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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Among Ziegler-Natta catalysts used for 1,3-butadiene (1,3-BD) polymerization, the advantage of a neodymium (Nd)-based catalyst is that it provides butadiene rubber (BR) with a high content of cis−1,4 configuration and a low amount of vinyl−1,2 units. Whereas, a cobalt (Co)-based catalyst can produce BR with a low content of trans−1,4 configuration. Thus, this research was aimed to prepare BR containing a high content of cis−1,4 configuration with low amounts of both trans−1,4 and vinyl−1,2 units using a combination of Nd- and Co-based Ziegler/Natta catalysts with triethyl aluminum (TEAL) and diethyl aluminum chloride (DEAC) acting as a co-catalyst and a chlorinating agent, respectively. The effects of the molar Co/Nd ratio, TEAL concentration, DEAC loading, 1,3-BD content, solvent type, and reaction temperature on % conversion, microstructures, molecular weight, and molecular weight distribution of the obtained BR (Co/Nd-BR) were evaluated. The Co/Nd-BR having >97% of cis−1,4 configuration, <2% of trans−1,4 structure, and <1% of vinyl−1,2 unit with >80% conversion was achieved when 3.01 M of 1,3-BD concentration was treated in a toluene/cyclohexane mixture (7/3 [w/w]). The Co/Nd-BR exhibited no gel formation with high mechanical performance, which was equivalent to commercial BR produced from a Nd-based catalyst system. POLYM. ENG. SCI., 55:14–21, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Butadiene rubber (BR), one of the most well-known synthetic elastomers, is normally utilized for tires (tire treads and side walls), thermoplastic elastomers (high impact polystyrene [HIPS] and acrylonitrile-butadiene-styrene terpolymer [ABS]), and technical rubber goods (golf ball cores, conveyer belts, shoe soles, and seals) [1]. BR is typically classified according to the polymerization technology and initiator/catalyst as three types: radical polymerization in an aqueous emulsion (E-BR), anionic polymerization in solution using Li-catalysts (Li-BR), and coordinative polymerization in solution using Co, Ni, and Nd-based Ziegler-Natta catalysts (Co-BR, Ni-BR, and Nd-BR, respectively) [1]. The Co-BR and Ni-BR have been commercialized since the 1960s.

For the Co-based system, it consists of cobalt (II) octanoate, diethyl aluminum chloride (DEAC) and water as an activator [2, 3]. This system provides Co-BR with a high branching level and low solution viscosity, making it appropriate for the production of ABS and HIPS [1, 4, 5]. Moreover, the high content of branching can decrease the cold flow effect of uncured BR to provide an advantage for preparation of compounded rubbers and rubber solutions [1, 4]. However, the drawback of the Co-BR arises from crosslinking resulting in gel formation which greatly affects the process behavior and surface appearance of the extruded products [2]. It also has a long optimum cure time and low tackiness [4]. Moreover, the low solubility of applied water as the activator for Co/DEAC in the solution polymerization induces instability in the process [2]. The Ni-based catalyst was developed by Bridgestone Tire in cooperation with Japan Synthetic Rubber (JSR). This system consists of nickel carboxylate, boron trifluoride diethyl etherate (BF3·Et2O), and triethyl aluminum (TEAL) [6, 7]. The BR produced by this system (Ni-BR) has higher linearity and a wider molecular weight distribution with a greater content of cis−1,4 configuration (>97%) than that of BR produced by the Co-based system (ca. 96%) [1, 4, 5].

The current technology applied in the tire industry relates to the development of tires with high energy-saving performance to reduce fuel consumption. Thus, the production of new tires with less rolling resistance and heat build-up including the improvement of wet skid and high durability is highly desired. Among the Ziegler-Natta catalysts for producing BR, a Nd-based catalyst exhibits the best performance to produce Nd-BR with the highest amount of cis-1,4-configuration (ca. >98%) and very low content of vinyl-1,2 structure (<1%). The high cis−1,4 structure content gives rise to a BR having the lowest glass transition temperature (Tg) (ca. −110°C) [1, 8], which is suitable for application in very low temperature circumstances. It also provides strain-induced crystallization properties to promote better green strength with a highly effective tackiness compared to other rubbers in the blends [4, 8, 9]. In addition, Nd-BR has lower heat build-up, greater abrasion resistance and higher linearity providing better elasticity and resilience properties [1, 5]. These properties are all required in the green tire industry. However, the relatively high viscosity and molecular weight of the Nd-BR is inappropriate for ABS production since its high linearity promotes high cold flow behavior resulting in poor extrudability and its molecular weight is difficult to control [1].

