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Latest Technical Report
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

 

Abstract

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 (TM5) 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.

Introduction

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.

Conclusions

1.The causes of the poor low-temperature flexibility of Hydrogenated NBR (HNBR) are:
(a) HNBR with high acrylonitrile content
Stretch-induced crystallites consisting of alternate sequences of acrylonitrile and tetramethylene unit which appear only at the stretching ratio higher than 100%.
(b) HNBR with low acrylonitrile content
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%.

2.Reduction of TM5 content is accomplished by copolymerization with third monomers or by of mercaptans.
(a) Copolymerization with third monomers having high Q and e values decreases TM5 content, resulting in an improved low-temperature flexibility.
(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.
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..
Firstly, NBRs are partially hydrogenated to pre-determined saturation degrees. Secondly, mercaptans are added randomly to form branched HNBR with a high saturation degree (Fs0.9). 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.
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)

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.

References

1. Y. Kubo, K. Hashimoto and N. Watanabe, Kautschuk + Gummi Kunstoffe,
No.2, 40, 118 (1987).
2. G. S. Whitby, C. C. Davis and R. F. Dunbook, "Synthetic Rubber," New
York, John Wiley & Sons, 356 (1954).
3. S. Hayashi et al., 140th Meeting of the Rubber Division, ACS Detroit,
MC (USA), October 8-11, 1991.
4. U. Eisele et al., J. Appl. Polym. Sci., Applied Polymer Symposium, 58,
185 (1992).
5. G. J. Arsenault et al., 145th Meeting of the Rubber Division, ACS,
Chicago, IL (USA), April 19-22 1994.
6. T. Kobatake, K. Kodama, S. Hayashi and A. Yoshioka, Rubber Chem.
Technol., 70, 839 (1997).