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Styrene/isoprene/butadiene integrated rubber prepared by anionic bulk polymerization in a twin-screw extruder.

INTRODUCTION

Natural rubber and synthetic rubbers were traditionally used to manufacture tires. The raw rubber was not a homogeneous polymer and separated into distinct microphases. The phase separation had a deleterious effect on the product's properties, limiting the performance of the vulcanized rubber. As a result, the tires made by the mixed rubber did not exhibit the desired integrated properties. In order to find a rubber with integrated properties, the "ideal rubber" concept was proposed by Nordsiek [1]. A styrene/isoprene/butadiene random copolymer was found to be such an integrated rubber [2] and approached the ideal rubber. This styrene/isoprene/butadiene rubber (SIBR) was prepared using anionic solution polymerization. The polymer chain of SIBR could be arranged into a variety of microstructures, which ultimately affect the properties of the corresponding tire products. Therefore, by adjusting the microstructure during the polymerization process, various application needs will be met.

In 1997, Goodyear manufactured the first SIBR industrial product, SIBRflex2550. The tires produced with this integrated rubber had excellent properties. After one and a half decades, SIBRflex2550 has still not been produced on a large industrial scale. The major obstacle to its widespread use was the high price.

The price of SIBR is 70% higher than that of ESBR (emulsion polymerized styrene/butadiene rubber), and 30% higher than solution SSBR (solution polymerized styrene/butadiene rubber). Improvement of the synthetic method may decrease the cost of SIBR. For example, the solvent concentration is usually maintained at 80 wt% during solution polymerization process. Recycling the solvent requires removal and purification in large chemical plants and consumes a great deal of money and resources. This is a high energy consumption process and is associated with high carbon dioxide emissions. The solution method can be replaced by the cheaper bulk method.

The main challenge with the bulk method performed in a traditional reactor is the control of heat transfer and the achievement of good mixing. The use of a twin-screw extruder with anionic bulk polymerization by reactive extrusion [3-8] can better control the temperature and provide good mixing. This method could at least partly overcome the problems associated with traditional anionic solution polymerization.

Michaeli et al. [3-5] first reported the use of twin-screw extruders to synthesize polystyrene and polystyrene copolymers using anionic bulk polymerization in 1993. Only a relatively small amount of solvent was required to dissolve the initiator in the reactive extrusion process. This solvent was easily removed by evaporation and additional equipment was not required. The high temperature in the extruder was associated with rapid polymerization. This efficient method was suitable for cost-effective industrial production.

We have studied anionic bulk polymerization by reactive extrusion for about 15 years. Our area of interest has been the polymerization of 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane [6] and the copolymerization of styrene and alkadiene [7-9]. We described the "bubble theory" which successfully explained the multiblock structure of the copolymer. Meanwhile, we had cooperated with Sinopec Yueyang Petrochemical Company Limited to produce high impact transparent styrene/butadiene resin (K resin) by reactive extrusion. From a twin-screw extruder with a diameter of 65 mm, 500 tons of K resin could be achieved in a year.

We previously reported the production of a styrene/isoprene copolymer with a weight content of isoprene above 50% using reactive extrusion [10]. We used this method to produce a high-performance synthetic rubber made of a styrene/isoprene/butadiene random copolymer.

EXPERIMENTAL

Materials

Styrene, butadiene, and isoprene used in this study were polymerization grade and provided by the Sinopec Shanghai Petrochemical Company Limited. All were stabilized with tertiary butylcatechol. All the monomers were purified by the physisorption of both aluminum oxide ([Al.sub.2] [O.sub.3] ) and molecular sieves. The styrene, isoprene, and butadiene monomers were mixed according to the weight ratio of 25/50/25 before polymerization.

N-Butyl lithium in hexane was provided by the J&K Scientific Company Limited, China at a concentration of 2.4 mol/L and used as the initiator. The initiator was diluted from 2.4 mol/ L to 0.1 mol/L, with purified cyclohexane. Three initiators were used, pure n-butyl lithium, n-BuLi/THF at a molar ratio of 1/5, and N-BuLi/TMEDA at a molar ratio of 1/1. These molar ratios were the most suitable for the desired microstructure and physical properties [11, 12]. The initiator solutions were stored in airtight steel cans that did not admit light. The final products were named SIBR-1, SIBR-THF, and SIBR-TMEDA, respectively.