To balance the advantages of the Co- and Nd-based catalyst systems, this research aimed to study the polymerization of 1,3-butadiene (1,3-BD) catalyzed using a mixed catalyst containing Co and Nd (Co/Nd) with DEAC and TEAL as co-catalysts. The effects of the molar Co/Nd ratio, TEAL content, DEAC concentration, 1,3-BD concentration, reaction temperature and solvent type on % conversion, microstructures, molecular weight, and molecular weight distribution of the resulting BR gum (Co/Nd-BR) were examined. Moreover, the vulcanization and mechanical properties of the Co/Nd-BR vulcanizates were investigated and compared to commercial BRs obtained from the Co- and Nd-based polymerization processes.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Materials

1,3-BD (96% purity), methanol (99.9% purity), and a solution mixture containing 2.0% (w/w) of cobalt octoate (Co), 15% (w/w) of DEAC, and 2.0% (w/w) of TEAL in the presence of toluene were supplied by BST Elastomer (Rayong, Thailand). The amount of water in toluene was controlled at <10 ppm by purging the system with nitrogen gas (99.99% purity) before charging into the polymerization unit. Neodymium versatate (NdV3) dissolved in cyclohexane (8.8% [w/v]) was purchased from Comar Petrochemical (Cape Town, South Africa). It was diluted to 1.0% (w/v) in the oxygen free-toluene before charging into the polymerization reactor. 2,6-di-tert-butyl-4-methyphenol (BMP) and phosphate of polyoxyethylene alkyl phenyl ether (PPA) were purchased from Lanxess Deutschland GbmH (Germany) and Twin Hart International (Taiwan), respectively. The 5.6% (w/w) of BMP/PPA solution mixture (10/3 [w/w]) was prepared by dissolving in toluene. The commercial BRs, Nd-BR, and Co-BR, were obtained from Lanxess (Germany) and BST Elastomer (Thailand), respectively. Sulfur (Tsurumi chemical industry, Japan), stearic acid (Kao Corporation, Japan), zinc oxide (ZnO, Sakai chemical industry, Japan), n-tert-butyl-benzothiazole sulfonamide (TBBS, Monflex PTE), sun oil (Sunoco, Japan), and carbon black IRB No. 7 (USA) were used as received for BR vulcanization.

Catalyst Preparation

A 1.0% (w/v) of NdV3 solution was mixed with desired contents of Co, DEAC, and TEAL in a degassed dried-glass ampoule capped with a neoprene rubber septum. The molar Co/Nd ratios were varied as 0.0/1.0, 0.2/0.8, 0.3/0.7, 0.4/0.6, 0.5/0.5, and 1.0/0.0 with 0.5–2.0 and 3.0–9.5 of molar DEAC/Nd and TEAL/Nd ratios, respectively. The catalyst solution was then aged in a water bath at 20°C for 1 h before charging into the polymerization reactor.

1,3-BD Polymerization Using Co/Nd-Based Catalyst

The solution polymerization for producing BR was carried out in a 1- and 2-L Parr reactor under a dried nitrogen atmosphere for studying the effect of reaction parameters on 1,3-BD polymerization and scaling up the BR production for physical and mechanical properties testing, respectively. The dried toluene (toluene/1,3-BD = 3.5–5.0 [w/w]) was initially transferred into the reactor. When the reactor was heated to the desired temperature (45–60°C), 2.53–3.31 M of 1,3-BD concentration ([1,3-BD]) was charged followed by the injection of 5.4 mL of aged catalyst solution using a syringe. The polymerization was allowed to proceed for 2 h before termination by adding 12 mL of BMP/PPA solution. The polymeric syrup was then transferred into methanol under stirring to obtain the BR crumb. The resulting BR product was dried in an oven at 80°C for 2 h. The % conversion of 1,3-BD was calculated using Eq. (1):

  • display math(1)

where TSC is the total solid content (g) and M is the weight of charged 1,3-BD monomer (g).

