The synthesis of high performance elastomers with the high thermal stability and chemical, inertness of perfluoroalkylene triazine and a low glass transition temperature is discussed. Perfluorether triazine elastomers were proposed as potentially superior. It is concluded that the difficulties experienced in fluoroalkytriazine elastomer synthesis can be overcome by a four-step reaction process involving chain extension, triazine ring closure, crosslinking, and elastomer curing. Molecular weight can be controlled in the initial polymer formation so that elastomer modulus can be determined. The final product elastomers exhibit a useful elastomeric range of approximately 45 to 325 C with an oxidative stability superior to other broad range elastomers

Korus, R. A.

English

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https://ntrs.nasa.gov/search.jsp?R=19810013661 2020-03-21T14:06:23+00:00Z
Project Monitor
Robert W. Rosser
NASA-Aloes Research Center
Moffett Field, California 94035
FINAL REPORT NASA GRANT NO. NSG 2367
Ames Control Number: 79-080
Period: i,wrch 1, 1980 to February 28, 1981
PERF.UOROETHER 'rRIAZINE ELASTOMERS
(HAEA-CH -164194) PERILU©ROBVIER T11IA'LIRE	 N61-22131
ELh ,TOMEhS Filial Report, 1 Liar. 1900 - 28
Feb. 1961 (Idaho Univ.) y p HC A02/MF° A01
CSCL 07C	 U 11cl asGJ/27 42083
Principal Investigator:
Roger A. Korus
Assistant Professor
Department of Chemical Engineering
University of Idaho
Moscow, Idaho 83843
(208) 885-6793
104.0 
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MY
RECEIVED	 j.
NASA S^FDEPZTM v^y'
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1
INTRDDUCTION
The weakest link in high performance equipment is often an elastomeric com-
ponent. Seals, gaskets, diaphragms and couplings are inherently less stable than
structural metals and ccioposites. This is true of such diverse applications as
fuel tank sealants in high speed aircraft, couplings for drill bits used in geo-
thermal
	 rmations, deep wells and welds exposed to sour gas, electrical connectors,
firewall seals, and tough-service parts in automobile engines, pumps, compressors,
pipelines and scientific equipment. All of these applications require elastomeric
materials with outstanding thermal and environmental stability.
Development of high performance, thermally stable elastomers has concentrated
on the fluorocarbons and fluorosilicones. Fluvrocarb.ns are generally superior to
fluorosilicones in thermal stability and fluid resistance. However, most fluoro-
carbons are not useful as elastomers at low temperatures (less that '40 0C) because
of their high glass transition temperatures. In particular, perfluoroalkylene-
triazina elastomers have promised a clear superiority over other known elastomers
in thermal and oxidative stabiliiy. Their reduction to practice has been frustrated
by the complex nature of the chemical system used for polymerization and the non-avail-
ability of desirable synthetic intermediates (Young, 1972).
In order to obtain high performance elastomers with the high ther~nal stability
and chemical inertness of perfluoroalkylene triazine and a low glass transition
temperature, perfluoroether triazine elastomers were proposed as potentially superior
high performance elastomers (Rosser et. al., 1975). However, perfluoroether synthesis
was limited to low molecular weight polymers, Elastomers with high tensile strength
and with a low glass transition temperature were not produced at this time. Before
an improved high performance elastomer could be produced, high molecular weight
linear perfluoroether craizine polymers were required that could be crosslinked
by a stable linkage with the average chain length between crosslinks controlled in
order to achieve desired mechanical properties.
Measurement of molecular weight distributions of fluorinated polymers is
difficult because of their insolubility in organic solvents. So fluorinated
solvents must be used. To determine the molecular weight distribution of per-
fluoroether pr.iymers size exclusion chromatography with small particle silica
column packing was used (torus and dosser, 1978). This rigid packing is com-
patible with fluorinated solvents whereas the polystyrene gel packings shrink
in the presence of fluorinated solvents. Thermally stable perfluoroalkylether
oxi4iazole elastomers have been crosslinked by tri-functional triazine and
characterized by size exclusion chromatography (Rosser et. al., 1979). This
stable triazine crostlink can also be used in the synthesis of perfluoroether
triazine elastomers.
Elastomer Synthesis
The starting material for the synthesis of perfluoroalkylether triazine
elastomers is a high molecular weight ether diacid fluoride. Perfluoroalkyl-
ether diacid fluorides and trifluoroacetic anhydridq were obtained from PCR, Inc.
These diacid fluorides .ire prepared by the step-wise addition of hexafluoro-
propylene oxide to hexafluoroglutaryl fluoride (Rosser et. al,, 1975). The
resulting oligomer has the following structure with the number of hexafluoro
propylene oxide units added equal to m + n + 2:
F3iI
CF3 C+3 CI3 0
F - C - CF - (OCF2CF) rrr -- 0 -	 (CF 2 ) 5 -- p -	 (Cr - CF 20) n
 - CF - CF
or
F-C- Py - C - F
i'erfluoroalkylether diacid fluorides were converted to diamides by the
addition of ammonia
^C - Rf C` + 2NN3 .^	 C _ Rf - ;	 + 2NF	 (1)
r	 1.1 2N	 NH 
Aninonia was bubbled through a solution of the diacid fluoride in 1, 1, 2-trichlore-
1, 2, 2, - trfluoroethane (Freon 113). The reaction flash was connected to a
reflux condenser loaded with dry ice so that ammonia was refluxed. Reaction
completion required 30-60 mii>utes and could be detected by a cooling of the
reaction flask as the exothermic reaction reached completion. There was a
­__ I'
I
phase sop-aration in the reaction product as the diamide has a very low solubility
in Freon 113. Methanol was add+od to dissolve the diamide, and the solution was
filtered under vacuum through a course filter paper to remove the aimnonium fluoride
by-product. Then Freon 113 and methane>'[ were removed by distillation, and the
diamide dried in a vacuum oven. Diamide is recovered with a yield of 80 to 901.
Perfluoroalkylether dinitrile was prepared by dehydration of the diamide
in the presence of P20 5. The P205 slurry was heated at 150 to 160°C for approxi-
mately five days and then vacuum distilled to remove the dinitrile. Oinitrile ;s
produced from diamide at a yield of 60 to 80%. Infrared spectroscopy was used
to monitor the above reactions since the acid fluoride, amide, and nitrite
infrared bands have higher frequencies than the ether, carbon-fluorine, and
carbon-carbon infrared bands (Figure 1).
Crosslinked perfluoroalkylether triazine elastomers were prepared from per-
fluoroalkylether dinitriles by the following four step reaction sequence:
^ N
NC-Rf —^ CN	 NIi3%____ NC- R f •-- ^Ci 	 	 Rf --CN
	
