ELASTOMERE UND KUNSTSTOFFE
ELASTOMERS AND PLASTICS
Low viscosity peroxide curable fluoroelastomers Advanced polymer architecture Processability Part quality Properties
Elastomers are specified in a variety of industrial, automotive and aerospace applications where large volumes of high quality
seals, tubes and hose are needed to contain
or transport a variety of gases and liquids. In
many instances, due to high operating
temperatures or reasons of chemical compatibility, fluoroelastomers are used. In the
case of high volume applications like seals
for automotive service, the need for easy
part manufacture is imperative to achieve
the required balance of part quality and
production economy. Ease of processing
also impacts part quality since processing
difficulties like mould sticking and fouling,
or problems with mould filling or extrusion,
can result in surface defects in seals and
tubes. In addition to easy processing
moulded and extruded parts must have
properties that provide long term reliable
service in use.
New fluoroelastomers based on Advanced
Polymer Architecture technology were originally disclosed in 2001 [1, 2] and comprise
a redesigned main chain molecular structure as well as new chain termination and
cure site chemistry. This paper introduces
several new low viscosity versions of these
peroxide curable fluoroelastomers, designed to deliver easy processing in the most
difficult moulding processes as well as excellent properties.
Neue peroxidisch vernetzbare
Fluorkautschuke mit außergewöhnlichen Gebrauchs- und
Verarbeitungseigenschaften
niedrigviskose, peroxidisch vernetzbare
Fluorkautschuke verbesserte Polymerarchitektur Verarbeitbarkeit Produktqualität Gebrauchseigenschaften
Wo Dichtungen, Leitungen und Schläuche hohen Temperaturen und unterschiedlichsten Chemikalien widerstehen
müssen, kommen oft Fluorkautschuke
zum Einsatz. Dabei wirkt sich die Verarbeitbarkeit der Materialien direkt auf die
Produktqualität aus, weil anhaftende
Teile oder Ablagerungen am Werkzeug
Oberflächenfehler bei den Formteilen
verursachen können. Abhilfe bei solchen
Problemen kann eine Reihe von 2001
erstmals vorgestellten Fluorkautschuken
schaffen. Kennzeichen dieser Werkstoffe
sind eine verbesserte Polymerarchitektur
[1, 2] sowie neuartige Vernetzungsstellen-Monomere. Der Artikel stellt neue,
niedrigviskose Typen dieser peroxidisch
vernetzbaren Fluorkautschuke vor, bei
deren Entwicklung eine leichte Verarbeitbarkeit selbst in anspruchsvollen
Formgebungsprozessen sowie sehr gute
Gebrauchseigenschaften im Mittelpunkt
standen.
New Peroxide Curable Fluoroelastomer Compositions
Properties and Processing Characteristics
In today’s industry, manufacturing costs
need to be minimized to maintain profitability. Injection moulds tend to be far more
demanding and often use cold feed systems which have the advantage that material in the feed system remains for the
most part unvulcanized and hence is not
wasted as in a traditional hot runner system. Transfer moulds can also present
very difficult flow conditions since relatively low temperature material flows under
high shear through transfer ports that
are usually rather small in cross section.
When elastomer compounds flow under
demanding conditions, relatively high
shear stresses are incurred which can cause
high injection pressures and significant
shear heating. The result can be an inability
to fill the mould, either due to a simple lack
of injection pressure or due to scorch of the
compound. Even in conventional direct injection hot runner systems problems can
arise when high viscosity compounds are
used, such as mineral filled compounds
or those designed to meet high hardness
requirements. Process aids, which are
usually incompatible with the polymer matrix and migrate to the surface, can improve flow and demolding but if added in excess can cause defects on the surface of
molded parts. The best solution is to use
a polymer that flows easily without the
need for high levels of process aids.
New Viton fluoroelastomer
grades
The first APA polymers had nominal Mooney viscosities of 65 ML 1 þ 10 at 121 8C
and are indicated in Table 1.
These 65 ML 1 þ 10 at 121 8C polymers
provide excellent processing in compression molding and extrusion, as well as in
many injection molding applications,
now confirmed in numerous industrial applications. For the reasons mentioned previously lower viscosity polymers are needed in some cases and three new polymers
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 1-2/2004
have recently been developed for use in
demanding molding processes. The new
polymers are VTR-8505, 8605 and 8655,
indicated in Table 2. They are intended
for use either alone or in blends with their
higher viscosity counterparts to provide an
optimal compound viscosity for a given
process.