Structure Characterization

The microstructures of Co/Nd-BR were quantitatively analyzed using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Perkin Elmer FT-IR spectrometer, Paragon 500 as ATR technique). The contents of cis−1,4, trans−1,4, and vinyl−1,2 configurations were calculated following Eqs. (2)-(4) according to ISO 12965:200(E) using the normalized peak height of the band absorbance at 738 (A738), 910 (A910), and 964 (A964) cm−1 for cis-, vinyl-, and trans-R–CH[DOUBLE BOND]CR–H, respectively.

  • display math(2)
  • display math(3)
  • display math(4)

To confirm the ATR-FTIR results, the microstructures of BR dissolved in CDCl3 at room temperature were also analyzed using proton (1H-NMR) and carbon (13C-NMR) nuclear magnetic resonance spectroscopy (Avance 500, Bruker) with 16 and 10,000 scans with trimethylsilane (TMS) as an internal standard.

Glass Transition Temperature (Tg)

The Tg of the Co/Nd-BR was evaluated using a dynamic mechanical analyzer (DMA, GABO/Eplexor) following ASTM 5279 with a tension mode under nitrogen atmosphere. The sample was prepared with a thickness of 1.75 mm and a width of 10 mm. The heating rate was controlled at 20°C/min with 1 Hz of applied frequency. The testing temperature was over the range −150–60°C.

Measurement of Gel Content

The gel content in the Co/Nd-BR was measured by putting 0.3 g of rubber sample on a container made from a metal screen sheet (80 mesh). Then, it was immerged in 100 mL toluene for 24 h. The rest of the sample in the container was dried in a vacuum oven at 60°C for 2 h. The gel content was calculated according to Eq. (5).

  • display math(5)

Molecular Weight and Molecular Weight Distribution

The molecular weight (Mw) and molecular weight distribution (MWD) of the Co/Nd-BR were determined using a gel permeation chromatograph (GPC, Waters 2690) equipped with a refractive index detector (Waters 2410). The mobile phase was tetrahydrofuran (THF) at a flow rate of 1 mL/min and 25°C. BR samples dissolved in THF at 0.1% (w/v) were injected into the GPC. A universal calibration curve was prepared using mono-dispersed polystyrene standards.

Cure Characteristics and Mechanical Properties of BR Vulcanizates

The recipe for preparation of the BR vulcanizates followed ASTM D3189-06. The Co/Nd-BR was compounded with three parts per hundred of rubber (phr) of ZnO, 2 phr of stearic acid, 1.5 phr of sulfur, 0.9 phr of TBBS, 15 phr of sun oil, and 60 phr of carbon black using an internal mixer at 30°C with a rotor speed of 50 rpm. Then, the compounded BR was sheeted with a 2-roll mixing mill at 40°C. The Mooney viscosity of the compounded BR was measured using a Mooney viscometer (Shimazu viscometer, Model SMV 202) at 100°C following ASTM D1646. The cure characteristics of the compounded BR were then analyzed using a moving die rheometer (MDR 2000, Alpha Technologies) at 160°C for 30 min to obtain the minimum (ML) and maximum torques (MH) with scorch time (ts1), and optimum cure time (tc90) according to ASTM D5289-07.

To evaluate the mechanical properties of the Co/Nd-BR vulcanizate, its resilience was determined using a Lupke rebound resilience tester (VR-6510, Ueshima Seisakusho) following JIS K 6255. Its tensile properties were also investigated using a tensile tester (Instron 5565A) with a crosshead speed of 500 mm/min following ASTM D412-06. The mechanical properties of the Co/Nd-BR vulcanizate were also compared to those of commercial Co- and Nd-BR vulcanizates.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

