Bromination modification of butyl rubber and its structure, properties

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Bromination modification of butyl rubber and its structure, properties and application Renwei Cao, Xiyuan Zhao, Xiuying Zhao, Xiaohui Wu, Xiaolin Li, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03491 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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Industrial & Engineering Chemistry Research

Bromination modification of butyl rubber and its structure, properties and application Renwei Cao1,2, Xiyuan Zhao1,2, Xiuying Zhao1,2*, Xiaohui Wu1*, Xiaolin Li1, Liqun Zhang1,2

1. Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China 2. State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

Keywords: BIIR; bromine content; bromine atom bonding structure; compatibility; gas permeability; cure reactivity

*To whom correspondence should be addressed.

Tel.: + 010-64434860；Fax: +86-10-64433964 (X. Zhao).

E-mail addresses: [email protected] (X. Zhao) [email protected] (X. Wu)

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

The properties of bromobutyl rubber (BIIR) are closely related to its bromine content and the bonding structure of the bromine atom with the BIIR molecular chain. We here investigated the effect of these two factors on BIIR properties. We prepared BIIR with a predominant

exo-methylene

allyl

(EXO)

or

endo-bromomethyl

olefin

(ENDO)

microstructure at different levels of bromine content by controlling the conditions of bromination. BIIRs with a high bromine content had low permeability coefficients: the permeability coefficient of BIIR-3# (bromine content, 1.58 wt%) was lower than that of butyl rubber (IIR) by 23.8%. BIIRs that were predominantly ENDO with a high bromine content had a high cure reactivity and excellent compatibility with natural rubber (NR), which markedly improved the mechanical performance of the NR/BIIR blend vulcanizate. A bridge between the microstructure and macro-performance of BIIR was successfully set up, which scientifically guide for fabrication of tire inner liner with BIIR.


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1. Introduction

Butyl rubber (IIR) is an elastomer obtained by the cationic polymerization of isobutylene and approximately 1–2 mol% isoprene. IIR has excellent properties such as high chemical stability, damping, and low permeability to gases and liquids, which is one of the highest among rubber 1. However, because of a tiny amount of double bonds (1–2 mol%) and its low cure reactivity, IIR has a low cure rate, poor adhesion and compatibility with unsaturated rubber. In order to improve their cure reactivity, adhesion, and compatibility, IIRs are halogenated, a reaction that has been investigated since the 1950s. Halogenated butyl rubber (HIIR), including chlorinated IIRs (CIIRs) and brominated IIRs (BIIRs), have been prepared and studied for several decades now 2. BIIR is widely used because of its superior properties. BIIR is obtained by introducing bromine atoms near the double bonds of the IIR molecular chain, which greatly increase the cure reactivity of the double bonds and the polarity of the molecular chain simultaneously. As a result, BIIR overcomes the disadvantages of IIR by exhibiting higher co-vulcanization, blending, and compatibility with other rubbers 3-6.


3


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The microstructure of IIR has been analyzed in detail using nuclear magnetic resonance (NMR) spectroscopy

7, 8.

It is seen that the isoprene unit predominantly exists in the

microstructure of IIR as the 1,4-trans-isomer and a trace amount of 1,4-cis-isomer. BIIR is usually prepared by the bromination of IIR in n-hexane 9-11. The bromination proceeds by an electrophilic substitution reaction rather than an addition reaction

5, 12-17

and the

mechanism of bromination is described as follows. When the Br2 molecule is close to the double bond of the isoprene unit, it undergoes heterolysis, which generates Br+ and Br-. The Br+ attacks the double bond to form a transition state, a bromonium ion of type B+. The numerous bulky methyl groups on the IIR molecular chain cause a strong steric hindrance preventing Br- from attacking the transition state. As a result, hydrogen bromide is eliminated, and the double bond is regenerated. It was found that

8, 13, 14, 18

the

bromination reaction first produced an exo-methylene allylic structure (EXO), but in the case of heating with an acid, a rearrangement reaction occurs to form the endobromomethyl structure (ENDO). Essentially, bromination of IIR is that Br atom is introduced near the IIR double bond, and the electronegativity of Br increases the reactivity of the double bond. The level of bromine content is directly proportional to the


