Available online at www.sciencedirect.com EUROPEAN POLYMER JOURNAL European Polymer Journal 43 (2007) 4706–4711 www.elsevier.com/locate/europolj Synthesis, spectroscopic characterization and vulcanization activity of a new compound containing the anion bis(4-methylphenylsulfonyldithiocarbimato)zincate(II) Roberta M. Mariano a, Marcelo R.L. Oliveira b, Mayura M.M. Rubinger b, Leila L.Y. Visconte a,* a Instituto de Macromole´culas Professora Eloisa Mano/Universidade Federal do Rio de Janeiro, P.O. Box 68525, Rio de Janeiro, RJ, Brazil b Departamento de Quı´mica, Universidade Federal de Vic¸osa, Vic¸osa MG, CEP 36570-000, Brazil Received 20 April 2007; received in revised form 6 July 2007; accepted 28 August 2007 Available online 2 September 2007 Abstract A new compound of the formula: (Bu4N)2[Zn(4-CH3C6H4SO2N@CS2)2] (Bu4N = tetrabutylammonium cation) was obtained by the reaction of CH3C6H4SO2N@CS2K2 Æ 2H2O with zinc(II) acetate dihydrate and tetrabutylammonium bromide in dimethylformamide. The elemental analyses of C, H, N and Zn are consistent with the proposed formula. The IR data point to the formation of a zinc(II)–bis(dithiocarbimato) complex. The 1H NMR and 13C NMR spectra showed the expected signals for the tetrabutylammonium cation and the dithiocarbimato moiety. This compound was evaluated as for the vulcanization activity and compared to the commercial accelerators CBS, MBTS and TMTD. These studies showed that the zinc(II)–ditiocarbimates are a new class of rubber vulcanization accelerators, worth of further investigation. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Dithiocarbimates; Zinc complexes; Vulcanization 1. Introduction Zinc(II)–bis(dithiocarbamato) complexes are used worldwide in rubber vulcanization [1–6] and are known as ultra-accelerators due to their extremely rapid acceleration properties [2]. Vulcanizing mixtures containing zinc(II)–tris(dithiocarbamato) complexes of the general formula: [Zn(R2NCS2)3] were found to be more accelerated than mixtures * Corresponding author. Tel.: +55 21 2562 7204; fax: +55 21 2270 1317. E-mail address: [email protected] (L.L.Y. Visconte). containing zinc(II)–bis(dithiocarbamato) complexes of the general formula [Zn(R2NCS2)2] [2,6]. The anionic species are more soluble in organic solvents [6] and stronger nucleophiles than the parent bis(dithiocarbamato) neutral complexes [2]. These facts can be responsible for their greater activity. Our interest in the research on dithiocarbimatometal-complexes is based on the similarities these compounds show when compared with the dithiocarbamate complexes, mainly as long as the presence of the Zn(S2C@NR)2 moiety in the former structures is concerned. However, small differences between these structures can be very important. 0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.08.013 R.M. Mariano et al. / European Polymer Journal 43 (2007) 4706–4711 4707 CH3C6H4SO2N@CS2K2 Æ 2H2O was prepared in dimethylformamide from the 4-toluenessulfonamide, CS2 and KOH, as described in the literature [7]. Microanalyses for C, H and N were obtained from a Perkin-Elmer 2400 CHN elemental analyzer. Zinc was analysed by atomic absorption with a Hitachi Z-8200 Atomic Absorption Spectrophotometer. The IR spectra were recorded with a Perkin-Elmer 283 B infrared spectrophotometer using CsI pellets. The 1H (200 MHz) and 13C (50 MHz) NMR spectra were recorded with a Bruker DPX200-AVANCE spectrophotometer in CDCl3 with TMS as internal standard. For example, the zinc(II)-bis(dithiocarbimato) complexes are, necessarily, anionic species. So, the improvement and/or modulation of the vulcanization activity is an interesting possibility, which can be accomplished either by