Chemical Structure, Tg and Gel Content of Co/Nd-BR

In the presence of toluene, the chemical structure of Co/Nd-BR obtained from the polymerization of 3.01 M of [1,3-BD] carried out using a 0.4/0.6 of molar Co/Nd ratio with 1.5 and 5.0 of molar DEAC/Nd and TEAL/Nd ratios, respectively at 50°C for 2 h was analyzed using ATR-FTIR spectroscopy (Fig. 1) and compared to those of commercial BRs: Co-BR and Nd-BR. The amount of each configuration in the chemical structure of all BRs is presented in Table 1. Figure 1 showed that the Co/Nd-BR structure had high intensity of the transmittance peak at 738 cm−1 indicating a high level of cis−1,4 configuration of ca. 98%. This implied that the Co/Nd mixed catalyst had similar potential to Co- and Nd-based catalyst systems to produce BR with a high level of cis−1,4 structure. However, the Co/Nd-BR exhibited a very low intensity of the transmittance signal attributed to the vinyl−1,2 configuration at 910 cm−1. From Table 1, the Co/Nd-BR had only 0.77% of vinyl−1,2 structure, which was lower than that of Co-BR (1.19%). This indicated that the Co/Nd mixed catalyst has a higher selectivity to produce BR with high cis−1,4 structure and a low amount of vinyl−1,2 configuration. This was mainly due to the influence of Nd in the mixed catalyst to provide a faster polymerization rate to form the external σ-allyl species than the internal ones, which provided less opportunity to generate the vinyl−1,2 structure in BR [10]. It was also observed that the existence of Co in the mixed catalyst induced Co/Nd-BR to have lower trans−1,4 configuration at 964 cm−1 (1.33%) than Nd-BR (2.12%). Thus, the lower amounts of both vinyl−1,2 and trans−1,4 units in Co/Nd-BR than that of Co-BR and Nd-BR, respectively were expected to promote easier crystallization under tension [4].

image

Figure 1. Comparative ATR-FTIR spectra of Co/Nd-BR and commercial BRs: Nd-BR and Co-BR.

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Table 1. Microstructures, Tg, and gel content of BRs.
BR typeMicrostructures (%)Tg (°C)Gel content (%)
Cis−1,4Trans−1,4Vinyl−1,2
  1. a

    Condition: [BD] = 3.01 M, molar Co/Nd ratio = 0.4/0.6, molar TEAL/Nd ratio = 5.0 and molar DEAC/Nd ratio = 1.5 at 50°C for 2 h in toluene.

Co-BR98.00.811.19−1000.04
Nd-BR97.62.120.28−980
(Co/Nd)-BRa97.91.330.77−1020

The microstructures of Co/Nd-BR were also comparatively determined by means of 1H-NMR and 13C-NMR spectroscopy. The 1H-NMR spectra of all BR sample (Fig. 2a) showed the signals at a chemical shift (δ) of 4.8–5.2 ppm for [DOUBLE BOND]CH2 of vinyl−1,2 configurations and 5.2–5.8 ppm for –CH[DOUBLE BOND] of 1,4-butadiene and 1,2-butadiene units [11]. It was found that the Nd-BR and Co/Nd-BR exhibited lower signals for [DOUBLE BOND]CH2 of vinyl−1,2 structure than Co-BR. 13C-NMR spectroscopy (Fig. 2b) shows important signals with a chemical shift of 27.4, 32.7, and 34.0 ppm attributed to 1,4-cis-methylene, –CH2– of 1,4-trans structure and vinyl-1,2-enchainment, respectively [12]. This observation was also consistent with the result from 1H-NMR spectroscopy which exhibited very low signal intensity for the vinyl−1,2 configuration of Co/Nd-BR.

image

Figure 2. (a) 1H-NMR and (b) 13C-NMR spectra of Co/Nd-BR, Nd-BR, and Co-BR.

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To evaluate the Tg value and gel content of Co/Nd-BR and compare it with the commercial Nd-BR and Co-BR (Table 1), it was found that Co/Nd-BR had the lowest Tg at −102°C, whereas the Tg values of Co-BR and Nd-BR were −100 and −98°C, respectively. This was due to the high portion of cis−1,4-configulation with very low vinyl−1,2 configuration of the Co/Nd-BR [1]. Moreover, Co/Nd-BR and Nd-BR with a very low amount of vinyl−1,2 configuration did not incur any gel formation, whereas, the commercial Co-BR containing a higher content of vinyl−1,2 structure had a gel content of ca. 0.04%. It is likely that the existing vinyl−1,2 configuration could easily induce the gel formation in BR via re-incorporation of internal unsaturation units for the Co-BR [5, 12].

Univariate Experiments for 1,3-BD Polymerization Using Co/Nd-Based Ziegler/Natta Catalyst

The effect of reaction parameters on the 1,3-BD polymerization using Co/Nd-Based Ziegler/Natta catalyst was individually investigated. The univariate experiments with the central condition of 3.01 M [1,3-BD] and 0.3/0.7 of molar Co/Nd ratio incorporated with the molar TEAL/Nd and DEAC/Nd ratios at 5.0 and 1.5, respectively, were carried out in a 1-L Parr reactor at desired temperatures for 2 h in the presence of toluene.