4

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degree of double bond activation of the IIR molecular chain. Besides, Br atoms also increase the polarity and mutual attraction between the molecular chains, which is demonstrated by the fact that the density of IIR is slightly smaller than that of BIIR. As reported earlier 12-14, Br atom is mainly present in the form of exo-methylene allyl or endo-bromomethyl olefinic bromide. However, our current research is restricted to the study of the bromination reaction conditions. We also observe that the EXO is predominant in the synthesized BIIR and its bromine content is constant. Until now, there is no study that reports that the properties of BIIR are influenced by the predominant ENDO. In a word, the effect of bromine content of BIIR and the bonding structure of bromine atoms with the molecular chain on its properties has been hardly reported. The blending of IIR with other rubbers is very poor because of the presence of a large number of methyl side groups on its carbon skeleton 19, 20. BIIR is mainly used in the inner liners of vehicle tires

21, 22

because of its excellent barrier properties. In order to improve

the adhesion and elasticity of these inner liners to casing ply, BIIR usually blends with natural rubber (NR), which involves the compatibility and properties complementarity between BIIR and NR.


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In this paper, BIIRs with different bromine contents and bonding structures were prepared by chemical modification. We then investigated the gas-permeability and cure reactivity of obtained BIIRs. The NR/BIIR blend was prepared, and its compatibility, mechanical properties, and thermal oxidative aging property were investigated, which established a theoretical foundation for the reasonable matching of bromine content and bonding structure in BIIR. 2. Experimental 2.1 Materials IIR (LANXESS IIR301; the molar percentage of isoprene units in the polymer chain was

𝑋𝑖𝑝=1.21% obtained by NMR spectroscopy using Equation (2)) was used as provided by Genway New Materials Company Ltd. (Tianjin, China). NR (SCR 1) was supplied by Yunnan Natural Rubber Industry Co. Ltd. (Kunming, China). Bromine (Br2, 99.9%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Sodium hypochlorite (NaClO, 99.9%) was brought from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The rest of the reagents, n-hexane (98%), sodium hydroxide


6

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(NaOH, 98.5%), and methylene chloride (CH2Cl2, 98%), were acquired from Beijing Chemical Works. (Beijing, China). 2.2 Preparation of BIIR Bromination was carried out at 50°C, and the reaction mixture was protected from sunlight. The reaction mechanism is shown in Scheme 1. The experimental conditions and the concentration of reagents are presented in Figure 1. A representative experimental procedure is as follows: (a) IIR (120 g) was cut into 0.5 cm2 pieces and taken in a three-neck flask. The IIR pieces were kept in a solution of hexane (540 ml) and methylene chloride (225 ml) for 24 h. The mixture was stirred to a uniform glue solution and heated to 50°C. (b) Hexane (5 ml) was added to the amber bottle followed by Br2 (0.420 ml) and NaClO (4.92 ml). NaClO, as an oxidant, can react with HBr (the byproduct in bromination reaction) to oxidize it back to molecular bromine and regenerating the bromine can continue to react with the carbon-carbon double bond.16, 23. Therefore, the Br2 utilization was increased. The reaction mixture was set. (c) Bromination was started by the injection of the reaction mixture into a three-neck flask. After 1 min, the reaction was terminated by the addition of pre-prepared sodium hydroxide solution (using 1.56 g


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NaOH). (d) The glue solution was subsequently washed with distilled water five times. The solvent was removed using a rotary evaporator and dried under vacuum at 50°C. The solid polymer was recovered.

Scheme 1. Bromination and rearrangement reactions.

2.3 Preparation of BIIR (IIR) vulcanizates The BIIR or IIR vulcanizates were prepared based on the compounding formulations shown in Table 1. The BIIR synthesized or IIR were mixed on a Φ 152.4-mm two-roll mill. Subsequently, the activators (zinc oxide, stearic acid), accelerator (DM), and sulfur were


8

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added to the rubber compounds. After 8 h, the cure characteristics of all the compounds were studied using the P3555B2 no rotor rheometer (Beijing Huanfeng Chemical Machinery Trial Plant, Beijing, China) at 151°C. After 24 h, all the compounds were hot pressed into samples with dimension 8 cm (diameter)×1 mm (thickness) and vulcanized at 151°C under a pressure of 15 MPa for (Tc90+2) min (Tc90, process curing time) using an XLB-D350×350-type automatic operation vulcanizing press (Huzhou Dongfang Machinery Co., Ltd., China). 2.4 Preparation of NR/BIIR (IIR) vulcanizates Rubber compounds were prepared based on the compounding formulations shown in Table 1. The NR and the BIIR synthesized or IIR were mixed in a ratio of 80:20 (mass ratio) on two-roll mill. Subsequently, the activators (zinc oxide, stearic acid), accelerators (TMTD, DM), carbon black (N550), and sulfur were added to the rubber compounds. NR/BIIR (IIR) compounds with and without carbon black filler were prepared. In addition, NR compounds with carbon black were also prepared. After 8 h, the cure characteristics of all compounds were studied using the P3555B2 no rotor rheometer at 151°C. After 24 h, all compounds were hot pressed into samples with dimension 13 cm (length)×11 cm


