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Polymer International
Polym Int 48:993±995 (1999)
Crosslinking of tetrafluoro-ethylene–perfluoro(methyl vinyl ether) perfluoro-elastomers with
electron beam irradiation
Anestis L Logothetis*
DuPont Dow Elastomers, LLC, Dupont Experimental Station, PO Box 80328, Wilmington, DE 19880-0328, USA
Abstract: Pre-shaped per¯uoro-elastomeric articles were crosslinked by electron-beam irradiation.
Doses of 10±15 Mrad are adequate to give a gel content of more than 95%. Irradiation experiments were
carried out on the pure polymer free of ®llers, processing aids or curing chemicals. The physical
properties of `O'-rings thus crosslinked are comparable to those chemically crosslinked. Addition of
free radical traps, such as triallyl isocyanurate, accelerates the crosslinking process and improves the
physical properties. Irradiation in addition to crosslinking also causes chain scissions. The per¯uoroether group appears to be the focus of the radiation attack. Radical intermediates and ions, produced
by the ionization and excitation steps, undergo chemical reactions which cause changes in the
molecular structure of the polymers. These changes are (a) formation of small molecules such as CF4,
COF2, CF3OCF3 and CO2 via side chain scission, (b) main chain scission of the polymeric chains, and
.
(c) crosslinking of the macromolecules. Y-type recombination of the per¯uoro-alkyl radicals ÐCF2
.
and ÐCF2CF CF2Ð is the most likely way that the crosslinking process occurs. The relatively high
mobility of the elastomeric chains at room temperature allows the radical to recombine.
# 1999 Society of Chemical Industry
Keywords: per¯uoroelastomers; electron beam; irradiation; crosslinking
INTRODUCTION
The sensitivity of ¯uoropolymers to ionizing irradiation is well known. Investigators found that polytetra¯uoroethylene (PTFE) and crystalline copolymers of
TFE mainly degrade when subjected to even mild
doses of irradiation at room temperatures. Amorphous
polymers such as per¯uoro(polyethers), irrespective of
their backbone composition, undergo main-chain
scission by an unzipping process of their polymeric
chains. Lyons1 has recently written an extensive review
describing the irradiation chemistry of ¯uoropolymers
including ¯uoro-elastomers. Uschold2 in 1984
reported that when he irradiated per¯uoro-elastomers
he found that the polymers had crosslinked but,
because of the extensive main chain scission, the
properties were poor. We found that by controlling the
experimental conditions, excellent properties are obtained from the irradiation of per¯uoroelastomers.3,4
RESULTS
The following observations were made when a TFE/
per¯uoro(methylvinyl ether) (PMVE) copolymer was
exposed to electron beam radiation.
There is a loss of weight proportional to the
irradiation dose ranging from 0.1 wt% loss at 4 Mrad
to 0.5 wt% at 20 Mrad to 5 wt% at 350 Mrad to 10 wt%
at 700 Mrad. The weight loss is due to the formation of
gaseous products identi®ed by infrared spectroscopy
and GC/MS as CF4, COF2, CF3OCF3, CO2 and low
molecular weight RÐCOF molecules.3,5
The polymer after irradiation shows formation of a
carbonyl ¯uoride functionality which hydrolyses to
carboxylic acid by atmospheric moisture during
handling of the ®lms. The carboxylic acid functionality
was identi®ed by the characteristic 1973 cmÿ1 peak in
the infrared spectrum.
The polymers which are originally soluble in the
¯uorinated solvent Fluorinert FC-75, become insoluble upon irradiation. Little difference was observed in
the results when the irradiation was carried out in the
presence or absence of air.
The tensile strength of irradiated `O'-rings or dumbbells show a decrease to a minimum value after 1±2
Mrad dose followed by an increase and eventual
leveling-off of the values as the dose increases to 20
Mrad. The elongation at break does the opposite,
increasing to a maximum value at 1±2 Mrad exposure
followed by a decrease and eventual ¯attening of the
curve as the dose increases.
When the TFE/PMVE copoplymer also contains a
nitrile and/or an iodine functionality, a similar weight
* Correspondence to: Anestis L Logothetis, DuPont Dow Elastomers, LLC, Dupont Experimental Station, PO Box 80328, Wilmington,
DE 19880-0328, USA
(Received 28 August 1998; accepted 22 January 1999)
# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50
993
AL Logothetis
Table 1. Changes in polymer composition upon irradiation
Table 4. Properties of TAIC-containing perfluoro-elastomers
Radiation
(Mrad)
Properties
0
1
2
4
8
16
20
Weight
loss (wt%)
PMVE
(wt%)
Nitrile
(wt%)
Iodine
(wt%)
±
±
0.074
0.105
0.141
0.347
0.518
43.3
42.4
41.9
44.7
42.3
41.8
41.2
1.8
1.7
1.7
2.2
1.8
1.9
1.9
0.12
0.11
0.11
0.10
0.10
0.10
0.10
M100 (lbf inÿ2)
Tb (lbf inÿ2)
Eb (%)
Compression set 150 °C/70 h
Crosslinking system
Irradiation (12 Mrad) Triazine
Compression set
336 h at 23 °C
70 h at 175 °C
336 h at 175 °C
70 h at 204 °C
Tensile properties
M100 (lbf inÿ2)
TB, (lbf inÿ2)
EB (%)
Hardness (Shore A)
35
22
28
72
23
25
25
30
305
1130
285
70
210
1250
240
68
Table 3. Chemical resistance of irradiation and chemically crosslinked
perfluoro-elastomers
Crosslinked system
swelling (%)
Chemical
Ethylene diamine
Nitric acid (70%)
Tri¯uoroacetic acid
Acetic acid
HF (5%)
HF (60%)
HF gas
Temperature for
70 h ( °C)
Irradiation
(12 Mrad)
Triazine
90
85
50
100
100
100
50
36
2
28
3
35
30
25
28
2
26
3
31
28
26
loss was observed but the functional groups remained
relatively unaffected when irradiated up to 20 Mrad
(Table 1).