Effect of Molar Co/Nd Ratio

The effect of the molar Co/Nd ratio on the % conversion, Mw, MWD, and microstructures of Co/Nd-BR gum is shown in Fig. 3 and Table 2. In the absence of Co (molar Co/Nd ratio = 0/1), using only Nd as catalyst for 1,3-BD polymerization, only 68% conversion was achieved. While, using only Co catalyst (molar Co/Nd ratio = 1/0) could not activate the 1,3-BD polymerization due to the lack of water which is required to convert DEAC as a reactive aluminoxane acting as an electron donor in the typical 1,3-BD polymerization using the Co-based catalyst [13, 14]. Figure 3 shows that an increase in the amount of Co in the mixed Co/Nd catalyst to 0.3/0.7 or 0.4/0.6 enhanced the conversion level from 68 to 73–74%. This exhibited a synergetic effect between Co and Nd in the mixed catalyst. The reason for this phenomenon is still unclear. However, these results were similar to those obtained for the Ni/Nd mixed catalyst system for 1,3-BD polymerization reported by Jang et al. [15]. They revealed that a molar ratio of NdV3/nickel octoate/triisobutylaluminum/borontrifluoride-diethylether at 0.7/0.3/30/1.0 was preferable to produce a very high BR yield (99%) with high content of cis−1,4 configuration (98.5%) when the polymerization was carried out in the presence of cyclohexane at 40°C for 2 h. At a critical Co content in the Co/Nd catalyst system, it was found that the molar Co/Nd ratio at 0.5/0.5 decreased the % conversion to 27.5%. Thus, it could be concluded that the appropriate molar Co/Nd ratio was 0.3/0.7 or 0.4/0.6 with the molar TEAL/Nd and DEAC/Nd ratios of 5.0 and 1.5, respectively to achieve >70% conversion.

image

Figure 3. Effect of molar Co/Nd ratio on % conversion of 1,3-BD polymerization: [1,3-BD] = 3.01 M, molar DEAC/Nd ratio = 1.5, molar TEAL/Nd ratio = 5.0 at 50°C for 2 h in toluene.

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Table 2. Effect of the molar Co/Nd, TEAL/Nd, and DEAC/Nd ratios on microstructures, Mw, and MWD of the obtained BR.
Co/NdTEAL/NdDEAC/NdMicrostructures (%)Mw (×10−5)MWD
Cis−1,4Trans−1,4Vinyl−1,2
  1. Condition: [BD] = 3.01 M at 50°C for 2 h in toluene.

0.0/1.05.01.596.32.830.878.252.5
0.3/0.75.01.596.92.270.837.942.3
0.5/0.55.01.597.71.570.737.333.4
0.3/0.77.01.596.03.160.845.352.7
0.3/0.79.51.594.84.380.824.713.0
0.3/0.75.01.096.12.970.936.442.5
0.3/0.75.01.797.32.040.667.593.1
0.3/0.75.02.096.42.111.499.284.3

To consider the microstructures, Mw and MWD of the (Co/Nd)-BR as shown in Table 2, an increase in the Co content in the mixed catalyst solution provided a BR with a higher content of cis−1,4 configuration and lower amount of both trans−1,4 and vinyl−1,2 units. This implied that the Co/Nd mixed catalyst system showed some unique properties for producing a BR structure with high cis−1,4 and low vinyl−1,2 contents from Nd species and low trans−1,4 structure induced by the Co portion [1, 16]. For Mw and MWD of the (Co/Nd)-BR, an increase in the amount of Co in the mixed catalyst solution decreased the Mw of BR from 8.25 × 105 to 7.33 × 105 when the molar Co/Nd ratio increased from 0/1 to 0.5/0.5 due to the effect of the Co catalyst producing BR with high branches and low Mw [5]. However, the increase in the Co content to 0.5/0.5 of molar Co/Nd ratio increased the MWD to 3.4. This means that this critical molar Co/Nd ratio of 0.5/0.5 could not provide a narrow MWD implying the loss of stereoregularity.

