Synthesis of Low Temperature Resistant Hydrogenated Nitrile Rubber Based on Esterification Reaction

Hydrogenated Nitrile Rubber (HNBR) is widely used in aerospace, petroleum exploration and other fields because of its excellent performances. However, there remains a challenge of balancing the oil resistance and the low temperature resistance for HNBR. In this work, a series of grafted carboxyl nitrile rubber (XNBR) was prepared by the esterification reaction between active functional groups (–COOH) of XNBR and alkanols of different molecular chain lengths (C8H17OH, C12H25OH, C16H33OH, C18H37OH) or Methoxypolyethylene glycols (MPEG) of different molecular weights (Mn = 350, 750, 1000). The structure and low temperature resistance of as-obtained grafted polymers were characterized by Fourier Transform Infrared (FTIR), 1H-NMR and Differential scanning calorimetry (DSC). It was found that the glass transition temperatures (Tg) of grafted XNBR were significantly decreased. MPEG grafted polymers with better low temperature resistance were then selected for hydrogenation. As-prepared hydrogenated XNBR grafted with MPEG-1000 (HXNBR-g-1000) showed the lowest Tg of −29.8 °C and the best low temperature resistance. This work provides a novel and simple preparation method for low temperature resistant HNBR, which might be used potentially in extremely cold environments.


Introduction
Nitrile butadiene rubber (NBR) is a random copolymer of butadiene and acrylonitrile [1] with excellent oil resistance and good mechanical strength, which has been widely used in oil fields, automobiles, wire and cable, adhesives and other fields [2,3]. However, due to the existence of unsaturated bonds on the molecular chain, it shows poor aging resistance. Hydrogenated nitrile rubber (HNBR) is the product of selective hydrogenation of NBR, which retains the original excellent performances of NBR and improves the ability of heat resistance, acid and alkali resistance and aging resistance [4][5][6][7]. However, HNBR is easy to crystallize due to the nearly ordered polyethylene structure of main chain [8,9], which causes an increase of T g . As a result, HNBR has poor low temperature resistance.
In order to improve the low temperature resistance of HNBR, physical modification and chemical modification [10,11] can be adopted. Physical modification includes adding small molecular plasticizer, adjusting vulcanization formula or rubber sharing. Kim D et al. [12] used various amounts and types of plasticizers to investigate the low temperature resistance of NBR materials. They found that T g of NBR decreased when the plasticizer was added. In particular, the addition of Dioctyl adipate (DOA) led to a lower T g than other plasticizers, which decreased T g from −43 to −52 • C. Although the introduction of plasticizers can reduce T g , the plasticizer is easy to migrate and exude, and excessive addition will result in deterioration of mechanical and dielectric properties [13,14]. Anna Laskowska et al. [15] investigated the effect of imidazole-based ionic liquids on the properties of NBR composites. Fifteen phr of hydrophobic 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) ionic liquid decreased T g from −23 to −31 • C. However, the mechanical properties of rubber were greatly affected. Chemical modification mainly involves the introduction of a flexible third monomer into the molecular chain by grafting or copolymerization. In our group [16], epoxidation and esterification reactions were used to introduce a flexible third monomer to HNBR molecular chain, as-obtained HNBR showed non crystallized structure, decreased T g of −34.1 • C and the better low temperature performances. It indicates that the addition of the third monomer can not only reduce T g , but also inhibit crystallization.
Carboxyl nitrile butadiene rubber (XNBR) is a special type of nitrile rubber containing carboxyl groups prepared by emulsion copolymerization of butadiene, acrylonitrile and a small amount of unsaturated acids [17]. It has good mechanical performances and resistance with abrasion and oil [18][19][20]. In addition, several kinds of active functional groups of XNBR, such as nitrile (-CN), carboxylic (-COOH) and alkene groups (-C=C) [17,21,22], can participate in reactions to form different types of chemical bonds. For example, the carboxylic functional group can react with several materials, such as metal oxides, amines, polyols and epoxies, which are widely used for the preparation of nano-composites and new materials [23,24].