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(width)×2 mm (thickness) and vulcanized at 151°C under a pressure of 15 MPa for (Tc90+2) min using an XLB-D350×350-type automatic operation vulcanizing press.


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Table 1 Compounding formulations (unit: parts per hundred rubber (phr)) Zinc

Stearic

Components (phr)

BIIR100

TMTDa

DMb

N550

Sulfur

oxide

acid

3

1

—

1

—

0.5

4

2

0.2

0.75

—

1.65

4

2

0.2

0.75

40

1.65

4

2

0.2

0.75

40

1.65

NR/BIIR (IIR)80/20 NR/BIIR (IIR)/CB80/20/40 NR/CB100/40 aTMTD, bDM,

accelerator, N,N’-tetramethylthiuram disulfide.

accelerator, 2,2'-dibenzothiazole disulfide.

2.5 Characterization 2.5.1 Structural characterization of the synthesized BIIR a. Determination of the chemical structure The BIIR synthesized were studied by NMR spectroscopy (1H-NMR, 400 MHz, Bruker AVANCE-600 spectrometer, Germany) in CDCl3, with chemical shifts referred to tetramethylsilane. b. Determination of molecular weight and molecular weight distribution


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The molecular weight and molecular weight distribution (MWD) of the BIIR synthesized were obtained using gel permeation chromatography (Waters RH7040, USA), calibrated using PL polystyrene standards and eluted with tetrahydrofuran. 2.5.2 Gas permeability characterization of synthesized BIIR The gas permeability coefficients of BIIR vulcanizates were obtained in accordance with ISO2566 using a VAC-V2 gas permeability tester (Jinan Labthink, China). Figure S1 shows schematic diagram of gas permeability test. The gas permeability coefficient is used to evaluate the material’s gas permeability. The test was carried out at 23±2°C with nitrogen as the test gas. The samples were wiped clean with alcohol and placed in a desiccator for 24 h before testing. The 𝑃𝑔 is obtained using the following equation:

Pg 

p V T D   0  (1) t S p0T ( p1  p 2 )

where ∆p/∆t is the arithmetic mean of the gas pressure change in the low-pressure chamber per unit time; 𝑉 is the volume of low-pressure chamber; 𝑆 is the superficial area of the sample; 𝑇0 and 𝑝0 are the temperature (273.15 K) and pressure (101.33 × 105 Pa),


12

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respectively, under standard conditions; T is the experimental temperature; D is the thickness of the sample; and 𝑝1 ― 𝑝2 is the pressure difference across the sample. 2.5.3 Characterization of the vulcanizates a. Compatibility The compatibility between NR and BIIR or IIR was studied using atomic force microscopy (AFM, Bruker Multi-Mode, Nanoscope IV Controller, Veeco Metrology). The microscope was operated in the PF-QNM (peak force quantity nanomechanical) mode. In order to prevent the influence of the filler on the observation, the NR/BIIR vulcanizate without filler was selected for AFM observation. The samples were cut into rod-like structures of dimension 15 mm (length)×2 mm (width)×2 mm (thickness) and were polished using a cryo-ultramicrotome (Leica EM UC7, Germany) at –120℃. The images were recorded under ambient temperature in air, and scanning was conducted at 10×10 μm and 1×1 μm. b. Mechanical properties The static mechanical properties of the vulcanizates (with carbon black filler) were tested in accordance with ASTM D368 using a CMT4104 Electrical Tensile Tester


13


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(Shenzhen SANS Test Machine Co., China) at a tensile rate of 500 mm/min at room temperature. The samples were cut into dumbbells whose working area was 25×6 mm2; five replicate specimens were tested for each sample. The mechanical properties tested include tensile strength, elongation at break, and stress at 100%. The hardness of the vulcanizates was tested at room temperature in accordance with ASTM 2240 using a Durometer Shore A. c. Thermal oxidative aging properties The thermal oxidative aging properties of the vulcanizates with carbon black filler were measured using an air circulating oven (GT-7017-E, Gotech Testing Machines Co., Ltd., China) at 100℃ for 48 h in accordance with ISO188:2011. The sample properties before and after thermal oxidative aging were compared. 3. Results and discussion 3.1 Microstructural analysis of BIIR


14

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Scheme 2. Chemical shifts of the 1H peaks in the synthesized BIIR.

