Study of solvent effect on the dissolution, size, structure and catalytic hydrogenation of nitrile butadiene rubber

doi:10.11949/0438-1157.20201045

Abstract

Abstract:

Nitrile butadiene rubber (NBR) is an important synthetic rubber made by copolymerization of butadiene and acrylonitrile. The hydrogenated nitrile rubber (HNBR), made by selective hydrogenation of NBR, not only maintains the original oil resistance and wear resistance, but also greatly improves its weather resistance and ozone resistance properties, which is widely used in weapon parts, aerospace seals and other fields. It is a key hot topic for the chemical modification of unsaturated polymers. In this paper, we systematically investigated the influence of solvent properties on the dissolution behavior, size distribution molecular structure of NBR and catalytic performance in solvent by using nanometer particle size analyzer, ubbelohde viscometer and Fourier transform infrared spectroscopy combined with density functional theory (DFT). The results show that strong electrophilic solvents cannot dissolve NBR, but electron donating solvents and some weak electrophilic solvents can dissolve NBR well. Especially in ketone solvents, NBR has small particle size with narrow distribution. DFT calculations reveal that compared with the gas atmosphere, the bond lengths of CC double bonds in NBR segment has increased in all solvents, and show an increasing trend as the polarity of the solvent increases. The dipole moment of NBR also increases with the increase of solvent polarity. Especially in ketone solvents, the highest occupied molecular orbital (HOMO) of NBR molecule moves from the edge to the inside of the molecule under the strong solvent effect, indicating that it may have a positive influence on the CC double bond inside the chain segment. The hydrogenation results show that the hydrogenation activity of NBR in the electron donating solvents is higher than that in weak electrophilic solvents, but no matter which solvent is selected, the hydrogenation selectivity to CC double bonds of NBR is 100%. This study provides a theoretical basis for the selection of solvents in the NBR heterogeneous solution hydrogenation system.

Key words: nitrile butadiene rubber, solvent effect, dissolution, structure, density functional theory

摘要：

丁腈橡胶（NBR）是由丁二烯和丙烯腈共聚而制得的一种重要的合成橡胶，而由NBR选择性加氢制得的氢化丁腈橡胶（HNBR），不仅保持了NBR原有的耐油和耐磨性，其耐侯性、耐臭氧性等均得到极大改善，被广泛用于武器部件、航天用密封件等领域，是不饱和聚合物化学改性领域的一个重要研究课题。本文采用纳米粒度仪、乌氏黏度计、傅里叶变换红外等表征手段，结合密度泛函理论计算（DFT），系统研究了溶剂性质对NBR在溶剂中的溶解行为、尺寸分布、分子结构及催化加氢性能等的影响。实验结果表明强亲电子溶剂不能溶解NBR，给电子溶剂及部分弱亲电子溶剂能较好地溶解NBR，特别是在酮类溶剂中，NBR粒径小且分布集中。DFT结果表明相比于气相氛围，在所有溶剂氛围中NBR链段里双键的键长均有所增长，且随着溶剂的极性增加而呈增长的趋势，NBR的偶极矩也随着溶剂极性增加而增加。特别是在酮类溶剂中，受溶剂效应的影响NBR分子的HOMO轨道由分子边缘向分子内部移动，表明对链段内部CC双键可能产生积极的影响。加氢结果表明在给电子溶剂中的加氢效果均优于弱亲电子溶剂，但无论选择何种溶剂，对NBR中CC双键的加氢选择性均为100%。该研究为NBR非均相溶液加氢体系中溶剂的筛选提供了理论依据。

关键词: 丁腈橡胶, 溶剂效应, 溶解, 结构, 密度泛函理论计算

CLC Number:

TQ 333.6

GE Bingqing, YIN Yixuan, WANG Yaxi, ZHANG Hongwei, YUAN Pei. Study of solvent effect on the dissolution, size, structure and catalytic hydrogenation of nitrile butadiene rubber[J]. CIESC Journal, 2021, 72(1): 543-554.

葛冰青, 阴义轩, 王亚溪, 张宏伟, 袁珮. 溶剂对丁腈橡胶溶解、尺寸、结构和催化加氢的影响研究[J]. 化工学报, 2021, 72(1): 543-554.

Figures/Tables 17

Fig.1 1H NMR spectrum of NBR

Table 1 Chemical shifts of H protons in different microstructures of NBR

Structure	Type of H	δ
1,4 unit	CH—	5.50
	—CH₂—	2.15
	—CH(CN)—	2.65
	—CH₂CH(CN)—	1.70
1,2 unit	CH— and CH₂	4.90—5.10

Fig.2 Pictures for NBR in strong electrophilic solvents before (a) and after (b) 24 h (Solvents from left to right: EtOH, EG, IPA, water and MeCN)

Fig.3 Pictures for NBR in weak electrophilic solvents before (a) and after (b) 24 h (Solvents from left to right: TOL，OX，MX，PX，TIPB，MCB，DCM，TCM and EDC)