Effect of TEAL Concentration

The effect of the TEAL concentration by varying the molar TEAL/Nd ratio from 3.0 to 9.5 on the % conversion, microstructures, Mw, and MWD of the obtained Co/Nd-BR gum was evaluated as shown in Fig. 4 and Table 2. From Fig. 4, using a molar TEAL/Nd ratio at 3.0 was not sufficient to promote the 1,3-BD polymerization. The increase in the amount of the molar TEAL/Nd ratio from 5.0 to 9.5 increased the conversion from 73 to ca. 82% due to the effect of TEAL acting as a scavenger for moisture and impurities possibly present in the process [17]. However, it was obvious that the high TEAL loading decreased the amount of cis−1,4 and vinyl−1,2 configurations, while the content of trans−1,4 structure of the resulting BR tended to be increased. This could be explained in that an overdose TEAL acting as an electron donor (donating alkyl group) competed with the new coming 1,3-BD monomer to coordinate with the metal catalyst using one double bond or “monodentate coordination” to form a trans2 configuration [17, 18].

image

Figure 4. Effect of molar TEAL/Nd ratio on % conversion of 1,3-BD polymerization: [1,3-BD] = 3.01 M, molar Co/Nd ratio = 0.3/7.0, molar DEAC/Nd ratio = 1.5 at 50°C for 2 h in toluene.

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From Table 2, an increase in the TEAL content in the system also decreased the Mw of the Co/Nd-BR since the TEAL was also used as a chain transfer agent for polymerization [17, 18]. For the MWD of the Co/Nd-BR, the previous literature [19] revealed that the alkyl aluminum containing hydride moiety such as diisobutyl aluminumhydride (DIBAH) had high efficiency to control the molar mass of BR than an alkyl aluminum such as triiobutylaluminum (TIBA). This implied that the use of TEAL in the Co/Nd-based catalytic system might not effectively control the MWD of BR. Thus, the MWD of the Co/Nd-BR slightly increased from 2.3 to 3.0 when the molar TEAL/Nd ratio increased from 5.0 to 9.5.

Effect of DEAC Concentration

The influence of the molar DEAC/Nd ratio on % conversion, microstructures, Mw and MWD of Co/Nd-BR was investigated by varying the molar DEAC/Nd ratio over the range of 0.5–2.0. From Fig. 5, the molar DEAC/Nd ratio at 0.5 was not sufficient to activate the 1,3-BD polymerization. The increase in the DEAC/Nd ratio from 1.0 to 1.7 enhanced the % conversion from 39.9 to 76.7%. This could be explained in that the DEAC was the chlorinating agent to transfer chloride onto Nd and Co to produce the active sites for the insertion of 1,3-BD as the cis−1,4 configuration [2, 20]. Thus, the amount of cis−1,4 units of the Co/Nd-BR slightly increased from 96.1 to 97.3% with a decreasing content of trans−1,4 and vinyl−1,2 configurations from 2.97 to 2.04% and from 0.93 to 0.66%, respectively (Table 2). However, an overdose of DEAC concentration (DEAC/Nd = 2.0) drastically decreased the level of conversion to 9.3% with lower content of cis−1,4 unit as 96.4% since the excess chlorinating agent might induce the precipitation of the catalyst to form insoluble particles of NdCl3 which diminished the catalytic activity [20, 21].

image

Figure 5. Effect of molar DEAC/Nd ratio on % conversion of 1,3-BD polymerization: [1,3-BD] = 3.01 M, molar Co/Nd ratio = 0.3/7.0, molar TEAL/Nd ratio = 5.0 at 50°C for 2 h in toluene.

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It was also observed that the Mw and MWD of (Co/Nd)-BR increased from 6.44 × 105 to 9.28 × 105 and 2.5 to 4.3, respectively, with increasing the molar DEAC/Nd ratio from 1.0 to 2.0 due to the over chlorination of catalyst. This behavior was also found in the polymerization of 1,3-BD in the presence of NdV3/tert-butyl chloride (t-BuCl)/DIBAH [22].

Effect of Monomer Concentration

The effect of 1,3-BD concentration on the % conversion and physical properties of Co/Nd-BR is presented in Table 3. When the amount of 1,3-BD increased from 2.53 to 3.31 M, the % conversion increased from 73.6 to 80.6% with increasing Mw of the obtained BR from 7.08 × 105 to 9.84 × 105. In addition, the increase in the 1,3-BD concentration to 3.31 M increased the amount of cis−1,4 configuration to 97.5%, while decreasing the content of trans−1,4 structure to 1.75%. This implies that the low 1,3-BD concentration provided the appropriate solution viscosity to promote the higher mobility of the monomer molecules favoring monodentate coordination with the metal and produced a syn complex (trans−1,4 form) with decreasing cis−1,4 configuration [23, 24]. However, the variation of the 1,3-BD concentration did not significantly affect the vinyl−1,2 content in the Co/Nd-BR (ca. 0.7–0.8%) [23].