The physical properties of radiation treated per¯uoro-elastomers compare favourably with those
chemically crosslinked as shown in Table 2 and 3.
Incorporation of triallyl isocyanurate (TAIC) in the
polymer improves the physical properties of irradiated
`O'-rings, particularly the M100, TB, and EB values
(Table 4). In addition, the properties reached their
optimum level at irradiation doses of 5±8 Mrad when
the TAIC was present compared with 12±25 Mrad for
the control, indicating that TAIC is helping the
crosslinking process.
994
232
742
308
25
1062
1447
117
28
Table 5. Fluorination of crosslinked polymers: physical properties of the
‘O’-rings
Table 2. Physical properties of irradiation crosslinked perfluoroelastomers
Property
Control Polymer with 2 wt%
(15 Mrad)
TAIC (Mrad)
Property
M100 (lbf inÿ2)
TB (lbf inÿ2)
EB (%)
Hardness (Short A)
Compression set
150 °C/70 h
200 °C/70 h
Before ¯uorination
After ¯uorination
(70 °C/6 h)
301
1202
243
73
236
1206
283
69
27
72
14
33
The crosslinked network is stable to elemental
¯uorine and no decrease in the crosslink density was
observed as judged by the gel content, tensile properties and compression set resistance. The only change
observed was that the carboxylic acid functionality
formed as end-groups upon irradiation was eliminated
upon exposure to the elemental ¯uorine. The fact that
the crosslinks are not attacked by elemental ¯uorine
strongly indicates that they are CÐC type bonds
(Table 5).
Solid state 13C and 19F NMR6 and infrared
spectroscopy give evidence that some new end-groups
are formed upon irradiation that have the following
structures: ÐCF2COOH, ÐCF=CF2, ÐCF2CF3
and ÐCF2OCF3.
DISCUSSION
Based on the above observations and by analogy to the
previous work with hydrocarbon and ¯uorocarbon
polymers, we propose that in the case of per¯uoroelastomers also, irradiation causes excitation and
ionization of the macromolecules. Intermediate radicals and ions are formed which in turn undergo
chemical reactions that transform the molecular
structure of the macromolecules. These changes
appear in the form of side-chain and main-chain
scissions (see Fig 1). All indications point to the
per¯uoro-ether functionality on the polymer as being
the focus of the scission reactions which result in (a)
formation of volatile products such as CF4, COF2,
CF3OCF3 and CO2 as a result of side-chain scissions,
and (b) formation of the new kinds of end-groups
(described above) as a result of main-chain scissions.
Y-type recombination of per¯uoro-alkyl radicals
.
.
(ÐCF2 and ÐCF2CF CF2Ð), one primary and the
Polym Int 48:993±995 (1999)
TFE/PMVE crosslinking with electron beam irradiation
Figure 1. Proposed steps for the
scission products during electron-beam
irradiation of a TFE/PMVE perfluoroelastomer.
that it is theoretically dif®cult to produce H-type crosslinks by recombination of two secondary per¯uororadicals. The Y-type structure is supported by the fact
that it contains a tertiary F atom at the branching site
of the main chain which appears as a new peak in the
solid state 19F NMR spectrum at ÿ183.3 ppm.6 This
peak appears only after the polymer is irradiated. The
relatively high mobility of the elastomeric chains at
room temperature allows radical combinations to
occur readily.
ACKNOWLEDGEMENTS
Figure 2. Proposed steps for crosslinking during electron-beam irradiation
of a TFE/PMVE perfluoro-elastomer.
The author wishes to thank Ta-Chen Mo for his
assistance with the experiments, and John S Forsythe
and David JT Hill of the University of Queensland for
making available their 13C and 19F NMR data and for
discussions which helped elucidate the mechanistic
aspects of the irradiation process.
REFERENCES
other secondary, is the most likely way that crosslinking proceeds (see Fig 2). The two radicals are
formed by a main-chain scission process. Recombination of a primary radical with another primary radical
is favoured, and does take place extensively, but it
results in chain extension and not crosslinking. The
recombination of a primary radical with a secondary
radical is also favoured and leads to a Y-type crosslink.
At high temperature (above its melting point) radiation crosslinking of Te¯on FEP7 and PTFE8 reported
recently is explained with a similar Y-type structure.
For steric reasons it was shown by Tsuda and Oikawa9
Polym Int 48:993±995 (1999)
1
2
3
4
5
6
7
8
9
Lyons BJ, Radiat Phys Chem 45(2):159 (1995).
Uschold RE, J Appl Polym Sci 29:1335 (1984).
Logothetis AL, J Appl Polym Chem 63:147±156 (1997).
Logothetis AL, US Patent 5, 260, 351 (1993), E I duPont de
Nemours and Co Inc.
Pacansky J, Waltman RJ, and Jebens D, Macromolecules 29:7699±
7704 (1996).
Forsythe JS, Hill DJT, Logothetis AL, Seguchi T and Whittaker
AK, Macromolecules, 20:8101±8108 (1997).
Zhong X, Sun J, Wong F and Sun Y, J Appl Polym Sci 44:639
(1992).
Tabata Y, Oshima A, Takashika K and Seguchi T, Radiat Phys
Chem 48:563 (1996).
Tsuda M and Oikawa S, J Polym Sci Polym Chem Ed 17:3759
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