modifying the solubility of the complexes salts with the use of different cations or different R groups on the dithiocarbimate structures, or by using active counter ions. In this work the vulcanization efficiency of a new compound containing the anionic complex bis(4methylphenylsulfonildithiocarbimato)zincate(II) with the formula (Bu4N)2[Zn(4-CH3C6H4SO2N@ CS2)2] (Bu4N = tetrabuthylammonium cation), ZNIBU, was investigated. This compound was obtained by the reaction of Zn(CH3COO)2 Æ 2H2O with potassium 4-methylphenylsulfonyldithiocarbimate dihydrate and tetrabutylammonium bromide and was characterized by IR, 1H and 13C NMR spectroscopies, and C, H, N and Zn elemental analyses. Standard compositions of natural rubber (NR), unfilled (gum) or filled with carbon black were studied as for their rheometric parameters. For comparison purposes, the commercial accelerators CBS (N-cyclohexylbenzothiazole sulfenamide), MBTS (dibenzothiazole disulfide) and TMTD (tetramethylthiuram disulfide) were also used. 2.2. Synthesis The synthesis was performed according to Fig. 1. Zinc(II) acetate dihydrate (74.41 g, 0.34 mol) and tetrabutylammonium bromide (219.83 g, 0.68 mol) were added to a solution of potassium 4-methylphenylsulfonyldithiocarbimate dihydrate (244.77 g, 0.68 mol) in methanol (300 mL) and water (900 mL). The mixture was stirred at room temperature for 1 h and the yellowish solid product was filtered, washed with distilled water (4 L), ethanol (300 mL) and diethyl ether (150 mL) and dried under reduced pressure, yielding (Bu4N)2[Zn(4CH3C6H4SO2N@CS2)2] (327.44 g, 0.31 mol). Tetrabutylammonium bis(4-methylphenylsulfonyldithiocarbimato)zincate(II), (Bu4N)2[Zn(4-CH3C6H4SO2N@CS2)2]: Elemental analyses: Found (calculated for C48H86N4O4S6Zn): C, 55.24 (55.38); H, 8.06 (8.33); N, 5.44 (5.38); Zn, 6.20 (6.28) %. M.p. (°C): 161.8-162.2. IR (most intense bands) (cm 1): 1372 m(C@N); 1266 mass(SO2); 1136 msym(SO2); 939 mass(CS2) and 312 m(ZnS). 1H NMR (d) J(Hz): 7.83 (d, 4H, J = 8.2, H2 and H6); 7.17 2. Experimental 2.1. Methods and materials The solvents were purchased from Merck and used without further purification. The chemicals sulfonamide, zinc acetate dihydrate and tetrabutylammonium bromide were purchased from Aldrich. Carbon disulfide and potassium hydroxide were purchased from Vetec. The compound 4- 2 p-CH3C6H4SO2N=CS2K2.2H2O + Zn(CH3COO)2.2H2O + 2 Bu4NBr CH3OH - CH3COOK - KBr H 2O O (Bu4N)2 H3C S O N C S S Zn O S S C N S O ZNIBU Fig. 1. Scheme for ZNIBU preparation. CH3 4708 R.M. Mariano et al. / European Polymer Journal 43 (2007) 4706–4711 (d, 4H, J = 8.1, H3 and H5); 2.34 (s, 6H, CH3); Tetrabutylammonium cation: 3.24, (m, 16H, CH2N); 1.60–1.50 (m, 16 H, CH2); 1.46–1.26 (m, 16 H, CH2); 0.95 (t, 24 H, J = 7.0, CH3). 13C NMR (d): 208.50 (N@CS2); 141.00 (C1); 127.72 (C2 and C6); 128.32 (C3 and C5); 140.14 (C4); 21.41 (CH3); Tetrabutylammonium cation signals: 58.60 (CH2N); 23.93 (CH2); 19.67 (CH2); 13.73 (CH3). 2.3. Vulcanization Natural rubber was compounded following the formulation (in phr): natural rubber, NR (100), stearic acid (2.5), zinc oxide (3.5), sulfur (2.5), carbon black (0 or 20), aminox (2.0), accelerator (0.8 or 1.2). Natural rubber, Mooney viscosity ML 1 + 4(100 °C) = 74.8, was supplied by Sociedade Michelin de Participac¸o˜es, Indu´stria e Come´rcio Ltda; the other ingredients were of grades commonly used in the industry. The mixture was carried out in a roll mill with 1:1.25 friccion rate, at room temperature. Rheometric parameters were determined in an oscillating disk rheometer from Tecnologia Industrial, at 150 °C and 1° arc, according to ASTM D 2084. The mixes were vulcanized and molded into sheets in a hydraulic press operating at 