NH	 N1,2
POLY(IMIOOYLAMIOINE)
+ 2CCF3C(0)12C
NC	
Rf rN
	Rf	 CN
N\ N	 + 3Cf3COOH
3 x
LINEAR POLYTRIAZINE
+ NN3
N
Rf	 N	 Rf ----^ C 	 \C	 R f,,,` 
)I
N'`N Nfi	 N^
C  Jx
EXTENDED O NEAR POLYMER
(G)
(3)
(4)
^/
^
^
^
^
x
|
!	 <
^	 ^
.^
Rf
EXTENDED LINEAR POLYMER
NEAT
Rf^N	 R f--
r._
NRf
N ^,^,N 	 J,N
't'
3
This reaction sequence involves:
(1) A step-growth polymerization which occurs with ammonia addition to
the dinitrile forming poly(imidoylamiline) (See reaction 2). It is
important to exclude water vapor duritrg this reaction to prevent amide
formation. Before ansnonia .addition, the re.-Action flask was purged with
nitrogen, Nitrogen and ammonia were purified by passage through dry-
ing tubes packed with caicium oxide. The extent of reaction was follow-
ed by IR spectroscopy and viscometry (Rosser and Korus, 1980). The
extent of this first polymerization reaction determines the eventual
molecular weigh-,s hc^cwec:n crosslinks. The terminal nitrile groups of
the poly -(imidoylaimi(line) tire the active sites for crosslink formation,
and all subsequent reactions were carr i ed out to completion.
(2) linear polytriazines were formed with the addition of trifluoroac(^tic
anhydride (See reaction 3). The polymer solution was added dropwise
to trifluoroacetic anhydride in an amber bottle with the weight of tri-
fluoroacetic anhydride at least 0.3 tiames the weight of the polymer. An
amber bottle is necessary to prevent photoinitiated decomposition of the
anhydride. Slow addition of poly (imidoyl amtidine) to the triftuoroacetic
anhydride solution is required in order to maintain a high concentration
of the anhydride during ring closure. The anhydride acts as a dehydrating
agent, and prevents hydrolysis products, especially amides, from forming.
hid des act as a polymer chain terminator and are formed if water is not
excluded from this reaction. The resultant solution was allowed to
stand overnight. Freon 113 was then removed by disti p lation. After
solvent removal a twig ;chase, poly (perfluoroalkyether triazine) and tri-
fluoroacetic acid, system resulted. The poly (perfluoroalkylether tria-
(b)
X
zine) layer was removed and placed under vacuum for complete tri-
fluoroacetic acid removal.
(3) Aivionia was introduced to reflux as in reaction 2 until completion
of poly (imidoylamidine) formation.
(4) The resultant polymer was heated for approximately one clay periods
at 100°C, 130°C and 150°C to form the crosslinked elastomer. Heating
of imidoyl amidine gives a mixture of imidoyl amidine, nitrile,
aumnonia, aA triazine. Amidines and imidoyl amidines condense to
form triazine with anrnonia being liberated as cyclization occurs
(Young, 1972).
RESULTS AND DISCUSSION
Using synthetic methods similar to those described here, Dorfman et al. (1966)
have synthesized high molecular weight perfluoroalkyl triazine polymers. These
linear polymers were crosslinked to form tough, rubbery elastorners with glass
transition temperatures from -17 to -5°C. Young (1972) stated that the materials
made by Dorfman, et al. represent the closest approaches to practicable perfluoro-
alkylene-triazine elastorners. However, only perfluoroalkyl monomers were used by
Dorfman et al. whereas perfluoroalkylether monomers have been used for perfluoro-
alkylether triazine synthesis. Using perfluoroalkylether groups produces improved
low temperature properties. Perfluoroalkylether triazine elastomers have glass
transition temperatures of approximately -45 0C or about 35 degrees less than the
corresponding perfluoroalkyl triazine elastorners,. Also, the method of perfluoro-
alkylether triazine crosslinking (reacttions 4 and 5) is novel. Dorfman et al. do
not even discuss crosslinking methods. Young discusses possible ways for incorporat-
ing sites for crosslinking in a linear triazine polymer. The methods that he
discusses are considerahly more cooplex, requiring higher curing temperatures, and
are not as s pecific as they crossiink reactions 4 and 5.
Perfluoroalkylether synthesis has previously been limited to low molecular
weight polymers, and elastomer synthesis was severely restricted. However, linear
polymers with weight average molecular weights from 20,000 to 28,000 can be pre-
pared following reactions 2 and 3 (Korus and Rosser, r 1978	 Rosser v, d Korus, 19£30).
Size-exclusion chromatography, viscometry, and infrared spectroscopy have been used
to monitor reactions 2 and 3. Previous polymerization reactions of imidoylamidine
and triazine derivatives gave products with molecular weights too low for good
physical properties. These products were also sensitive to the reaction conditions
and therefore were not easily reproduced (Young, 1972).
The properties of ttte cured pertluoroalkylether triazine elastomers are
affected by the length of the alkylether region of the polymer, m + n value,
	