VTR-8655 and VTR-8605 are new GBL-S
and GF-S type polymers with nominal
Mooney viscosities of 25 ML 1 þ 10 at
121 8C. They are based on the main monomers hexafluoroproplyene (HFP), tetrafluoroethylene (TFE), and vinylidene fluoride
(VF2) with a cure site monomer (CSM) to
allow peroxide curing. They have nominal
fluorine contents of 68 % and 70 % respectively and the general chemical structure:
ÿCF2ÿCFÿCH2ÿCF2ÿCF2ÿCF2ÿðCSMÞ
|
CF3
VTR-8505 is a new GLT-S type polymer of
nominal Mooney viscosity 25 ML 1 þ 10 at
121 8C based on the main monomers perfluoromethylvinylether (PMVE), TFE and
VF2 with a cure site monomer. It has the
general chemical structure:
ÿCF2ÿCFÿCH2ÿCF2ÿCF2ÿCF2ÿðCSMÞ
|
OCF3
VTR-8505, VTR-8605 and VTR-8655 are
low viscosity analogues of VTR-8500,
VTR-8600 and VTR-8650, respectively. Since fluid resistance and low temperature
properties are primarily controlled by the
S. Bowers, Le Grand-Saconnex
(Switzerland)
Corresponding author:
Stephen Bowers
Du Pont Dow Elastomers S.A.
Chemin du Pavillon 2
1218 Le Grand-Saconnex,
Switzerland
37
Tab. 1. 65 ML at 1218C APA polymer designations
65 ML at 1218C Polymers Already Introduced
VTR-8500
GLT type
PMVE LT type
64 % fluorine
VTR-8525
GBLT type
PMVE LT type
66 % fluorine
VTR-8550
GFLT type
PMVE LT type
67 % fluorine
VTR-8600
GF type
HFP Type
70 % fluorine
VTR-8650
GBL type
HFP Type
68 % fluorine
Tab. 2. 25 ML at 121 8C APA polymer designations
New 25 ML at 1218C Polymers
VTR-8505
GLT type
PMVE LT type
64 % fluorine
VTR-8605
GF type
HFP Type
70 % fluorine
main monomer composition, the new low
viscosity polymers give comparable performance in this respect to their higher viscosity counterparts.
Curing of new lower viscosity
fluoroelastomers
The curing characteristics of the new lower
viscosity polymers are very similar to the
VTR-8655
GBL type
HFP Type
68 % fluorine
higher viscosity grades – fast and efficient.
MDR 2000 cure data are presented in Figures 1 through 3 for GLT, GF and GBL family
polymers of old and new (APA) generation.
The compounds used contained 100 parts
of polymer, 30 phr MT carbon black, 3 phr
zinc oxide, 3 phr triallylisocyanurate (TAIC)
and 3 phr of a 45 % organic peroxide dispersion. Figure 4 illustrates how the APA
polymers can be compounded with lower
levels of peroxide and still achieve equivalent cure rate to the old generation polymers (example given for GBL family polymers).
The bar charts (Figures 1 through 3) show
the massive reduction in cure time for APA
polymers but also that the new lower viscosity polymers cure just as well as the higher viscosity counterparts. Faster curing
can be translated into shorter cure cycles
during molding. The fast cure rate of the
new polymers can be regulated by reducing the level of peroxide in the recipe. Typically for APA polymers the recommended level is in the range 1.25 to 1.5 phr
of 45 % active peroxide dispersion compared to between 2 and 3 for older generation Viton grades. Figure 4 shows that
even with only 1.25 phr of 45 % active
peroxide dispersion the APA polymers provide equivalent cure rates and much higher
maximum torque levels than the old generation polymers. It is well known that
mould release is strongly influenced by
the efficiency of the curing process. Fast,
Fig. 1. MDR cure times for GLT types
Fig. 2. MDR cure times for GF types
Fig. 3. MDR cure times for GBL types
Fig. 4. MDR cure curves for GBL types
38
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 1-2/2004
Fig. 5. Orifice die pressure – GLT types
Fig. 6. Orifice die pressure – GF types
Fig. 7. Injection of GLT types
Fig. 8. Injection of GBL types
efficient cures give lower mould sticking
and lower mould fouling.
Extrusion rheometer testing
Comparison of extrusion rheometer test
data for the various polymers, in full compound form, helps to illustrate why APA
polymers flow so well and why they provide low die swell in extrusion processes. The
new molecular structure of Viton polymers made using APA technology provides
improved flow. This is immediately visible
in injection molding processes where injection times and pressures are much lower
than for older generation Viton polymers. In extrusion processes we see reduced head pressures, higher extrusion rates
and lower die swell. One laboratory test
that illustrates these differences is extrusion rheometry conducted using zero length
(orifice) dies. Pressure drop vs. shear rate
curves measured at 100 8C are given in Fi-
gures 5 and 6 for old generation and APA
GLT and GF family polymers of high and
low Mooney viscosity.