In this work, the flexible monomer with hydroxyl group were grafted to XNBR by the reaction between -COOH and −OH. Subsequently, these grafted XNBR were hydrogenated to fabricate a series of grafted HXNBR ( Figure 1). Among them, HXNBR-g-1000 showed the lowest T g of −29.8 • C and the best low temperature resistance. The reaction between functional groups ensures grafting efficiency and avoids the generation of by-products and gels. This work provides a novel and simple method to improve the low temperature resistance for HNBR while maintaining its oil and aging resistance. plasticizers can reduce Tg, the plasticizer is easy to migrate and exude, and excessive addition will result in deterioration of mechanical and dielectric properties [13,14]. Anna Laskowska et al. [15] investigated the effect of imidazole-based ionic liquids on the properties of NBR composites. Fifteen phr of hydrophobic 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) ionic liquid decreased Tg from −23 to −31 °C. However, the mechanical properties of rubber were greatly affected. Chemical modification mainly involves the introduction of a flexible third monomer into the molecular chain by grafting or copolymerization. In our group [16], epoxidation and esterification reactions were used to introduce a flexible third monomer to HNBR molecular chain, asobtained HNBR showed non crystallized structure, decreased Tg of −34.1 °C and the better low temperature performances. It indicates that the addition of the third monomer can not only reduce Tg, but also inhibit crystallization.
Carboxyl nitrile butadiene rubber (XNBR) is a special type of nitrile rubber containing carboxyl groups prepared by emulsion copolymerization of butadiene, acrylonitrile and a small amount of unsaturated acids [17]. It has good mechanical performances and resistance with abrasion and oil [18][19][20]. In addition, several kinds of active functional groups of XNBR, such as nitrile (-CN), carboxylic (-COOH) and alkene groups (-C=C) [17,21,22], can participate in reactions to form different types of chemical bonds. For example, the carboxylic functional group can react with several materials, such as metal oxides, amines, polyols and epoxies, which are widely used for the preparation of nanocomposites and new materials [23,24].
In this work, the flexible monomer with hydroxyl group were grafted to XNBR by the reaction between -COOH and −OH. Subsequently, these grafted XNBR were hydrogenated to fabricate a series of grafted HXNBR ( Figure 1). Among them, HXNBR-g-1000 showed the lowest Tg of −29.8 °C and the best low temperature resistance. The reaction between functional groups ensures grafting efficiency and avoids the generation of byproducts and gels. This work provides a novel and simple method to improve the low temperature resistance for HNBR while maintaining its oil and aging resistance.

Preparation of Grafted XNBR with Different Side Groups
Ten grams of XNBR was dissolved in 250 mL chlorobenzene, and 10 mL thionyl chloride was then added to the polymer solution dropwise. The reaction was carried out at 78 • C for 4 h. After that, an appropriate amount of C 8 H 17 OH, C 12 H 25 OH, C 16 H 33 OH, C 18 H 37 OH or MPEG (Mn = 350/750/1000) (the molar ratio of −COOH: −OH = 1:1.2) and triethylamine (the molar ratio of −COOH:Et 3 N =1:1.2) were added to the above solution, and the reaction was run at 55 • C for 5 h. The whole reaction process was carried out in nitrogen atmosphere. The products were coagulated in ethanol, washed three times, and dried at 55 • C. As-obtained grafted polymers are called XNBR-g-8, XNBR-g-12, XNBR-g-16, XNBR-g-18, XNBR-g-350, XNBR-g-750, and XNBR-g-1000, respectively.

Hydrogenation of Grafted XNBR with Different Side Groups
Eighteen grams of grafted XNBR with MPEG (Mn = 350/750/1000) and 282 mL of chlorobenzene were added into an autoclave. Then, the required catalyst was transferred into the reactor. The autoclave was slowly heated and maintained at 120 • C for 8 h. The whole reaction process was carried out in hydrogen atmosphere. The products were coagulated in ethanol, washed thoroughly and dried at 55 • C. As-prepared products are marked as HXNBR-g-350, HXNBR-g-750, HXNBR-g-1000, respectively.