150 °C and 3.0 MPa. From the sheets, specimens for stress strength (ASTM D412) and tear resistance (ASTM D 624) tests were cut. 3. Results and discussion The compound here studied is quite stable at the ambient conditions. It is slightly soluble in water, methanol and ethanol, and is soluble in chloroform and dichloromethane. A strong band at 1372 cm 1 observed in its vibrational spectrum was assigned to the mC@N vibration. The massCS2 was observed at higher frequency in the spectrum of the parent potassium salt of dithiocarbimate (955 cm 1) [8] than in the spectrum of the tetrabutylammonium bis(4-methylphenylsulfonyldithiocarbimato)zincate(II) (939 cm 1), confirming the complexation [9]. The spectrum of the title compound also shows the expected medium band in the 312 cm 1, assigned to the Zn–S stretching vibration, indicating a bidentade coordination by two sulfur atoms of the dithiocarbimato ligand [10,11]. The 1H NMR spectrum showed all the signals for the hydrogen atoms of the tetrabutylammonium cation. The remaining signals could be assigned to the CH3 group of the aromatic moiety and to the aromatic hydrogen atoms. The integration curves on the 1H NMR spectrum were consistent with a 2:1 proportion between the tetrabutylammonium cation and the complex anion. The 13C NMR spectrum showed the six expected signals due to the dithiocarbimato moiety. The N@CS2 (C1) signal is shifted in the spectrum of the compound to higher field if Table 1 Rheometric parameters for NR mixes compounded with ZNIBU, CBS, MBTS or TMTD as the accelerator Accelerator Accelerator amount (phr) ZNIBU 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 CBS MBTS TMTD Carbon black (phr) 0 20 0 20 0 20 0 20 Minimum torque, Ml (dN m) Maximum torque, Mh (dN m) (Mh– Ml) t90 (min) 4.85 5.53 6.66 5.31 2.82 2.93 4.29 6.21 3.39 6.00 6.10 6.32 3.05 4.63 5.31 6.55 25.29 25.06 31.72 32.06 25.50 27.09 30.60 34.00 21.34 23.03 26.42 27.54 25.49 27.55 29.12 34.40 20.44 19.53 25.06 26.75 22.68 24.16 26.31 27.79 17.95 17.03 20.32 21.22 22.44 22.92 23.81 27.85 35.4 30.0 29.4 21.0 11.6 8.7 11.4 8.3 12.0 7.6 12,0 10.2 4.8 3.6 4.4 3.8 t90: optimim vulcanization time; CBS: N-cyclohexyl-2-benzothiazol-2-sulfenamide; MBTS: benzothiazol disulfide; TMTD: tetramethylthiuram disulfide. R.M. Mariano et al. / European Polymer Journal 43 (2007) 4706–4711 compared to the spectrum of the ligand (225.19 d) [8] due to the complexation by two sulfur atoms. The spectroscopic data for the compound here studied are very similar to those observed for the analogous compound tetraphenylphosphonium bis(4-methylphenylsulfonyldithiocarbimato) zincate(II) [9] and tetraphenylphosphonium bis(methylsulfonyldithiocarbimato)zincate(II) [10] that have a distorted tetrahedral configuration around the zinc(II) cation due to the bidentate chelation by two sulfur atoms of the dithiocarbimato ligands. The efficiency of ZNIBU as an accelerator for natural rubber vulcanization was investigated by rheometry. Table 1 shows data from rheometric curves obtained for ZNIBU as well as for CBS, MBTS and TMTD as accelerators. These three 4709 compounds were chosen because they pertain to different classes of accelerators and therefore they will have different influences on the rubber towards vulcanization [12–14]. Consequently, characteristics such as cure rate, safety scorch, number and type of the crosslinks formed will be dictated by the choice of the accelerator. From data of t90 in Table 1 it can be seen that vulcanization with ZNIBU proceeds at slower rates than with any of the other compounds. An acceleration is accomplished by increasing the amount of ZNIBU, specially in the presence of carbon black. In all cases, the addition of this filler has a positive effect on (Mh–Ml), which is related to the degree of crosslinking, particularly when using ZNIBU as the accelerator. For this compound, the values of Fig. 2. Rheometric curves for NR compounded with 0.8 phr of different accelerators 1: ZNIBU; 2: CBS; 3: MBTS; 4: TMTD. Fig. 3. Rheometric curves for NR compounded with ZNIBU. 