Solution polymerized styrene/butadiene rubber SSBR2003 was obtained from the Sinopec Gaoqiao Chemical Co (Shanghai, China) to be used as the benchmark. Properties of synthesized products and SSBR2003 were evaluated at the same time. SSBR2003 was chosen, because there was no commercially available SIBR, while the structure and properties of SSBR and SIBR were similar.

Polymerization

The reactor used was a co-rotating closely intermeshing twin-screw extruder with a 36 mm diameter and a length/diameter ratio of 56. The feeding system consisted of an initiator pump (#6, Fig. 1) and a monomer mixture pump (#7, Fig. 1). The extrusion screw was rotated at a constant speed of 64 rpm during synthesis. The extruder had 13 barrels. The temperature of each barrel was set from 40[degrees]C to 200[degrees]C, as shown in Table 1. During the polymerization, the flow rates of styrene/isoprene/ butadiene monomer mixture and initiator solution were 70 mL/ min and 5 mL/min, respectively. The entire process (Fig. 1) was performed in an argon atmosphere to prevent contamination of the reaction system by oxygen and moisture. The screw profile was revealed in Fig. 2. The screw was composed of traditional conveying elements and kneading blocks while no reverse conveying elements were used.

Oxidative Degradation of the Copolymer

The distribution of the polystyrene blocks in the final polymer chain was evaluated after completely degrading the polymer using oxidative degradation process, as previously described [13]. Polystyrene blocks were not affected by the degradation reaction and remained as residue solid.

Vulcanization

The synthesized rubber and other reagents were mixed in a two-roll mill. Carbon black N220 was used as a reinforcing filler. The mixing formulation is shown in Table 2. The vulcanization was completed in a plate vulcanizing press. The time, temperature, and pressure of the vulcanization were 40 min, 145[degrees]C, and 10 MPa, respectively.

Gel Permeation Chromatography

The absolute molecular weight was measured using a DAWN EOS small-angle light scattering instrument with a Water-244 gel permeation chromatography (GPC) spectrometer. The molecular weight range of the device was from [10.sup.3] to [10.sup.6]. THF was used as the solvent, with a sample content of 5 mg/ mL. The test was carried out at 25[degrees]C.

Proton Nuclear Magnetic Resonance

Microstructure of the polymerized materials was characterized using proton nuclear magnetic resonance ([sup.1]H-NMR) spectroscopy. NMR spectra of the polymers were obtained using a Bruker DRX-400 spectrometer. The spectra were calibrated using tetramethylsilane as an internal standard. All the samples were dissolved in deuterated chloroform.

Dynamic Mechanical Analysis

The dynamic viscoelastic properties of the copolymers were measured using a TA Instruments Q800 Dynamic Mechanical Analyzer. The samples were scanned at temperatures ranging from -100 to 100[degrees]C with a frequency of 1 rad/s. The temperature was increased at a rate of 3 K/min.

Transmission Electron Microscopy

The polymer samples were dissolved in tetrahydrofuran to 0.5 wt%. The polymer solution was dripped on a copper grid, leaving a polymer film when the solvent evaporated. The sample film was stained in Os[O.sub.4] vapor for 25 min. A Jeol JEM-1400 transmission electron microscopy (TEM) was used for observation and micrographing.

Tensile Stress Tests and Tear Resistance

The vulcanized rubber products were cut to standard dumbbell-shaped spline with a width of 4 mm, thickness of 2 mm, and scale distance of 25 mm. The tests were carried out using a SANS CMT2203 universal testing machine with a speed of 500 mm/min.

RESULTS AND DISCUSSION

Polymerization

The chemical reaction formula was briefly revealed in Fig. 3. Both isoprene and butadiene could be polymerized to 1,4 and vinyl (3,4 for isoprene, 1,2 for butadiene) structure. The content of each structure could be regulated by changing the polarity of the polymerization atmosphere.