Characterization
Fourier Transform Infrared (FTIR) spectra were obtained on a Bruker Tensor 27 spectrophotometer (Bruker, Karlsruhe, Germany) in the range of 400 to 4000 cm −1 with a resolution of 4 cm −1 , with Attenuated Total Reflectance (ATR) mode. Nuclear Magnetic Resonance (NMR) measurements were carried out on an AV600-400MHZ spectrophotometer (Bruker, Karlsruhe, Germany ) using CDCl 3 and C 3 D 6 O solution as solvents. The element content was determined using the CHNSO element analyzer produced by Elementor, Hanau, Germany. Differential scanning calorimetry (DSC) was performed on Mettler Toledo (Zurich, Switzerland ). Firstly, the sample was heated from 25 to 80 • C at a rate of 10 • C/min and kept at 80 • C for 5 min. Then, it was cooled from 80 to −100 • C at a rate of 10 • C/min and kept at −100 • C for 5 min under nitrogen atmosphere. Ultimately, the sample was heated from −100 to 100 • C at a rate of 10 • C /min. The glass transition temperature was determined based on the heating curve from −100 to 100 • C at a rate of 10 • C /min. Thermogravimetric analysis (TGA) measurements were performed on Mettler Toledo equipment from 25 to 600 • C at a heating rate of 10 • C /min under nitrogen atmosphere.

Structures of Grafted XNBR
A series of alkanols (C 8 H 17 OH, C 12 H 25 OH, C 16 H 33 OH, C 18 H 37 OH) and Methoxypolyethylene glycols (Mn = 350, 750, 1000) were grafted to XNBR, and the structures of the grafted polymers were characterized using FTIR and 1 H-NMR. The FTIR spectra of XNBR and grafted XNBR are shown in Figure 2. The absorption peaks at 2920 and 2850 cm −1 are attributed to the stretching vibration of -CH 2 . The bands at 967 and 915 cm −1 are assigned to the bending vibration of =C-H of the butadiene units. The peak at 2237 cm −1 is attributed to cyano group [25,26]. The band at 1725 cm −1 is due to the stretching vibration of C=O [27]. In the case of XNBR, the weak peak at 1725 cm −1 , attributed to C=O, and the strong peak at 1698 cm −1 , ascribed to the migration absorption peak of hydrogen bond in the carboxyl group, are observed. After grafting, the peak at 1698 cm −1 disappears completely and the peak intensities at 1725 cm −1 increase. In addition, the peaks at 1179 and 1120 cm −1 are attributed to the stretching vibration of -COOC-and -C-O-C-, respec-tively [28]. It indicates that the grafting reaction was successful and ester groups were formed by the grafted side chains.