Figure 1. 1H-NMR spectra of IIR and the synthesized BIIR. Experimental conditions: BIIR1#: 0.420 ml Br2, 4.92 ml NaClO, 1.56 g NaOH, reaction time: 1 min; BIIR-2#: 0.835 ml Br2, 9.85 ml NaClO, 3.12 g NaOH, reaction time: 1 min; BIIR-3#: 1.250 ml Br2, 14.77 ml


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NaClO, 4.68 g NaOH, reaction time: 1 min; and BIIR-4#: 1.250 ml Br2, 14.77 ml NaClO, 4.68 g NaOH, reaction time: 45 min.

Table 2 Bromine content, conversion rate of the unsaturated double bond of IIR, and molar ratio of EXO to the sum of EXO and ENDO in the synthesized BIIR

Sample

𝑋𝑏𝑟(wt%)

𝑋𝑐𝑜𝑛𝑣 (%)

𝜔(%)

BIIR-1#

0.57

24.1

95.2

BIIR-2#

1.09

53.2

96.6

BIIR-3#

1.58

92.8

87.7

BIIR-4#

1.62

100

29.5

The bromine content and the bonding structure of bromine with the molecular chain were characterized by NMR spectroscopy. NMR spectroscopy has been used to study the microstructure of BIIR quantitatively

4, 7, 13, 14, 24.

The review

25

shows analytical

methods and characterization of Butyl and Bromobutyl Rubber in detail. The 1H-NMR spectra of IIR and the synthesized BIIR are shown in Figure 1. Some important information about microstructure, such as the molar percentage of isoprene units of IIR(𝑋𝑖𝑝), the bromine content of BIIR (𝑋𝑏𝑟), conversion rate of the unsaturated double bond of IIR (𝑋𝑐𝑜𝑛𝑣), and the molar


16

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ratio (𝜔) of EXO to the sum of EXO and ENDO, can be obtained from the 1H-NMR spectra. 4, 13, 24.

The 𝑋𝑏𝑟, 𝑋𝑐𝑜𝑛𝑣, and 𝜔 can be calculated using Equations. (3) – (5): 𝐻𝑐

𝑋𝑖𝑝 =

𝐻𝑐 +

X br 

𝐻𝑎

× 100% (2)

2

 H H f  g  

X conv 

 H  Hh  H f  g   79.9   2    100%  Hh  Hb   68.1  1.0  79.9   56.1  2 6 

nIIR  n BIIR nIIR

  Hf 

      H Hc c     Ha  Hg  Hh  H  a  Hc    Hc  Hf  2 IIR  2 2       H c   Ha    Hc   2 IIR 

      BIIR

(3)

 100% (4)

Hf  100% (5) Hg  Hh 2

where 𝐻𝑎 ― 𝐻ℎ are the integral areas of the different 1H peaks of BIIR and IIR. The 𝑋𝑏𝑟, 𝑋𝑐𝑜𝑛𝑣, and 𝜔 of the synthesized BIIRs are listed in Table 2. According to scheme 2, single peaks at 1.11 and 1.42 ppm were derived from protons b and a of the methyl and methylene, respectively, of the isobutylene structural unit. The triple peak at 5.07 ppm was derived from proton c of the carbon–carbon double bond of


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the isoprene structural unit. The BIIR synthesized existed in two forms: exo-methylene allyl bromide and endo-bromomethyl olefin. The single peaks at 5.40 and 5.02 ppm were derived from protons d and e, respectively, of the carbon–carbon double bond of EXO, and the triple peak at 4.34 ppm was derived from proton f of EXO. The single peaks at 4.08 and 4.04 ppm were derived from protons g and h, respectively, of ENDO in which the two protons are different in their chemical environments due to the cis-trans isomerization of the double bond. The triple peaks at 5.75 and 5.41 ppm corresponded to protons i and j, respectively, of the carbon–carbon double bond of ENDO similarly due to the cis-trans isomerization of the double bond. 3, 4, 7, 8, 18 As seen from the 1H-NMR spectra, the peaks of proton f appear in BIIR-1#, BIIR-2#, and BIIR-3#. Besides, the peaks of protons g and h are very weak, even hardly appearing in BIIR-1# and BIIR-2#. The observation indicates that bromine atoms exist in the EXO in BIIR-1#, BIIR-2#, and BIIR-3#. The 𝜔 of BIIR-1#, BIIR-2#, and BIIR-3# are 95.2%, 96.6%, and 87.7%, respectively. The peaks of protons d, e, and f gradually become stronger and that of proton c becomes weaker from BIIR-1# to BIIR-3#, suggesting that the bromine content of the BIIR is increasing continuously. Using Equation (3), the 𝑋𝑏𝑟 of BIIR-1#,