The polymerization mechanism in this study was different from that in the solution. In the nonpolar solution at 40[degrees]C, the reactivity ratio of styrene ([r.sub.s]) and isoprene was ([r.sub.i]) 0.04 and 2.6 [14], respectively. The [r.sub.s] for styrene and [r.sub.b] for butadiene at 25[degrees]C was 0.04 and 15.5, respectively [14]. Therefore, when monomer mixture was polymerized in the solution, styrene could not be consumed unless both the alkadienes were almost polymerized to build a chain without styrene units. In this study, when the monomer mixture was added into the screw, part of the alkadiene monomers would gasify because the barrel temperature (40[degrees]C) was higher than the boiling point of isoprene (34.3[degrees]C) and butadiene (-4.5[degrees]C). However, some of the liquid alkadiene dissolved in the styrene monomers. As soon as n-BuLi was added into the screw, liquid alkadiene began to polymerize. Styrene followed after the liquid alkadiene were completely consumed. Along with the polymerization, when the viscosity of the copolymer would be sufficient to contain the small alkadiene bubbles, the short polystyrene and polyalkadiene segments would be further polymerized. Consequently, there would be a long polystyrene block on the final copolymer chain, according to the "bubble theory" [8]. When n-BuLi was previously mixed with THF or TMEDA and then added into the screw, styrene could be copolymerized with alkadiene and the reaction speed would be increased. As a result, the total amount of long polystyrene block would be reduced. This would be proved in the followed GPC analysis.

The final conversion was about 90-95%. The residual molecules, including monomers, solvent for the initiator were removed by dissolution method.

Microstructure Analysis of SIBRs and SSBR2003

The [sup.1]H-NMR spectra of SIBRs and SSBR2003 are shown in Fig. 4. The phenyl peaks, double bond peaks, and saturated bond peaks appeared at 6.05-7.50, 4.00-6.00, and 1.00-3.00 ppm, respectively. Blocks of styrene usually produced separate resonance peaks. The ortho protons produced a peak at ~6.55 ppm, while the meta and para protons produced a resonance peak near 7.05 ppm [15]. There would be no peak at 6.55 ppm if the polystyrene segments had a random distribution [16]. The integrated areas from 4.25 to 5.85 ppm corresponded to the double bond protons of the polyisoprene and polybutadiene segments (Table 3). Polyisoprene had 1,4, 1,2, and 3,4 structures while polybutadiene had 1,4 and 1,2 structures. The integrated areas from 4.00-6.00 and 1.00-3.00 ppm were quite complex. As a result, the cis-trans isomerism could not be detected by [sup.1]H-NMR. From Table 3, the microstructure distribution of polyisoprene and polybutadiene was calculated and is shown in Table 4.

As shown in Table 4, the weight content of each product sample was slightly different from the monomer mixtures. The mixing procedure and the feeding system were not accurate enough to reach 100% conversion (about 90-95%), introducing some error. The addition of the randomizer such as THF or TMEDA to the initiator system increased the vinyl content. The weight content of 1,2 and 1,4 polybutadiene units in both SIBRs and SSBR2003 were quite similar. This suggests that SSBR2003 polymerization was initiated by n-BuLi, without any polar regulators in the nonpolar solution.

Molecular Weight and Polydispersity of SIBRs and SSBR2003

The GPC results of the SIBRs and SSBR2003 before and after oxidative degradation are shown in Figs. 5 and 6. The double bond in the polymer chain was completely fractured by the degradation process, while the polystyrene segments were completely preserved [13]. The light scattering curves of GPC (Fig. 5) consisted of the laser light signals from the scatterometer, which is very sensitive to large molecule weight polymers. The refractive index curves of GPC (Fig. 6) consisted of signals from the differential refractometer, which is very sensitive to low molecular weight polymers. Therefore, the calculation of the degraded samples was based on the refractive index curves, while that of the pristine samples was based on the light scattering curves. The calculated results are shown in Table 5.