The extrusion rheometer tests were conducted on full compounds using 1.0 mm
diameter zero length (orifice) dies. The
compounds used contained 100 parts of
polymer, 30 phr MT carbon black, 3 phr
zinc oxide, 3 phr TAIC and 3 phr a 45 %
organic peroxide dispersion. The pressure
losses resulting from flow through the orifice die for older technology, higher viscosity GF and GLT polymers are very high
compared to the higher viscosity APA polymers VTR-8600 and VTR-8500. This is a direct result of the re-designed molecular
structure of the APA polymers and translates into easier injection and extrusion.
More impressive is that the higher Mooney
APA polymers provide an even lower orifice die pressure loss than the lower Mooney old generation polymers (compare
VTR-8500 to GLT-305 and VTR-8600 to
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 1-2/2004
GF-300). These same trends are seen in
production scale injection molding and extrusion operations where higher viscosity
APA polymers process more easily than lower viscosity old generation products of
the same family. The new low viscosity
APA polymers flow extremely easily, making them suitable for very difficult processing situations.
Injection molding comparison
Flow of compounds can be assessed using
equipment such as an extrusion rheometer
but the best test is to use production scale
injection molding equipment. Figures 7
through 9 compare the injection behavior
of APA polymers to old generation Viton
GLT, GF and GBL. Data are presented as
contours of fill time vs. pressure measured
while molding into a 40 cavity chrome plated AS-214 O-ring mould with a conventional direct injection hot runner system.
39
Fig. 10. Compression set of GLT types
Fig. 9. Injection of GF types
Tab 3. Recipes and property comparisons of GLT family products
GLT-305
VTR-8505
VTR-8500
Polymer
MT Carbon Black
ZnO
TAIC
45 % organic peroxide
100
30
3
3
3
100
30
3
3
3
100
30
3
3
3
Physical Properties at 238C
Press Cure 5 mins at 1778C
Modulus 10 % (MPa)
Modulus 100 % (MPa)
Tensile Strength (MPa)
Elongation (%)
Shore A
Tg by DSC 8C
PC 16 hrs
at 232 8C
0.7
6.9
15.3
159
70
ÿ 30.5
PC 2 hrs
at 232 8C
0.7
3.7
15.5
236
67
ÿ 32.3
PC 2 hrs
at 232 8C
0.7
3.9
17.4
269
66
ÿ 32.8
Tab 4. Recipes and property comparisons of GF family products
GF-300
VTR-8605
VTR-8600
Polymer
MT Carbon Black
ZnO
TAIC
45 % organic peroxide
100
30
3
3
3
100
30
3
3
3
100
30
3
3
3
Physical Properties at 238C
Press Cure 5 mins at 1778C
Modulus 10 % (MPa)
Modulus 100 % (MPa)
Tensile Strength (MPa)
Elongation (%)
Shore A
Tg by DSC 8C
PC 2 hrs
at 232 8C
0.8
5.4
16.8
227
72
ÿ 6.1
PC 2 hrs
at 232 8C
0.98
7.3
20.2
220
74
ÿ 5.2
PC 2 hrs
at 232 8C
0.88
7.0
20.3
249
72
ÿ 6.5
Tab 5. Recipes and property comparisons of GBL family products
GBL-200
VTR-8655
VTR-8650
Polymer
MT Carbon Black
ZnO
TAIC
45 % organic peroxide
100
30
3
3
3
100
30
3
3
2
100
30
3
3
2
Physical Properties at 238C
Press Cure 5 mins at 1778C
Modulus 10 % (MPa)
Modulus 100 % (MPa)
Tensile Strength (MPa)
Elongation (%)
Shore A
Tg by DSC 8C
PC 16 hrs
at 232 8C
0.8
5.4
20.6
247
70
ÿ 17.8
PC 4 hrs
at 232 8C
0.7
4.2
19.6
289
70
ÿ 18.7
PC 4 hrs
at 232 8C
0.71
3.7
17.6
278
69
ÿ 17.2
40
A 135 ton MIR industrial-scale injection
molding machine was used for these tests.
The data correlate very well with the extrusion rheometer tests. The new low viscosity
APA polymers provide exceptionally easy
injection. Higher viscosity old generation
polymers cannot be injected into this
mould. The recipes used for all injection
molding trials contained 100 parts polymer, 30 phr MT carbon black, 3 phr ZnO,
3 phr TAIC and 1.6 phr conventional process aid. Peroxide levels were 1.25 phr
for all APA polymers and 3 phr for old generation materials. It is also important to
mention that in all cases the APA polymers
demolded easily and cleanly compared to
the control compounds based on older generation Viton types.