18

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BIIR-2#, and BIIR-3# was calculated to be 0.57, 1.09, and 1.58 wt%, respectively. The 𝑋𝑐𝑜𝑛𝑣 of BIIR-1#, BIIR-2#, and BIIR-3# is 24.1%, 53.2%, and 92.8%, respectively. As a result, the synthesized BIIR had a microstructure that was predominantly the EXO, with the bromine content increasing continuously from BIIR-1# to BIIR-3#. The peak of proton f is very weak, and those of protons g and h are strong in BIIR-4#, which is exactly in contrast to that found in BIIR-1#, BIIR-2#, and BIIR-3#. The observation indicates that the bromine of BIIR-4# is mainly present in ENDO. Using Equation (5), the 𝜔 value of BIIR-4# was calculated to be 29.5%, that is, the molar ratio of ENDO to the sum of EXO and ENDO (1- 𝜔) was 70.5%. Using Eq. (3), the 𝑋𝑏𝑟 of BIIR-4# was calculated to be 1.62wt%. Remarkably, the peak of proton c did not appear in BIIR-4#; as a result, 𝑋𝑐𝑜𝑛𝑣 of BIIR-4# was 100%, which indicated that all the carbon–carbon double bonds of IIR were brominated when bromine content was up to 1.62wt%. In addition, the appearance of the peak of protons i and j in BIIR-4# further demonstrated the existence of the ENDO on the molecular chain of BIIR-4#. Overall, BIIR having a microstructure that was predominantly the EXO and ENDO were prepared successfully.


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Table 3 Mn and MWD of IIR and BIIR synthesized

Sample

Mn◊104

MWD

IIR301

21.5

2.45

BIIR-1#

18.6

2.83

BIIR-2#

20.4

2.61

BIIR-3#

21.3

2.47

BIIR-4#

21.4

2.42

The number average molecular weight (Mn) and MWD of IIR and its brominated samples BIIR-1#BIIR-4# are shown in Table 3. The results show that when compared with those of IIR, the Mn and MWD of BIIR-3# and BIIR-4# did not change much, but the Mn of BIIR-1# and BIIR-2# decreased slightly, and their MWD became wider. This is mainly because, during bromination, hydrogen bromide is generated and gets accumulated gradually, and the polymer chain is prone to chain breakdown during catalysis by hydrogen bromide

12-14

, which affects the mechanical strength and gas

permeability of the rubber. 3.2 Cure reactivity and gas permeability of BIIR


20

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Figure 2. (a) Torque versus time traces. (b) Permeability coefficient of IIR and the synthesized BIIR

Cure traces of BIIR compounds are shown in Figure 2(a). The important cure parameters, which can be obtained from cure traces, are maximum torque (MH), minimum torque (ML), the difference between MH and ML (M), and the time required for the torque to rise by 1 dN.m from the minimum torque (Ts1). Ts1 is the time required to start the vulcanization reaction. As can be seen from the cure traces, the Ts1 of the BIIR compounds were obviously different, and it gradually shortened from BIIR-1# to BIIR-4#. Besides, during the stage in which the torque rises significantly, the slope of the curve also had a significant difference, gradually becoming larger from BIIR-1# to BIIR-4#. The slope of the curve reflects the cure rate; the greater the slope of the curve, the higher the