As shown in Fig. 5, the light scattering curves of these four rubbers consisted of one main peak although three of them showed slight bimodal distribution tendency. However, their molecular weight distribution was quite similar, ranged from 1.6 to 1.8 (Table 5). The number-average molecular weight ([M.sub.n]) of the SIBRs synthesized by reactive extrusion was significantly larger than the [M.sub.n] of SSBR2003. Although a higher molecular weight rubber should have better physical properties, it is more difficult to process. As a result, the molecular weight of synthetic rubber is usually kept below a certain value. The use of anionic polymerization allowed the molecular weight of the polymer to be easily adjusted by altering the initiator and the molar ratio of the monomers.

The polystyrene segments of the degradation products from the four kinds of rubber were very different, as shown bye refractive index curves (Fig. 6). After degradation, the polystyrene segments were divided into long blocks and micro blocks, each with a multimodal distribution. Since GPC is a size-exclusion chromatography, the long blocks came out of the GPC column first (shorter elution time) and the micro blocks came out later (longer elution time). Hence, the boundaries of long blocks and short blocks could be easily determined by the refractive index curves. As shown in Fig. 6, the polystyrene segments in the rubber products consist mainly of micro blocks rather than long blocks. The [M.sub.n] of the polystyrene micro blocks present in the three SIBRs were essentially the same and were much lower than the [M.sub.n] of polystyrene micro blocks found in SSBR2003 (Table 5). SSBR2003 was prepared in a nonpolar solution, without any polar regulators. In that atmosphere, [r.sub.b] was much larger than [r.sub.s]. Therefore, during the polymerization of SSBR2003, the butadiene monomers would be polymerized first, forming a long block. When most of the butadiene monomers were consumed, a transition boundary called a "tapered block" structure was formed [17, 18].

According to the "bubble theory" [8], SIBR-1 formed in a nonpolar atmosphere would exhibit a multiblock structure. There would be a long polystyrene block at one end of a number of polymer chains. Addition of a "randomizer," such as THF or TMEDA, would balance the ratio of styrene and alkadiene. As a result, the number of long polystyrene block in SIBR-THF or SIBR-TMEDA was much fewer than that in SIBR-1. This conjecture was supported by the following integral data.

The integral curves in Fig. 7 and the data in Table 6 indicate the distribution of the polystyrene segments in each rubber product. SIBR-THF and SIBR-TMEDA were mainly composed of quite short blocks of styrene. Styrene blocks below 2000 g/mol comprised 88.5% and 86.9%, respectively, of the total styrene content in SIBR-THF and in SIBR-TMEDA. The degree of randomness was greater in SIBR-TMEDA than in SIBR-THF. Styrene micro blocks less than 500 g/mol comprised 56.7% of the total styrene content in SIBR-TMEDA and 6.7% of the total styrene content in SIBR-THF. SSBR2003 was mainly composed of long styrene blocks. 29.3% of styrene blocks in SSBR2003 have molar masses higher than 10,000 g/mol, and 57.5% of styrene blocks in SSBR2003 have molar masses between 2000 and 10,000 g/mol. In contrast, both the long and micro styrene blocks in SIBR-1 ranged between SSBR2003 and SIBR-THF (or SIBRTMEDA). As a result, SIBR-1 prepared in a nonpolar atmosphere exhibited a multiblock structure rather than a "tapered block." The use of polar regulators changed SIBR-THF and SIBR-TMEDA from multiblock polymers to random copolymers.

Dynamic Mechanical Properties of SIBRs and SSBR2003

The "ideal rubber" theory [1] relates the performance of tires to the tan [delta] of the rubber. From -20 to 0[degrees]C, the values of tan [delta] correspond to the wet grip of the rubber tread and a higher value is associated with a better wet grip [19]. The values of tan [delta] from 60 to 80[degrees]C correspond to the rolling resistance, and a lower value is associated with a lower rolling resistance [20]. An ideal rubber should exhibit a low tan [delta] value from 60 to 80[degrees]C and a high value from -20 to 0[degrees]C.