Table 3. Effect of BD concentration on % conversion, microstructures, Mw, and MWD of (Co/Nd)-BR gum.
[BD] (M)% ConversionMicrostructures (%)Mw (×10−5)MWD
Cis−1,4Trans−1,4Vinyl−1,2
  1. Condition: molar Co/Nd ratio = 0.3/0.7, molar TEAL/Nd ratio = 5.0 and molar DEAC/Nd ratio = 1.5 at 50°C for 2 h in toluene.

2.5373.696.62.750.657.082.5
3.0173.196.92.270.837.942.3
3.3180.697.51.750.759.842.7
Effect of Polymerization Temperature

The reaction temperature strongly affected the polymerization process in terms of % conversion, Mw and MWD of Co/Nd-BR as shown in Table 4. The maximum conversion of 81.8% was obtained when the polymerization temperature was increased to 55°C. Above this point (60°C), the % conversion significantly decreased to 56.9% possibly due to catalyst deactivation. This indicated that the Co/Nd mixed catalyst in cooperation with DEAC and TEAL was easier to be thermally deteriorated than the NdV3/tert-butyl chloride (t-BuCl)/DIBAH system, which was deactivated above 80°C [23]. However, the higher 1,3-BD conversion (81.8%) promoted by the Co/Nd mixed catalyst was achieved at lower temperature (55°C) than that promoted by the NdV3/tert-butyl chloride (t-BuCl)/DIBAH system requiring a more severe polymerization temperature (>70–80°C) to reach the same % conversion over the same reaction time [23].

Table 4. Effect of polymerization temperature on % conversion, microstructures, Mw, and MWD of (Co/Nd)-BR gum.
Temperature (°C)% ConversionMicrostructures (%)Mw (×10−5)MWD
Cis−1,4Trans−1,4Vinyl−1,2
  1. Condition: [BD] = 3.01 M, molar Co/Nd ratio = 0.3/0.7, molar TEAL/Nd ratio = 5.0, and molar DEAC/Nd ratio = 1.5 for 2 h in toluene.

4566.997.72.110.198.562.5
5073.196.92.270.837.942.3
5581.897.02.430.577.532.6
6056.996.52.690.816.863.0

To focus on the microstructures of Co/Nd-BR, an increase in the reaction temperature from 45 to 60°C slightly decreased the amount of cis−1,4-configuration from 97.7 to 96.5% with increasing formation of trans−1,4 and vinyl−1,2 microstructures. This could be explained in that the higher reaction temperature promoted cis–trans isomerization to enhance both the trans−1,4 and vinyl−1,2 units at the expense of the cis-1,4 isomer [23, 25]. An increase in the reaction temperature also decreased the Mw of the Co/Nd-BR since the higher reaction temperature induced chain transfer, which led to a lower Mw with a broader MWD of the resulting BR [1, 23].

Effect of Toluene/Cyclohexane Ratio on 1,3-BD Polymerization Using Co/Nd-Based Ziegler/Natta Catalyst

The selection of solvent as the media for the 1,3-BD polymerization is also an important factor for the level of conversion including the microstructures of the resulting BR. The effect of the toluene/cyclohexane (w/w) ratio is presented in Table 5. The 1,3-BD polymerization was performed under the central condition as described above. The results indicated that the increase in the portion of cyclohexane increased the % conversion up to 90% when the cyclohexane was only applied as the media for the reaction. This indicated that the benzyl-H atom of toluene could be transferred to the allyl-end of the poly(butadiene)yl chain resulting in termination of the polymeric chain propagation [26]. However, the use of toluene, cyclohexane or a toluene/cyclohexane mixture had no significant effect on a change in the microstructures of the obtained BR product (cis−1,4 = 96–97%, trans−1,4 = 2–3% and vinyl−1,2 = 0.8–0.9%). This could be explained in that the solubility parameter (∂) of 1,3-BD (17.6) was quite similar for both toluene (18.3) and cyclohexane (16.8) [27]. The use of cyclohexane as the solvent also promoted a higher Mw and MWD of the (Co/Nd)-BR to 11.5 × 105 and 3.30, respectively. This observation for the Mw of the resulting BR was different from the previous literature using NdV3/t-BuCl/DIBAH as the catalyst for the 1,3-BD polymerization providing a low Mw of 2.23 × 105 due to the effect of chain transfer promoted by cyclohexane [27]. Thus, the existence of Co in the Co/Nd mixed catalyst possibly retarded the chain transferring effect when 1,3-BD polymerization was performed in cyclohexane.