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cure rate. M reflects the degree of cross-linking of the vulcanizates. From cure traces, the degree of cross-linking of the BIIR compounds considerably increased with increasing bromine content. For example, BIIR-4# has a higher degree of cross-linking than BIIR3#. Table S1 shows the cure characteristics of the compounds. The observation proves that the cure reactivity was gradually increasing from BIIR-1# to BIIR-4#, that is, as the bromine content increases, the cure reactivity of the BIIR compounds gradually increased, and the reactivity of the BIIR compounds with predominantly ENDO was greater than that of the BIIR compounds that were made predominantly with EXO. The permeability coefficients of BIIR and the IIR vulcanizates are shown in Figure 2(b). The coefficients showed a decreasing trend from BIIR-1# to BIIR-3#, which suggested that the gas permeability gradually increased with the increase in the bromine content. As reported earlier

, the diffusion of small molecular gas in polymer materials is

26-32

extremely sensitive to free volume. After IIR was brominated to form BIIR, a certain amount of bromine atoms was introduced into the IIR molecular chain. Since bromine has a strong electronegativity, the interaction between molecular chains is significantly enhanced, which reduces not only the free volume fraction of the material but also the


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size of the free volume cavity inside the material. However, when the BIIR (BIIR-1#, 0.57 wt%) had a low bromine content, its gas permeability coefficient was higher than that of IIR. The reason for the above phenomenon, we believe, is that the synthesized BIIRs have a lower molecular weight and a broader MWD than the IIR, which results in an increase in the number of molecular ends, which in turn leads to an increase in the free volume of the material and a decrease in the gas permeability. With the increase in the bromine content, the gas permeability coefficient of the BIIR (BIIR-2#, 1.09 wt%; BIIR-3#, 1.58 wt%) began to decrease when compared with that of the IIR. For example, gas permeability coefficient of BIIR-3# decreased by 23.8%, which indicates that the gas permeability of BIIR improved remarkably. When the bromine content was basically the same, the gas permeability coefficients of BIIR-3# and BIIR-4# were approximately the same, indicating that different bromine atom bonding structures have little effect on the gas permeability of the BIIR vulcanizates. 3.3 Properties of NR/BIIR blended rubber 3.3.1 Compatibility analysis between NR and BIIR


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In recent years, the PF-QNM technique has been used to characterize the interfacial properties of polymer composites 33. Compared to the traditional tapping mode, the PFQNM model quantitatively characterizes the mechanical and interface interactions

.

34, 35

Utilizing the PeakForce Tapping technology, PF-QNM maps and distinguishes between nanomechanical properties with up to atomic resolution in topography as well as in the property channels. Based on this efficient technique, this study characterized the thickness of the interfacial layer according to the gradient change in the LogDMTMoudulus (Young’s modulus logarithm) (see Figure 3). AFM images of NR/IIR and NR/BIIR vulcanizates are presented in Figure 3. After simple mechanical blending, NR was the continuous phase (darker regions), and IIR or BIIR was the dispersed phase (lighter regions) because the blend ratio of NR to BIIR or IIR was 80:20 (mass ratio). It is seen from Figure 3(a) that the dispersed phase particle size of the IIR can be up to 5 μm, while that of BIIR decreased significantly, becoming roughly 2 μm (see Figure 3(b)–(e)). In addition, it was seen that the dispersed phase had very small particles in NR/BIIR-1-3#, the size being about several hundred nanometers only. In NR/IIR and NR/BIIR-4#, such small particles were hardly found. It may be noted that the


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dispersed phase formed by NR/BIIR-1-3# was smaller than that formed by NR/IIR. The size distribution was uneven in NR/BIIR-1-3#, while that formed by NR/BIIR-4# was uniform. We believe that during the mechanical blending of NR with BIIR or IIR, a small dispersed phase is formed. Once the mechanical blending is completed, the blend rubber would melt and become larger dispersed particles during storage and vulcanization 36. IIR was more prone to this fusion effect than BIIR. The LogDMTMoudulus of the interfacial region between NR and BIIR or IIR was varied gradually, which was because of the interdiffusion between the molecular chains of NR and BIIR or IIR. Therefore, the thickness of the interfacial layer was characterized according to the gradient change in the LogDMTMoudulus. The thicker the interface, the easier the mutual diffusion of the molecular chains between the two phases, that is, the better the compatibility of the two phases. As shown in Figure 3(a), the interfacial thickness of NR/IIR was the narrowest in all samples, being only 215 nm. However, after IIR was brominated to BIIR, the interfacial region became wider and wider with the increasing bromine content of BIIR. When the bromine content of BIIR was up to 1.58 wt% (BIIR-3#), the interfacial thickness was 430 nm. The interfacial thickness of NR/BIIR-3# was 100% more than that of NR/IIR, which


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indicated that the compatibility between NR and BIIR got better with the increase in the bromine content of the BIIR. In the case of an approximate content of bromine in BIIR, the interfacial layer of NR/BIIR-4# was thicker than that of NR-BIIR-3#. The interfacial thickness of NR/BIIR-4# was 169.8% more than that of NR/IIR, which indicated that the BIIR with predominantly ENDO had the best compatibility with NR compared to all other BIIR samples.