1: 0.8 phr ZNIBU; unfilled 2: 0.8 phr ZNIBU; filled 3: 1.2 phr ZNIBU; unfilled 4: 1.2 phr ZNIBU; filled. 4710 R.M. Mariano et al. / European Polymer Journal 43 (2007) 4706–4711 (Mh–Ml) are almost of the same magnitude as those obtained for CBS, one of the most used accelerators for NR. The rheometric curves recorded during vulcanization of the compounds in the presence of 0.8 phr of accelerator are shown in Fig. 2. These curves show the reversion tendency of NR under continuous heating. In the presence of any of the commercial accelerators, torque reaches its max- a CBS TMTD ZNIBU MBTS Stress strength (MPa) 25 20 15 10 5 0 0 200 400 600 800 Elongation (%) Stress strength (MPa) b 25 20 15 10 5 0 0 100 200 300 400 500 600 700 800 Elongation (%) Stress strength (MPa) c 25 20 Table 2 Tear resistance of NR compounds vulcanized with different accelerators 15 10 5 100 200 300 400 500 600 700 800 Elongation (%) Stress strength (MPa) Accelerator Accelerator amount (phr) ZNIBU 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0.8 1.2 0 0 d imun value and then starts to decrease. This is a well known NR characteristic and is the result of degradation brought about by oxidation of the remaining double bonds in the rubber main chain. Interestingly this seems not happen in the presence of ZNIBU, during the duration of the experiment. Upon incorporation of a larger amount of accelerator, 1.2 phr, the same trend is observed in the unfilled compounds (curves 1 and 3 in Fig. 3), and still no reversion tendency is observed. This begins to show up at 1.2 phr of ZNIBU in the filled composite. Fig. 4 shows data of stress strength for the compounds in the presence of the different accelerators. From the curves in Fig. 3a it can be seen that ZNIBU and CBS have similar behavior and present the highest results. In the presence of carbon black (Fig. 3b) these two accelerators give rise to compounds which still behave similarly. An increment in these accelerators content for the unfilled compounds (Fig. 3c) leads to a slight decrease in elongation, a small increase in 300% modulus but the stress resistance remains practically unchanged. In the presence of carbon black, ZNIBU, CBS and MBTS give results which are very close to each other. However, considering ZNIBU alone, the addition of this filler did not bring any improvement in the property. For the unfilled compounds vulcanized with 0.8 phr of accelerator, shown in Table 2, the highest values of tear resistance are given by ZNIBU and CBS. On increasing the amount of these two 25 CBS 20 15 10 MBTS 5 0 0 100 200 300 400 500 600 700 800 Elongation (%) Fig. 4. Stress resistance of the filled and unfilled compounds vulcanized with different amounts of accelerators: (a) 0.8 phr, unfilled; (b) 0.8 phr, filled; (c) 1.2 phr, unfilled; (d) 1.2 phr, filled. TMTD Carbon black (phr) 0 20 0 20 0 20 0 20 Tear resistance (kN/m) 34.8 28.0 36.7 37.1 34.7 33.9 39.1 40.0 30.9 33.1 33.7 34.4 30.2 31.2 38.4 34.6 R.M. Mariano et al. / European Polymer Journal 43 (2007) 4706–4711 accelerators, the property worsens in opposition to what happens with the other compounds. The positive effect of carbon black is seen in all cases and concerning this property, ZNIBU behaves very much in the same way as the commercial accelerators. 