to the bending vibration of =C-H of the butadiene units. The peak at 2237 cm is attrib to cyano group [25,26]. The band at 1725 cm −1 is due to the stretching vibration of [27]. In the case of XNBR, the weak peak at 1725 cm −1 , attributed to C=O, and the str peak at 1698 cm −1 , ascribed to the migration absorption peak of hydrogen bond in carboxyl group, are observed. After grafting, the peak at 1698 cm −1 disappears comple and the peak intensities at 1725 cm −1 increase. In addition, the peaks at 1179 and 1120 are attributed to the stretching vibration of -COOC-and -C-O-C-, respectively [2 indicates that the grafting reaction was successful and ester groups were formed by grafted side chains.   to cyano group [25,26]. The band at 1725 cm is due to the stretching vibration of [27]. In the case of XNBR, the weak peak at 1725 cm −1 , attributed to C=O, and the s peak at 1698 cm −1 , ascribed to the migration absorption peak of hydrogen bond i carboxyl group, are observed. After grafting, the peak at 1698 cm −1 disappears comp and the peak intensities at 1725 cm −1 increase. In addition, the peaks at 1179 and 1120 are attributed to the stretching vibration of -COOC-and -C-O-C-, respectively [2 indicates that the grafting reaction was successful and ester groups were formed b grafted side chains.   In order to further confirm the success of grafted reactions, elemental analysis was performed for XNBR grafted with different side groups. As shown in Table 1, XNBR has the highest carbon and nitrogen content as well as the lowest oxygen content. The content of nitrogen for the grafted polymers decreases from 6.67% to 6.23% when the Polymers 2021, 13, 4096 5 of 10 molecular weight of the grafted side groups increases. It indicates that the grafting reaction was successful. The nitrogen element only comes from the nitrogen atom of the cyano group, and there is no nitrogen element in the grafted side groups. As the molecular weight of the grafted side group increases, the nitrogen element in the grafted polymer decreases correspondingly. In order to characterize the esterification rate of nitrile rubber with different alkanols and methoxypolyethylene glycols, deuterated acetone was selected as the nuclear magnetic reagent to compare the content of residual carboxyl groups before and after the grafting reaction. As shown in Figure 4, XNBR shows a carboxyl proton absorption peak at 7.3 ppm before grafting. However, the peak at 7.3 ppm disappears after grafting, indicating that the esterification reaction was complete. performed for XNBR grafted with different side groups. As shown in Table 1, XNBR the highest carbon and nitrogen content as well as the lowest oxygen content. The co of nitrogen for the grafted polymers decreases from 6.67% to 6.23% when the mole weight of the grafted side groups increases. It indicates that the grafting reaction was cessful. The nitrogen element only comes from the nitrogen atom of the cyano group there is no nitrogen element in the grafted side groups. As the molecular weight o grafted side group increases, the nitrogen element in the grafted polymer decreases respondingly. In order to characterize the esterification rate of nitrile rubber with different alk and methoxypolyethylene glycols, deuterated acetone was selected as the nuclear netic reagent to compare the content of residual carboxyl groups before and after the g ing reaction. As shown in Figure 4, XNBR shows a carboxyl proton absorption peak ppm before grafting. However, the peak at 7.3 ppm disappears after grafting, indic that the esterification reaction was complete. Glass transition temperature (Tg) is a significant physical parameter that has a influence on the application performance of rubber. Figure 5 shows DSC curves of X and XNBR grafted with different long-chain alkanols. XNBR shows a Tg of −25.2 °C. all grafted XNBR are much lower than that of XNBR. As the carbon atoms of alkan creases from 8, 12, 16 to 18, the Tg of the grafted XNBR gradually increases from − −32.5, −31.4 to −30.6 °C. When the length of the molecular chain for alkanol grows, Glass transition temperature (T g ) is a significant physical parameter that has a great influence on the application performance of rubber. Figure 5 shows DSC curves of XNBR and XNBR grafted with different long-chain alkanols. XNBR shows a T g of −25.2 • C. T g of all grafted XNBR are much lower than that of XNBR. As the carbon atoms of alkanol increases from 8, 12, 16 to 18, the T g of the grafted XNBR gradually increases from −35.5, −32.5, −31.4 to −30.6 • C. When the length of the molecular chain for alkanol grows, T g of grafted XNBR gradually increases. As the length of the alkanol chain increases, the steric hindrance effect increases, which hinders the internal rotation of the molecular chain, resulting in a decrease of segment flexibility and an increase of T g .
Polymers 2021, 13, x FOR PEER REVIEW 6 of 1 grafted XNBR gradually increases. As the length of the alkanol chain increases, the steric hindrance effect increases, which hinders the internal rotation of the molecular chain, re sulting in a decrease of segment flexibility and an increase of Tg.  Figure 6 shows DSC curves of XNBR and XNBR grafted with MPEG of different mo lecular weights. Compared to the pristine XNBR, the grafted XNBR show lower Tg. As the molecular weight of MPEG increases from 350, 750 to 1000, Tg of the grafted XNBR grad ually decreases from −35.6, −36.2 to −41.1 °C. MPEG contains flexible ether groups. As the flexible side groups increase, the distance between polymer molecules increases and the interaction weakens. In another words, the "internal plasticization" is produced, hence T of grafted XNBR eventually decreases. Therefore, we choose MPEG grafted polymer with better low temperature resistance for hydrogenation.