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Figure 3. Representative AFM LogDMTmodulus image of (a) NR/IIR, (b) NR/BIIR-1#, (c) NR/BIIR-2#, (d) NR/BIIR-3#, and (e) NR/BIIR-4# blends, and their corresponding LogDMTModulus-scanning location curve representing the interfacial thickness between NR and IIR or BIIR.

3.3.2 Mechanical properties of NR/BIIR/CB

Figure 4. (a) Stress–strain curves of NR/BIIR (IIR)/CB vulcanizates. (b) Strength and elongation at break of NR/BIIR (IIR) vulcanizates. (c) Stress at 100% and hardness of NR/BIIR (IIR) vulcanizates. (d) Tear strength of NR/BIIR (IIR) vulcanizates.


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The mechanical properties and the stress–strain curves of NR/BIIR/CB and NR/IIR/CB are shown in Figure 4. From Figure 4(a), it can be seen that the stress–strain curves of NR/IIR/CB, NR/BIIR-1–3#/CB, and NR/BIIR-4#/CB are different. The stress–strain curves of NR/BIIR-1–3#/CB are basically coincident. However, as the bromine content of BIIR increased, the elongation at break and the tensile strength of the blended rubber continued to increase. The stress–strain curve of NR/BIIR-4#/CB was below those of NR/BIIR-1–3#, indicating a small tensile stress, but a higher tensile strength and elongation at break. From Figure 4(b)–(d), the stress strength, elongation at break, stress at 100%, hardness, and tear strength of NR/IIR/CB are 9.9 MPa, 204%, 3.9 MPa, 69, and 28.0 KN/m, respectively. After IIR was brominated to BIIR, the stress strength, elongation at break, and tear strength obviously increased with the increase in the bromine content of BIIR. When the bromine content of BIIR increased to 1.58% (BIIR-3#), the stress strength, elongation at break, and tear strength of NR/BIIR-3#/CB increased by 76.8%, 105.9%, and 43.2%, respectively, when compared with those of NR/IIR/CB. Remarkably, the stress strength, elongation at break, and tear strength of NR/BIIR-4#/CB increased by 89.9%, 130.9%, and 54.3%, respectively, when compared with those of NR/IIR/CB.


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The observation suggests that BIIR-4#, with predominantly ENDO performed better than BIIR-3# with predominantly EXO for any level of bromine content of BIIR. As is well known, in order to improve the mechanical properties of blended rubbers, the compatibility must be improved since a very good compatibility produces strong interfacial adhesion 37-39. Poor compatibility of the blended rubber causes stress concentration at the interface during the stretching process, resulting in the generation of a microscopic “cavity” at the interface, with the “cavity effect” worsening the mechanical properties of the material. As seen from AFM analysis, the compatibility between NR and BIIR remarkably improved with the increase in the bromine content of BIIR. Simultaneously, it is seen that BIIR-4# has better compatibility with NR than BIIR-3#. 3.3.3 Thermal oxidative aging properties


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Figure 5. (a) Strength and elongation at break before and after thermal oxidative aging of NR/BIIR (IIR)/CB vulcanizates. (b) Tensile product aging coefficient of NR/BIIR (IIR)/CB vulcanizates.

Figure 5(a) shows the tensile strength and elongation at break of NR/CB, NR/IIR/CB, and NR/BIIR/CB vulcanizates before and after aging. The tensile strength and elongation at break of all samples decreased after thermal oxidative aging. However, when compared with that of the NR/CB vulcanizate, the falling range of NR/IIR/CB and NR/BIIR/CB vulcanizates decreased obviously. Generally, the tensile product is used to evaluate the aging properties of a rubber because it is a comprehensive measure of the material strength and toughness 40, 41. In this work, tensile product retention (𝑅𝑡) is as the aging coefficient and calculated as follows:

Rt 

Ts  E b Ts0  E b0

(6)

where 𝑇𝑠0 and 𝑇𝑠 are the stress strengths; and 𝐸𝑏0 and 𝐸𝑏 are the elongation at break before and after thermal oxidative aging. A higher 𝑅𝑡 means a better anti-thermal oxidative aging property. Figure 5(b) shows the tensile product aging coefficient (𝑅𝑡). When


