flexures until
failure for three samples.
[0033] The examples below describe the invention in more detail.
Percentages and ratios are based on weight unless otherwise
stated.
COMPARATIVE EXAMPLE 1
[0034] 25 L of demineralized water and 122 g of ammonium
perfluorooctanoate in the form of a 30% strength solution were
charged to a polymerization reactor whose total capacity was 40 L,
provided with an impeller stirrer. Once the reactor had been
sealed, alternating evacuation and nitrogen-flushing were used to
remove atmospheric oxygen, and the vessel was heated to 70.degree.
C. After an evacuation stage, 11.0 bar of HFP were pumped in. The
stirrer rate was set to 240 rpm. TFE was then introduced until the
total pressure had reached 17.0 bar. The polymerization was
initiated by pumping in 35 g of ammonium peroxodisulfate
(hereinafter APS), dissolved in 100 mL of demineralized water. As
soon as the pressure began to fall, supplementary gaseous TFE and
HFP were added with an HFP/TFE feed ratio of 0.11 to maintain the
total pressure at 17.0 bar. The heat generated was dissipated by
cooling the vessel wall, keeping the temperature constant at
70.degree. C. After a total of 7.2 kg of TFE had been fed into the
reactor the monomer feed was stopped, the pressure in the reactor
released, and the reactor flushed several times with N.sub.2.
[0035] The resultant 31.5 kg of polymer dispersion with a solids
content of 22:8% were discharged at the base of the reactor. The
dispersion was transferred to a 180 L precipitation vessel and made
up to 100 L with demineralized water, mixed with 200 mL of
concentrated hydrochloric acid and stirred until the solid had
separated from the aqueous phase. The flocculent powder
precipitated was granulated with 6.9 L of petroleum spirit, and the
petroleum spirit was driven off using steam. The product was then
washed six times, each time with 100 L of demineralized water, with
vigorous stirring. The moist powder was dried for 12 hours in a
drying cabinet under nitrogen at 200.degree. C. This gave 7.1 kg of
a copolymer which had an HFP content of 15%, a melting point of
252.degree. C. and an MFI of 24.
[0036] The product was subjected to melt granulation and then 3 kg
of product were charged to a 4 L fluorination reactor. During
heating to 200.degree. C., alternating evacuation and
nitrogen-flushing were used to remove atmospheric oxygen and
moisture. The reactor was then filled with a F.sub.2/N.sub.2
mixture comprising 20% of F.sub.2. The reaction ran for 5 hours,
and the F.sub.2/N.sub.2 mixture was renewed after each hour. During
cooling from 200.degree. C. to room temperature, alternating
evacuation and nitrogen-flushing was used to remove unconverted
fluorine. The resultant product had only 11 remaining thermally
unstable end groups per 10.sup.6 carbon atoms.
COMPARATIVE EXAMPLE 2
[0037] The procedure was similar to that of Comparative Example 1,
except that 8 bar of HFP were pumped in after the evacuation and 10
g of APS were employed, and the HFP/TFE feed ratio was 0.7. The
resultant product had an HFP content of 6.9%, a melting point of
301.degree. C. and an MFI of 0.01.
EXAMPLE 1
[0038] 0.2% of the dispersion of Comparative Example 2, based on
solids, were added into the dispersion of a product from
Comparative Example 1, and the materials were then worked up
together as described in Comparative Example 1. The resultant
product had an MFI of 23 and had 7 remaining thermally unstable end
groups per 10.sup.6 carbon atoms.
EXAMPLE 2
[0039] After 30 h of running time, the occurrence of melt cone
break-off using the product from Example 1 at very high take-off
speeds and high die temperatures and screw rotation rates was
reduced by a factor of 5 over that of a comparable FEP copolymer
product from Comparative Example 1. This was attributable to
reduced accumulation of die deposits.
1 Extruder: 45 mm, L/D 30:1 with mixing head Die: 0.965 cm Mandrel:
0.558 cm Copper wire: 0.0515 cm Insulation: 0.0175 cm
[0040]
2TABLE 1 Cable extrusion with products from Example 1 and
Comparative Example 1 Comparative Example 1 Example 1 MFI(g/10 min)
23 24 Copper wire temperature (.degree. C.) 190 190 Cone length
(cm) 5.1 5.1 Die temperature (.degree. C.) 404 404 Screw rotation
rate (rpm) 42.5 42.5 Take-off speed (m/min) 611 611 Melt cone
break-off after 30 h of running 1 5 time
COMPARATIVE EXAMPLE 3
[0041] Polymerization and work-up took place as described in
Comparative Example 1, but to produce a lower MFI only 12 g of APS
were used. The resultant product had an MFI of 6.9, a melting point
of 256.degree. C., HFP content of 14% and 4 thermally unstable end
groups per 1 06 carbon atoms.
COMPARATIVE EXAMPLE 4
[0042] Polymerization and work-up took place as described in
Comparative Example 1, but to produce a lower MFI only 12 g of APS
were used, and the initial charge of HFP was reduced to 10.5 bar,
and perfluoro n-propyl vinyl ether (PPVE) was also fed at a
PPVE/TFE feed ratio of 0.011. The resultant product had an MFI of
7.4, a melting point of 254.degree. C., HFP content of 13.1%, PPVE
content of 0.9% and 7 thermally unstable end groups per 10.sup.6
carbon atoms.
EXAMPLE 3
[0043] A dispersion product from Comparative Example 3 was mixed
with 0.2% (based on solids) of the dispersion product from
Comparative Example 2 with its MFI of 0.01, and the materials were
worked up together, as described in Comparative Example 1. The
resultant product had an MFI of 6.8 and 5 remaining thermally
unstable end groups per 10.sup.6 carbon atoms.
[0044] The products from Comparative Examples 3 and 4 were compared
for flexural fatigue strength with the product from Example 3. The
product from Example 3 showed an improvement by a factor of 4 in
flexural fatigue behavior over a standard FEP copolymer of
Comparative Example 3, and had also been improved over the
PPVE-modified FEP material from Comparative Example 4.
3TABLE 2 Comparison of flexural fatigue strengths Cycles prior to
fracture Example 3 46000 Comparative Example 3 11000 Comparative
Example 4 3000
* * * * *