Fig.4 Pictures for NBR in electron donating solvents before (a) and after (b) 24 h (Solvents from left to right: ACE，MEK，MIBK，CYC，NMP，MAC，EA，IBAC and DMF)

Fig.5 The size distribution of NBR in weak electrophilic solvents (a) and electron donating solvents (b)

Table 2 Viscosity of NBR solutions in weak electrophilic solvents (a1—a9) and electron donating solvents (b1—b9)

编号	溶剂	[η]/(L/g)	η_sp	编号	溶剂	[η]/(L/g)	η_sp
a1	DCM	1.84×10^-1	2.55	b1	ACE	6.88×10^-2	1.13
a2	TCM	1.32×10^-1	2.17	b2	MEK	6.17×10^-2	0.92
a3	EDC	1.13×10^-1	2.00	b3	MIBK	5.68×10^-2	0.97
a4	MCB	1.03×10^-1	1.48	b4	CYC	1.21×10^-1	1.63
a5	TOL	4.90×10^-2	0.61	b5	NMP	1.35×10^-1	1.68
a6	OX	4.91×10^-2	0.60	b6	MAC	6.59×10^-2	1.06
a7	MX	2.72×10^-2	0.38	b7	EA	6.85×10^-2	1.07
a8	PX	3.83×10^-2	0.41	b8	IBAC	5.13×10^-2	0.78
a9	TIPB	2.31×10^-2	0.24	b9	DMF	9.56×10^-2	1.38

Fig.6 The relationship between the characteristic viscosity and the specific viscosity of NBR solutions in weak electrophilic solvents (a) and electron donating solvents (b); the relationship between specific viscosity of NBR solutions and absolute difference of solubility parameters in weak electrophilic solvents (c) and electron donating solvents (d) (absolute difference of solubility parameters ?δ=|δNBR-δsolvent|)

Fig.7 NBR geometric patterns in gas phase (White: H, gray: C, and blue: N)

Fig.8 The CC and CN bonds distribution in NBR chain

Table 3 The length of CC double bonds at different positions of NBR molecule in different solvents

溶剂种类	溶剂	Bond length/?
		1,4-CC—					1,2-CC—
		(1)	(2)	(3)	(4)	(5)	1,2-CC—
弱亲电子溶剂	TOL	1.3391	1.3355	1.3400	1.3370	1.3371	1.3348
	OX	1.3391	1.3355	1.3401	1.3371	1.3371	1.3348
	TCM	1.3395	1.3359	1.3405	1.3375	1.3376	1.3351
	MCB	1.3397	1.3359	1.3406	1.3376	1.3376	1.3351
给电子溶剂	EA	1.3396	1.3359	1.3406	1.3376	1.3376	1.3352
	MAC	1.3397	1.3360	1.3407	1.3376	1.3377	1.3352
	CYC	1.3398	1.3362	1.3409	1.3378	1.3379	1.3354
	MEK	1.4000	1.3364	1.3411	1.3379	1.3381	1.3356
	ACE	1.3399	1.3363	1.3410	1.3378	1.3380	1.3355
气相	NBR	1.3384	1.3348	1.3392	1.3365	1.3365	1.3346

Fig.9 The bond length of the CC double bonds at different positions of NBR molecule in weak electrophilic solvents (a) and electron donating solvents (b)

Fig.10 The stretching vibration peak position of the 1, 4-CC— double bond of NBR chain in weak electrophilic solvents (a) and electron donating solvents (b)

Fig.11 Frontier orbital of NBR chain in gas phase

Fig.12 The HOMO orbital distribution of the NBR molecule in weak electrophilic solvents (a) and electron donating solvents (b)

Table 4 Frontier orbit energy, energy gap, solvation energy and dipole moments of NBR in different solvents

溶剂类型	溶剂	介电常数	E_LUMO/ a.u.	E_HOMO/a.u.	?E_gap/a.u.	?G/(kcal/mol)	偶极矩/D
弱亲电子溶剂	TOL	2.24	0.01201	-0.22966	0.24167	-19.14	5.99
	OX	2.27	0.01264	-0.23036	0.24300	-18.69	6.07
	TCM	4.90	0.01176	-0.22935	0.24111	-25.87	6.62
	MCB	5.65	0.00988	-0.22837	0.23825	-24.77	6.79
给电子溶剂	EA	6.02	0.01037	-0.22847	0.23884	-22.19	6.80
	MAC	6.68	0.00996	-0.22838	0.23834	-22.29	6.89
	CYC	18.30	0.01206	-0.22974	0.24180	-22.20	7.35
	MEK	18.51	0.01245	-0.23065	0.24310	-26.31	7.50
	ACE	20.49	0.01226	-0.23006	0.24232	-25.47	7.42
气相	NBR	0	0.00420	-0.22990	0.23410	0	5.01

Fig.13 FT-IR spectra and hydrogenation degree of HNBR obtained in weak electrophilic solvents [(a), (b)] and electron donating solvents [(c), (d)]

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