The tan [delta]-temperature curves of the vulcanized SIBRs and SSBR2003 were determined (Fig. 8). SSBR2003 had a bimodal distribution, with the first peak at -69[degrees]C and the second at 84[degrees]C. Both peaks were located between the glass transition temperature ([T.sub.g]) of LCBR (-105[degrees]C) (low-c/s butadiene rubber, prepared by anionic solution polymerization without any polar regulator in a nonpolar solution) and polystyrene (110[degrees]C). This bimodal distribution indicated that SSBR2003 consisted of separate polystyrene and polybutadiene microphases and this observation was compatible with the GPC analysis. SIBR-1 had a similar tan [delta]-temperature curve to SSBR2003, demonstrating the separation of the polyalkadiene and polystyrene phases. The random distribution of styrene microblocks in SIBR-THF and SIBR-TMEDA resulted in an almost unimodal distribution, even though there seemed to have a small peak in both curves (a) and (b) above 50[degrees]C.

As shown in Table 7, SIBR-1 and SSBR2003 had two distinct [T.sub.g]. The higher [T.sub.g] was related to long blocks of polystyrene which reduced the elasticity and increased heat generation in the tires. The average tan [delta] values of SIBR-1 and SSBR2003 from 60 to 80[degrees]C were 0.212 and 0.230, respectively, higher than SIBR-THF (0.145) and SIBR-TMEDA (0.156). Tires made of SIBR-1 and SSBR2003 would thus have more heat generation and higher rolling resistance. The average tan [delta] values of SIBR-THF and SIBR-TMEDA from -20 to 0[degrees]C were 0.434 and 0.390, respectively, significantly higher than SIBR (0.096) and SSBR2003 (0.085). Tires made of SIBR-THF and SIBR-TMEDA would have an excellent grip on wet surfaces.

Microdomain Morphology of SIBRs and SSBR2003

TEM photographs of the four rubber products are shown in Fig. 9. The polystyrene component of all four products was in the dispersed phase. The continuous phase was composed of polyalkadiene. As discussed previously, the distribution of the polystyrene blocks in the four rubber samples was quite different. However, their TEM images appeared very similar. TEM morphology was mainly determined by the weight content of each phase and not the length of each block. The dynamic mechanical analysis and GPC analyses demonstrated that SIBR-THF and SIBR-TMEDA were composed of high weight content of random copolymers with polystyrene micro blocks. TEM photographs showed that these very short polystyrene segments aggregated into spherical or elliptical particles with a diameter of 20-50 nm. Although SIBR-1 and SSBR2003 were composed of multiblock or "tapered block" copolymers, the size of their dispersed phase was not larger than that found in the random copolymers SIBR-THF and SIBR-TMEDA.

Mechanical Properties of Vulcanized SIBRs and SSBR2003

There were no noticeable differences in the mechanical properties of the SIBRs and SSBR2003, even though the latter was an industrial product (Table 8). The tearing strength of SIBR-1 and SSBR2003 were larger than that of SIBR-THF and SIBR-TMEDA because the former contained long blocks of polystyrene in the polymer chain.

CONCLUSION

Although SIBRs have been under study for nearly twenty years, there are still not any SIBR industrial products on the market. We aimed to solve this problem by successfully synthesizing inexpensive SIBRs using bulk polymerization. Two almost randomized SIBRs we manufactured had desired tire properties very similar to the ideal rubber. From the experience about the previous production of K resin by reactive extrusion, the further work of the industrial application for the reactive extrusion of SIBR is followed.

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Xieyao Yuan, Jiming Wang, Dong Shan, Anna Zheng

Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

Correspondence to: Anna Zheng; e-mail: zan@ecust.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grand number: 50390091, 50933002.

DOI 10.1002/pen.23987

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Temperature setting for extruder.

Barrel                      1     2     3     4     5     6     7

Temperature ([degrees]C)   40    50    60    80    100   120   140
Barrel                      8     9    10    11    12    13    Die
Temperature ([degrees]C)   160   180   180   200   200   200   200

TABLE 2. Vulcanization formula.