Properties of new low viscosity
Viton products
Despite their relatively low viscosity, the
new polymers also provide tensile properties, compression set resistance and aging
characteristics comparable to their higher
viscosity counterparts and superior to the
old generation products in many respects.
Property test data are given in the following Tables. Note that the APA products
are only given a light post cure compared
to the old generation polymers. APA products cure so much more efficiently in
the molding process they do not require
a severe post cure to achieve optimal properties. Post cure (PC) times are denoted in
the Tables 3 – 5.
Compression set resistance
As illustrated in Figures 10, 11 and 12 APA
polymers offer significantly improved performance over old generation polymers
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 1-2/2004
Fig. 11. Compression set of GF types
Fig. 12. Compression set of GBL types
Fig. 13. Influence of cure site strategy 70 hrs at 250 8C heat aging
Fig. 14. Influence of cure site strategy 70 hrs at 275 8C heat aging
and the low viscosity APA polymers are
marginally superior.
The improved compression set resistance
of the low viscosity APA polymers is exactly
as expected and is due to the particular
structure of these new products.
Heat resistance
APA polymers have good heat resistance
despite the fact that they are peroxide cured. The heat resistance of this kind of polymer is strongly influenced by the quantity
and position of cure sites within the polymer chains, as well as their chemical nature. Peroxide curable fluoroelastomers can
be made in several different ways. Some
examples of positioning are:
– cure sites on chain ends only
– cure sites mid chain but not on chain
ends
– cure sites mid chain and on chain ends
The choice strongly impacts the heat resistance of the polymer, or compounds
made from it. Figures 13 through 15
show a comparison of the heat resistance
of two polymers, one having cure sites only
on chain ends and the other having cure
sites on chain ends and at mid chain positions. In a 70 hrs at 250 8C test both polymers show good retention of properties.
Figure 13 indicates the start of degradation for the end to end cross linked polymer
(loss of tensile strength) but possibly further curing of the end to end and mid
chain linked polymer. Figure 14 shows
the result of testing for 70 hrs at 275 8C,
a much more demanding test which is outside the generally recommended longterm service temperature for fluoroelastomers. In this test the tensile loss of the end
to end linked polymer is already very significant, with 60 % retained, but the polymer containing end to end and mid chain
cure sites still performs well.
Only on increasing the test time at 275 8C
to 168 hrs do we start to see the influence
on the polymer that contains cure sites
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 1-2/2004
both at chain ends and mid chain. Figure 15 shows that even in this very severe
test the polymer with mid chain cure sites
still retains 60 % of its original tensile
strength, compared to only 40 % retention of tensile strength for the end to end
only cross linked polymer.
APA polymers are designed with cure sites
at chain ends and at mid chain positions
specifically to ensure good heat resistance.
Figures 16 through 18 illustrate the tensile
strength and elongation retention after
heat aging at 250 8C for low viscosity polymers of both old and new generation
(APA). These data show very stable behaviour for Viton made with APA grades.
Conclusion
The new fluoroelastomers presented in
this paper are designed for difficult molding processes where they provide excellent flow characteristics and easy mould release. The polymers deliver the required
41
Fig. 15. Influence of cure site strategy 168 hrs at 275 8C heat aging
Fig. 16. Heat Resistance of GLT types 70 hrs at 250 8C air aging test
Fig. 17. Heat Resistance of GF types 70 hrs at 250 8C air aging test
Fig. 18. Heat Resistance of GBL types 70 hrs at 250 8C air aging test
ease of processing without compromising
physical properties, compression set resistance or heat aging characteristics. Optimal properties and compression set resistance are generally achieved after only
2 to 4 hrs at 232 8C compared to 16 hrs
for a traditional Viton polymers. The
new polymers provide comparable fluid re-
42
sistance to their higher viscosity counterparts.
References
[1] R.D. Stevens and D.F. Lyons, Paper #29, „New, Improved Processing HFP-Peroxide Cured Types of Vitonâ“, 160th meeting of the Rubber Div./ACS, October 2001.
[2] D.F. Lyons and R.D. Stevens, Paper #30, „New, Improved Processing PMVE-Peroxide Cured Types of
Vitonâ“, 160th meeting of the Rubber Div./ACS,
October 2001.
[3] R.D. Stevens, SAE Paper 2002-01-0632, „A New
Fluoroelastomer for Fuel Systems Seals“, SAE International Congress, March 2002.
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 1-2/2004