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compared with that of NR/CB vulcanizates, 𝑅𝑡 of the NR/IIR/CB and NR/BIIR/CB vulcanizates increased by a wide margin. This suggested that both IIR and BIIR can significantly improve the thermal aging resistance of the blended rubber. 𝑅𝑡 of NR/BIIR/CB vulcanizates was higher than that of the NR/IIR/CB vulcanizate. With the increasing bromine content of BIIR, 𝑅𝑡 of NR/BIIR vulcanizates decreased gradually. In the case of an approximate bromine content of BIIR, 𝑅𝑡 of NR/BIIR-4#/CB was higher than that of NR/BIIR-3#/CB, which indicated that the BIIR with predominantly ENDO performed better than the BIIR with predominantly EXO in improving the thermal aging properties of blended rubbers. 4. Conclusions In this work, by reasonably controlling the conditions of the bromination reaction, the BIIR with a microstructure that was predominantly EXO with a bromine content of 0.57 wt%, 1.09 wt%, and 1.58 wt% and with a microstructure that was predominantly the ENDO with a bromine content of 1.62 wt% were successfully prepared. The result from NMR analysis indicated that all the double bonds of isoprene units were brominated when the bromine content was up to 1.62 wt%. It was found that the high bromine content of


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the BIIR compounds made them prone to vulcanization, resulting in a high degree of cross-linking, superior gas permeability, and excellent compatibility between BIIR and NR. When the bromine content was up to 1.58 wt%, the permeability coefficient of the BIIR decreased by 23.8% with respect to IIR. The difference in the bonding position of bromine atom on the molecular chain also had a great influence on the properties of the BIIR. BIIRs with predominantly ENDO showed higher cure reactivity than those with predominantly EXO. The interfacial layer of BIIR with predominantly ENDO and NR was thicker than that with predominantly EXO and NR, and the interfacial layer of NR/BIIR-4# was 169.8% thicker than that of NR/IIR. As a result, the mechanical properties of NR/BIIR/CB gradually increased, which could be attributed to the improvement in the compatibility between BIIR and NR. The tensile strength, elongation at break, and tear strength of NR/BIIR-4#/CB increased by 89.9%, 130.9%, and 54.3%, respectively, when compared with those of NR/IIR/CB. However, BIIR with a high bromine content and predominant ENDO microstructure had high double bond reactivity, resulting in their thermal aging resistance not as good as that of BIIR with a low bromine content, but it still exhibited good thermal aging resistance.


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ASSOCIATED CONTENT

Supporting Information The following files are available free of charge. Schematic diagram of gas permeability test, the cure characteristics of synthesized BIIR, a plot of interfacial thickness of NR/BIIR(IIR) blend (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Tel.:(010)64434860. Fax: (010)64433964. *E-mail: [email protected]. Tel.: (010)64433964. Fax: (010)64433964. ACKNOWLEDGMENT

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51320105012).


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Figure 1. 1H-NMR spectra of IIR and the synthesized BIIR. 169x149mm (300 x 300 DPI)



Figure 2. (a) Torque versus time traces. (b) Permeability coefficient of IIR and the synthesized BIIR 159x58mm (600 x 600 DPI)


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Figure 3. Representative AFM LogDMTmodulus image of (a) NR/IIR, (b) NR/BIIR1#, (c) NR/BIIR-2#, (d) NR/BIIR-3#, and (e) NR/BIIR-4# blends, and their corresponding LogDMTModulus-scanning location curve representing the interfacial thickness between NR and IIR or BIIR.


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Figure 4. (a) Stress–strain curves of NR/BIIR (IIR)/CB vulcanizates. (b) Strength and elongation at break of NR/BIIR (IIR) vulcanizates. (c) Stress at 100% and hardness of NR/BIIR (IIR) vulcanizates. (d) Tear strength of NR/BIIR (IIR) vulcanizates. 159x112mm (600 x 600 DPI)



Figure 5. (a) Strength and elongation at break before and after thermal oxidative aging of NR/BIIR (IIR)/CB vulcanizates. (b) Tensile product aging coefficient of NR/BIIR (IIR)/CB vulcanizates. 159x59mm (600 x 600 DPI)


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Bromination modification of butyl rubber and its structure, properties

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