Rubber              100   Accelerator CZ   1
Sulfur               2    Antiager 4010    1
Zinc oxide           5    Coumarone        5
Carbon black N220   60    Stearic acid     2

TABLE 3. Chemical shift and number of protons on double bonds.

Protons              Number   Chemical shift (ppm)

1,4 PB -CH=            2            5.35
1,2 PB =C[H.sub.2]     2            4.95
1,2 PB -CH=            1            5.55
1,4 PI -CH=            1            5.13
3,4 PI =C[H.sub.2]     2           4.68, 4.76

TABLE 4. Microstructure distributions of SIBRs and SSBR2003.

Samples     1,2 PB  1,4 PB  1,4 PI  3,4 PI  St (wt)  Ip (wt)  Bd (wt)

SIBR        11.1%   88.9%   90.1%    9.9%    25.7%    49.1%    25.2%
SIBR-THF    25.6%   74.2%   70.6%   29.4%    22.7%    53.1%    24.2%
SIBR-TMEDA  40.6%   59.4%   59.0%   41.0%    23.4%    56.1%    20.5%
SSBR2003    10.5%   89.5%     --      --     26.2%     --      73.8%

TABLE 5. GPC data for SIBRs and SSBR2003.

                          Before degradation

Samples      [M.sub.n] x [10.sup.-4]   [M.sub.w]/[M.sub.n]

SIBR                  14.9                     1.7
SIBR-THF              17.3                     1.8
SIBR-TMEDA            15.7                     1.6
SSBR2003               7.5                     1.6

                           After degradation

                              Long block

Samples      [M.sub.n] x [10.sup.-4]   [M.sub.w]/[M.sub.n]

SIBR                   1.9                     1.6
SIBR-THF               3.4                     1.9
SIBR-TMEDA             3.0                     1.5
SSBR2003               3.3                     1.2

                           After degradation

                              Micro block

Samples      [M.sub.n] x [10.sup.-2]   [M.sub.w]/[M.sub.n]

SIBR                   7.5                     2.1
SIBR-THF               8.1                     2.0
SIBR-TMEDA             3.3                     1.3
SSBR2003              34.1                     1.8

TABLE 6. Composition of the polystyrene segments.

                Distribution of PS blocks
                    (weight content)

                 Below            Below
Samples      5 x [10.sup.2]   2 x [10.sup.3]

SIBR-THF          6.7%            88.5%
SIBR-TMEDA       56.7%            86.9%
SIBR-1           14.3%            55.2%
SSBR2003          0.2%            13.2%

                Distribution of PS blocks (weight content)

                                                    Above
Samples      2 x [10.sup.3]-10 x [10.sup.3]    10 x [10.sup.3]

SIBR-THF                  10.5%                      1.0%
SIBR-TMEDA                 2.1%                     11.0%
SIBR-1                    17.5%                     27.3%
SSBR2003                  57.5%                     29.3%

TABLE 7. [T.sub.g] and tan [delta] value for vulcanized SIBRs and
SSBR2003.

                                  Average tan [delta] values
              [T.sub.g]
Samples      ([degrees]C)   -20 to 0[degrees]C   60 to 80[degrees]C

SIBR-1         -46, 74            0.096                0.212
SIBR-THF         -15              0.434                0.145
SIBR-TMEDA        -7              0.390                0.156
SSBR2003       -69, 84            0.085                0.230

TABLE 8. Mechanical properties of vulcanized SIBRs and SSBR2003.

             Tensile                                     Tearing
             strength    Elongation    300% stretching   strength
Samples       (MPa)     at break (%)   strength (MPa)     (kN/m)

SIBR-1         15.5         580             11.5           54.2
SIBR-THF       16.0         560              9.5           42.9
SIBR-TMEDA     18.7         495             12.7           41.8
SSBR2003       17.2         705              9.2           56.8
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Author:Yuan, Xieyao; Wang, Jiming; Shan, Dong; Zheng, Anna
Publication:Polymer Engineering and Science
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Date:May 1, 2015
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