and by the degree of polymerization between crosslinks, x 	 Increasing the alkyl
ether portion of the elastomer gives more chain flexibility and a lower glass
transition temperature (Table I). Increasing the degree of polymerization between
TABLE 1: Glass Transition Temperatures
m + n	
_	
X	 .
2	 -36
4	 -41
6	 -45
crosslinks reduced the elastomer modulue (Table II),
TABLE .II	 tlastomer Moduli for m + n = 6
x
	
Modulus A)-) 2)
4	 107
6	 106
8	 105
10	 20	 104
Perfluoroalkylether triazine elastomers show lower isothermal weight losses
than poly (trifluoropropylmethane siloxane), a thermally stable polyester sealant,
and perfluoroalkylether oxadiazole elastomers (Rosser and Korus, 19€30a). The
high temperature oxidative stal;flity of the perfluoroether triazine elastomers
show a very marked improvement over state-of-the-art thermally stable elastomers.
Initial weight losses of perfluoroalkylether triazine elastomers in air at 300°C
are only about 2A"; of the weight losses experienced by poly (trifluoropropyl methyl
siloxane). After 6-12 hours in air at 300 0 0 the triazine elastomers show little
change in physical properties while the siloxant %. rlastomners are reduced to a char.
Triazine polymer synthesis is not limited to the perfluoroalkylether
triazine polymers described. Similar synthesis procedures have been used for
several perfluorodinitrile monomers and ring-closing reagents other than tri-
fluoroacetic anhydride (Rosser and Korus, 1980 b).
3
, 4
1
CONCLUSION
The diff =iculties experienced in fluoroalkyltriaxine elastomer synthesis
can be overcome by a four-step reaction process involving chain extension, aria
xine ring closure, crosslinking, and elastomer curing. Molecular weight can be
controlled in the initial polymer formation so that elastomer modulus can be
determined, The final product elastomers exhibit a useful ':lastomeric range of
approximately -450C to 32500 with an oxidative stability superior to other broad
range elastomer.s.
REFERENCES
1. Dorfman, E., Emerson, W.G., Carr, R.L.K., and Bean, C.T., Rubber
Chem, Technol. (1966), 39(4), 1175,
2. Korus, R.A. and (tosser, R.W., Anal. Chem. (1978), 50, 249.
3. Rosser, R.W. and Korus, R.A., J. Polym. Sci.: Polym, Letters Ed.
(1980a), 1P, 135..
4. Rosser, R.W. and Korus, R.A., U.S. Patent #4,242,498 (1980b).
5. Rosser, R.W., Korus R.A., Sha'houb, I.M. and Kwong, H., J. POVIII,
Sci. , Polynr. Letters Ed, (1979.1 17, 635.
6. Rosser, R.W., Parker, J.A., DePasquale, R.J., and Stump, E.C., Jr.)
ACS Symposium Series, No. 6, Polyethers, 135 (1975).
7. Young, J.A., "Fluoropolynaers," in "Cligh Polymer Series," Wiley-Inter-
science, New York, Vol. 25, Chap. 9 (1972).


Perfluoroether triazine elastomers

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