4. Conclusion A new salt of bis(4-methylphenylsulfonyldithiocarbimato)zincate(II) complex was prepared (see Fig. 1). The tetrabutylammonium salt was isolated and characterized by IR, 1H, and 13C NMR and by elemental analyses. The addition of the complex to a standard natural rubber formulation has shown that it behaves as a slow accelerator for vulcanization. Nevertheless, as far as mechanical properties, stress and tear resistances, are concerned, NR compounds formulated with ZNIBU give values which are of the same magnitude as those found for NR compounds vulcanized in the presence of three of the most used commercial accelerators, CBS, MBTS and TMTD. By increasing the amount of ZNIBU from 0.8 to 1.2 phr, stress strength of the unfilled compounds did no change and decreased in the presence of carbon black. Concerning the optimum vulcanization time, t90, larger ZNIBU contents lead to reduced values, but still too long when compared to the commercial accelerators. ZNIBU is the first example of a novel class of rubber vulcanization accelerators. Analogous compounds are being prepared to extend the studies on their vulcanization activities. Acknowledgement This work has been supported by the brazilian research council, CNPq. References [1] Coucouvanis D. The chemistry of the dithioacid and 1,1dithiolate complexes. Prog Inorg Chem 1969;11:233. 4711 [2] Nieuwenhuizen PJ, Reedijk J, van Duin M, McGill WJ. Thiuram- and dithiocarbamate-accelerated sulfur vulcanization from the chemist’s perspective; methods, materials and mechanisms reviewed. Rubber Chem Technol 1997;70:368. [3] Hogarth G. Transition metal dithiocarbamates: 1978–2003. Prog Inorg Chem 2005;53:71. [4] Nieuwenhuizen JP, Timal S, Haasnoot JG, Spek AL, Reedijk J. Zinc(II)-catalyzed disproportionation in rubber: The mechanism of sulfur vulcanization revisited. Chem Eur J 1997;3:1846. [5] Nieuwenhuizen PJ, Ehlers AW, Haasnoot JG, Janse SR, Reedijk J, Baerends EJ. The mechanism of zinc(II)-dithiocarbamate-accelerated vulcanization uncovered; theoretical and experimental evidence. J Am Chem Soc 1999;121:163. [6] MaCleverty JA, Spencer N, Bailey NA, Shackleton SL. Aspects of the inorganic chemistry of rubber vulcanization Part 1. Reactions of zinc bis(dithiocarbamates) and bis(benzothiazole-2-thiolates) with carboxylates, and the structure of [NBun 4 ] [{Zn(S2-CNMe2)2}2 (l-OCOMe)]. J Chem Soc Dalton Trans 1980;10:1939. [7] Hartke K. Darstellung von sulfonylisothiocyanaten. Arch Pharm 1966;299:164. [8] Oliveira MRL, De Bellis VM. Preparation of novel cobalt(III) complexes with dithiocarbimates derived from sulfonamides. Trans Met Chem 1999;24:127. [9] Perpe´tuo GJ, Oliveira MRL, Janczak J, Vieira HP, Amaral FF, De Bellis VM. Syntheses, crystal structure and spectroscopic characterization of novel N–R-sulfonyldithiocarbimate zinc(II) complexes. Polyhedron 2003;22:3355. [10] Oliveira MRL, Perpe´tuo GJ, Janczak J, Rubinger MMM. Synthesis, structural and spectroscopic characterization of novel zinc(II) complexes with N-methylsulfonyldithiocarbimato and N-methylsulfonyltrithiocarbimato ligands. Polyhedron 2007;26:163. [11] Nakamoto K. Infrared and Raman of inorganic and coordination compounds. New York: John Wiley & Sons, Inc.; 1978. p. 339. [12] Travas-Sejdic J, Jelencic J, Bravar M, Fro¨be Z. Characterization of the natural rubber vulcanizates obtained by different accelerators. Eur Polym J 1996;32(12):1395. [13] da Costa HM, Visconte LLY, Nunes RCR, Furtado CG. Aspectos histo´ricos da vulcanizac¸a˜o. Polı´meros: Cieˆncia e Tecnologia 2003;13(2):125. [14] da Costa HM, Visconte LLY, Nunes RCR, Furtado CG. Rice husk ash filled natural rubber compounds III. The role of metal oxides on the kinetics of sulfur vulcanization. J Appl Polym Sci 2003;90:1519.
© Copyright 2024