Structures of Grafted HXNBR
XNBR grafted with different molecular weights of MPEG were selected for hydro genation, and the structure of the hydrogenated products were characterized by FTIR and 1 H-NMR (Figures 7 and 8). As can be seen from IR spectra in Figure 7, the absorption  Figure 6 shows DSC curves of XNBR and XNBR grafted with MPEG of different molecular weights. Compared to the pristine XNBR, the grafted XNBR show lower T g . As the molecular weight of MPEG increases from 350, 750 to 1000, T g of the grafted XNBR gradually decreases from −35.6, −36.2 to −41.1 • C. MPEG contains flexible ether groups. As the flexible side groups increase, the distance between polymer molecules increases and the interaction weakens. In another words, the "internal plasticization" is produced, hence T g of grafted XNBR eventually decreases. Therefore, we choose MPEG grafted polymer with better low temperature resistance for hydrogenation. grafted XNBR gradually increases. As the length of the alkanol chain increases, the steric hindrance effect increases, which hinders the internal rotation of the molecular chain, re sulting in a decrease of segment flexibility and an increase of Tg.  Figure 6 shows DSC curves of XNBR and XNBR grafted with MPEG of different mo lecular weights. Compared to the pristine XNBR, the grafted XNBR show lower Tg. As the molecular weight of MPEG increases from 350, 750 to 1000, Tg of the grafted XNBR grad ually decreases from −35.6, −36.2 to −41.1 °C. MPEG contains flexible ether groups. As the flexible side groups increase, the distance between polymer molecules increases and the interaction weakens. In another words, the "internal plasticization" is produced, hence T of grafted XNBR eventually decreases. Therefore, we choose MPEG grafted polymer with better low temperature resistance for hydrogenation.

Structures of Grafted HXNBR
XNBR grafted with different molecular weights of MPEG were selected for hydro genation, and the structure of the hydrogenated products were characterized by FTIR and 1 H-NMR (Figures 7 and 8). As can be seen from IR spectra in Figure 7, the absorption

Structures of Grafted HXNBR
XNBR grafted with different molecular weights of MPEG were selected for hydrogenation, and the structure of the hydrogenated products were characterized by FTIR and 1 H-NMR (Figures 7 and 8). As can be seen from IR spectra in Figure 7, the absorption peaks at 967 and 915 cm −1 of XNBR indicate the presence of unsaturated C=C in the As shown in Figure 8, XNBR shows the proton absorption peaks of -CH=CH− in the range of 4.8-5.8 ppm. However, the hydrogenated grafted products have no absorption peaks in this range, indicating that the C=C bond is saturated to the C-C bond. According to the integral area of nuclear magnetic peaks, the hydrogenation degree of the hydrogenated grafted products is above 90%. The thermogravimetric test was used to analyze the thermal properties of XNBR and HXNBR grafted with MPEG of different molecular weights. TG curves of grafted and hy-  As shown in Figure 8, XNBR shows the proton absorption peaks of -CH=CH− in the range of 4.8-5.8 ppm. However, the hydrogenated grafted products have no absorption peaks in this range, indicating that the C=C bond is saturated to the C-C bond. According to the integral area of nuclear magnetic peaks, the hydrogenation degree of the hydrogenated grafted products is above 90%. The thermogravimetric test was used to analyze the thermal properties of XNBR and HXNBR grafted with MPEG of different molecular weights. TG curves of grafted and hy- As shown in Figure 8, XNBR shows the proton absorption peaks of -CH=CH− in the range of 4.8-5.8 ppm. However, the hydrogenated grafted products have no absorption peaks in this range, indicating that the C=C bond is saturated to the C-C bond. According to the integral area of nuclear magnetic peaks, the hydrogenation degree of the hydrogenated grafted products is above 90%.