Table 5. Effect of toluene/cyclohexane ratio (w/w) on % conversion, microstructures, Mw, and MWD of (Co/Nd)-BR gum.
Toluene/cyclohexane% ConversionMicrostructures (%)Mw (×10−5)MWD
Cis−1,4Trans−1,4Vinyl−1,2
  1. Condition: [BD] = 3.01 M, molar Co/Nd ratio = 0.3/0.7, molar TEAL/Nd ratio = 5.0, molar DEAC/Nd ratio = 1.5 at 50°C for 2 h.

1/073.196.92.270.877.942.3
7/381.497.41.840.769.053.1
5/581.696.12.930.977.972.7
3/791.696.23.210.598.392.5
0/190.096.62.630.7711.53.3

Cure Characteristics and Mechanical Properties of BR

Table 6 shows a comparison of cure characteristics and mechanical properties of Co/Nd-BR with the commercial BRs: Nd-BR and Co-BR. The results showed that the Mooney viscosity of Co/Nd-BR gum (42.4) was similar to that of Nd-BR (42.9) and Co-BR (41.1). Comparing the ML and MH, there was no significant difference between Co/Nd-BR and commercial BRs. However, the MH values of Nd-BR and Co/Nd-BR were slightly higher than that of Co-BR reflecting the higher hardness of the Nd-BR and Co/Nd-BR vulcanizates. Possibly, the polymerization using the Nd-catalyst resulted in a BR with lower vinyl content and more linearity in its chemical structure [1, 4]. This also yielded a higher Mooney viscosity of the compounded BR. Moreover, the scorch time (ts1) and optimum cure time (tc90) of all BRs were similar in the range 2.7–3.5 min and 9.6–10.6 min, respectively.

Table 6. Cure characteristics and mechanical properties of BRs.
PropertiesCo-BRNd-BRCo/Nd-BR
Mooney viscosity (ML1+4@100°C)
BR gum41.142.942.4
Compounded BR66.477.483.9
Curing properties at 160°C
ML (dN m)3.363.293.11
MH (dN m)18.320.220.1
ts1 (min)3.182.753.43
tc90 (min)10.69.6310.4
Mechanical properties
Tensile strength (MPa)15.218.819.2
Elongation at break (%)400440470
Hardness (cure @145°C) (IRHD)626666
Rebound (%)545760
Abrasion (%wt loss)8.87.98.3

With regards to the mechanical properties, the Co/Nd-BR had the highest tensile strength (19.2 MPa) and ultimate elongation (470%). The improvement might be due to the influence of the Nd species in the mixed Co/Nd catalyst to provide the high cis−1,4 with low vinyl−1,2 configuration and a high linear structure compared with the Co species producing BR with high vinyl−1,2 content and long chain branching [4]. Moreover, the Co/Nd-BR also exhibited a high rebound directly relating the low heat build-up and good abrasion properties as that of the commercial Nd-BR.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

BR having high content of cis−1,4 configuration (>97%) with a low amount of vinyl−1,2 structure (<1%) could be produced using a Co/Nd/DEAC/TEAL catalytic system in the presence of toluene/cyclohexane mixture (7/3 [w/w]). When the 1,3-BD polymerization proceeded at 50°C and 2 h, the optimized catalyst composition was between 0.3/0.7 and 0.4/0.6 of the molar Co/Nd ratio where the molar DEAC/Nd and TEAL/Nd ratios was 1.5 and 5.0, respectively. The Co/Nd mixed catalyst also possibly retarded the chain transfer effect for 1,3-BD polymerization in the presence of cyclohexane. The mechanical performance such as tensile, rebound and abrasion properties of Co/Nd-BR was as good as that of the commercial Nd-BR.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES