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Improvement Of Low-Temperature Flexibility
Of Hydrogenated Nitrile-Butadiene Rubber (HNBR)
Takishi Kobatake, Kazumi
Kodama, Sachio Hayashi and Akira Yoshioka
ZEON Corporation, R&D Center, Kawasaki 210, Japan
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Abstract
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Improvement of the
low-temperature flexibility of Hydrogenated Nitrile Butadiene
Rubber (HNBR) has been required for industrial applications.
In this study, the relationship between low-temperature flexibility
and polymer structure such as higher order structure and monomer
unit distribution along a polymer chain was examined for HNBRs
having various acrylonitrile contents by using X-ray diffraction,
nuclear magnetic resonance computer simulation and other methods.
It was found that the length of tetramethylene sequence in
a polymer chain is the key factor for improving low-temperature
flexibility. Tetramethylene sequences having five or more
than five continuously linked tetramethylenes (TM 5)
facilitate crystallization upon stretching, which leads to
a poor low-temperature flexibility. Consequently, it was deduced
that the TM5 content must be as
low as possible to improve the low-temperature flexibility.
To methods were tried to decrease the TM5
content: (1) copolymerization with third monomers and (2)
addition of mercaptanes. It was confirmed that branched HNBRs
made by each of the two methods have lower Tgs and an improved
low-temperature flexibility.
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Introduction
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Hydrogenated Nitrile
Butadiene Rubber (HNBR) is a new oil-resistant elastomer made
by a selective hydrogenation reaction of butadiene units in
NBR and has an improved heat resistance, ozone resistance,
resistance to oxidized fuel, and resistance to lubricating
oil containing various additives.1 HNBR is used
for various rubber parts, including seals and packings, where
an improvement of low-temperature flexibility is required.
It is well known that NBR demonstrates the linear relation
between glass transition temperature (Tg) and molar fraction
(Fa) of acrylonitrile units.2 In the case of HNBR
with high acrylonitrile content, it was reported that high
acrylonitrile content leads to a high Tg which causes poor
low-temperature flexibility. On the other hand, in the case
of HNBR with low acrylonitrile content, the poor flexibility
is caused by crystallization of tetramethylene unit in spite
of the good flexibility of NBR, which has a low acrylonitrile
content originally.3,4 Recently, Arsenaultl et
al.5 discussed the low-temperature flexibility
of HNBR focusing only on the molar ratio of ethylene unit
to other monomer units. However, no report has clarified the
relationship between the length of tetramethylene sequences
and low-temperature flexibility.
The purposes of this study are (1) to elucidate
the cause of the difference between original NBR and HNBR
with respect to low-temperature flexibility; (2) to develop
new HNBRs with an improved low-temperature flexibility.
This study is composed of two parts. The first part covers
the structure analysis experiments designed to find the
sequence length of tetramethylene taking part in stretch-induced
crystallization. The second part relates to the improved
low-temperature flexibility of branched HNBR whose length
of tetramethylene sequence is shortened by copolymerization
with third monomers or addition of mercaptans.
The full details of this study are available in a paper6
that is under publication by Rubber Chem. Technol.
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Conclusions
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1.The causes of
the poor low-temperature flexibility of Hydrogenated NBR (HNBR)
are:
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(a) |
HNBR with high acrylonitrile content |
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Stretch-induced crystallites consisting of
alternate sequences of acrylonitrile and tetramethylene unit
which appear only at the stretching ratio higher than 100%.
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(b) |
HNBR with low acrylonitrile content |
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Continuously linked tetramethylene sequences
in the polymer chain which form disordered aggregate structure
under no stretching and form stretch-induced crystallites
under stretching. These higher order structures (aggregate
structure and stretch-induced crystallites) lead to the reduction
of low-temperature flexibility. The length of the continuously
linked tetramethylene sequence that takes part in stretch-induced
crystallites at -30°C is five or more than five (TM5).
The low-temperature flexibility of HNBR decreases when the
content of tetramethylene sequences having five or more than
five continuously linked tetramethylenes (TM5
content) is more than 20%.
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2.Reduction of TM5
content is accomplished by copolymerization with third monomers
or by of mercaptans.
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(a) |
Copolymerization with third monomers having
high Q and e values decreases TM5
content, resulting in an improved low-temperature flexibility.
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(b) |
Mercaptans additions decreases T5
content also resulting in an improved low-temperature flexibility.
Figure 1 illustrates the preparation scheme of branched HNBR
by addition of mercaptans.
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Fig.1Mercaptan addition
reaction scheme.
K: number of acrylonitrile unit, M:
number of tetremethylene unit that is hydrogenated butadiene
unit, N': number of butadiene units which undergo mercaptan
addition, (L'-M-N'-K): number of residual butadiene units..
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Firstly, NBRs are partially hydrogenated
to pre-determined saturation degrees. Secondly, mercaptans
are added randomly to form branched HNBR with a high saturation
degree (Fs 0.9).
The saturation degree is the ratio of the number of saturated
double bonds of modified polymer to the total number of unsaturated
double bonds of original polymer.
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Fig.2. Change of
glass transition temperature (Tg) with acrylonitrile content
(Fa) for NBRs ( : Fs=0), HNBRs ( :
Fs0.9) and
branched NBRs saturated with only n-butyl mercaptan
( : Fs 0.9)
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Figure 2 shows that a linear
relation is observed between acryrlonitrile and Tg in the
case of NBR
(). On the other hand, in the case
of HNBR() with a high saturation
degree (Fs0.9), the linear relation
between acrylonitrile content and Tg is observed only in the
region of high acrylonitrile content (Fa0.37).
The Tg of HNBR is not lower than -35°C in any level of
acrylonitrile content, which indicates poor low-temperature
flexibility. The Tgs for the various acrylonitrile content
branched NBRs () highly saturated
with only n-butyl mercaptans are also shown in Figure 2. The
Tg of branched NBR (Fs0.9)
highly saturated with only n-butyl mercaptan has a linear
relation with acrylonitrile content in the region of low acrylonitrile
and shows good low-temperature flexibility.
Furthermore, even a small number of branches introduced into
low acrylonitrile content HNBR sharply reduces TM5
content. Low-temperature flexibility is further improved with
the addition of higher degree and/or with the extension of
length of branch.
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References
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1. |
Y. Kubo, K. Hashimoto and N. Watanabe, Kautschuk
+ Gummi Kunstoffe,
No.2, 40, 118 (1987). |
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2. |
G. S. Whitby, C. C. Davis and R. F. Dunbook,
"Synthetic Rubber," New
York, John Wiley & Sons, 356 (1954). |
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3. |
S. Hayashi et al., 140th Meeting of the Rubber
Division, ACS Detroit,
MC (USA), October 8-11, 1991. |
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4. |
U. Eisele et al., J. Appl. Polym. Sci., Applied
Polymer Symposium, 58,
185 (1992). |
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5. |
G. J. Arsenault et al., 145th Meeting of
the Rubber Division, ACS,
Chicago, IL (USA), April 19-22 1994. |
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6. |
T. Kobatake, K. Kodama, S. Hayashi and A.
Yoshioka, Rubber Chem.
Technol., 70, 839 (1997). |