The thermogravimetric test was used to analyze the thermal properties of XNBR and HXNBR grafted with MPEG of different molecular weights. TG curves of grafted and hydrogenated products is shown in Figure 9, respectively. Grafted HXNBR start to decompose at above 380 • C, while the initial decomposition temperatures for grafted XNBR are in the range of 150-180 • C. All hydrogenated products show higher thermostabilities than grafted XNBR before hydrogenation. The thermal decomposition temperature is less affected by different side groups. It can be found that the thermal stability of hydrogenated products is improved obviously.
Polymers 2021, 13, x FOR PEER REVIEW 8 of 10 drogenated products is shown in Figure 9, respectively. Grafted HXNBR start to decompose at above 380 °C, while the initial decomposition temperatures for grafted XNBR are in the range of 150-180 °C. All hydrogenated products show higher thermostabilities than grafted XNBR before hydrogenation. The thermal decomposition temperature is less affected by different side groups. It can be found that the thermal stability of hydrogenated products is improved obviously. DSC curves of HXNBR and HXNBR grafted with MPEG of different molecular weights are shown in Figure 10. Compared to Tg of −25.2 °C for the pristine XNBR, HXNBR shows a higher Tg of −19.2 °C. Due to the decrease of unsaturated bonds on the main chain after hydrogenation, the flexibility of the molecular chain segment decreases. However, Tg of HXNBR grafted with different MPEG side groups ranges from −26.8 to −29.8 °C. It can be seen that the low temperature resistance of the grafted HXNBR has been greatly improved. Among them, HXNBR-g-1000 with the lowest Tg of −29.8 °C has the best low temperature resistance. The side group of HXNBR-g-1000 is MPEG with the largest molecular weight of 1000, which contains the most flexible ether bond. Therefore, the molecular chain segment of HXNBR-g-1000 with more flexible side groups exhibits the best flexibility.  DSC curves of HXNBR and HXNBR grafted with MPEG of different molecular weights are shown in Figure 10. Compared to T g of −25.2 • C for the pristine XNBR, HXNBR shows a higher T g of −19.2 • C. Due to the decrease of unsaturated bonds on the main chain after hydrogenation, the flexibility of the molecular chain segment decreases. However, T g of HXNBR grafted with different MPEG side groups ranges from −26.8 to −29.8 • C. It can be seen that the low temperature resistance of the grafted HXNBR has been greatly improved. Among them, HXNBR-g-1000 with the lowest T g of −29.8 • C has the best low temperature resistance. The side group of HXNBR-g-1000 is MPEG with the largest molecular weight of 1000, which contains the most flexible ether bond. Therefore, the molecular chain segment of HXNBR-g-1000 with more flexible side groups exhibits the best flexibility. drogenated products is shown in Figure 9, respectively. Grafted HXNBR start to decom pose at above 380 °C, while the initial decomposition temperatures for grafted XNBR are in the range of 150-180 °C. All hydrogenated products show higher thermostabilities than grafted XNBR before hydrogenation. The thermal decomposition temperature is less af fected by different side groups. It can be found that the thermal stability of hydrogenated products is improved obviously. DSC curves of HXNBR and HXNBR grafted with MPEG of different molecular weights are shown in Figure 10. Compared to Tg of −25.2 °C for the pristine XNBR HXNBR shows a higher Tg of −19.2 °C. Due to the decrease of unsaturated bonds on the main chain after hydrogenation, the flexibility of the molecular chain segment decreases However, Tg of HXNBR grafted with different MPEG side groups ranges from −26.8 to −29.8 °C. It can be seen that the low temperature resistance of the grafted HXNBR has been greatly improved. Among them, HXNBR-g-1000 with the lowest Tg of −29.8 °C has the best low temperature resistance. The side group of HXNBR-g-1000 is MPEG with the larg est molecular weight of 1000, which contains the most flexible ether bond. Therefore, the molecular chain segment of HXNBR-g-1000 with more flexible side groups exhibits the best flexibility.