Zinc Chelates as Activators for Sulphur Vulcanization of Natural Rubber

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1 Zinc Chelates as Activators for Sulphur Vulcanization of Natural Rubber Vidya V.G 1, Sadasivan V 2 & Mini S 3 1 epartment of Chemistry, University College, Trivandrum , Kerala, India 2, 3 epartment of Chemistry, University College, Trivandrum , Kerala, India Abstract: Rubber articles are usually vulcanized using conventional accelerators like zinc oxide and tetra methyl thiuram disulphide. These materials are carcinogenic and zinc oxide is found to be dangerous for the environment also. In search of finding out a new non toxic material for activator in rubber vulcanization we have synthesized four new zinc complexes using different azodyes and Schiff base and analysed the vulcanization of rubber using these zinc chelates instead of conventional activators. The curing and characterization of the mixes were done using rheometer and the results indicates the cure characteristics depend on the availability of the Zn 2+ ions from the chelates. The mechanical properties of the vulcanizates before and after ageing were also determined. The results of the study indicate that by making better structural changes the chelates can be used as activators. Key words: Vulcanization; Zinc chelates; activators Introduction Zinc is a ubiquitous component of rubber compounds cured by accelerated sulphur vulcanisation. About one lakh tonnes of ZnO, together with smaller amounts of other zinc chemicals are consumed annually by rubber industry which is roughly 40% of total ZnO consumption. ZnO primarily acts as an activator in sulphur vulcanization [1]. Although zinc is considered as one of the least harmful of heavy metal, it s also one of the most widespread. It occurs naturally in environment and is essential for humans and for plant nutrition. However there are some aquatic species that are very sensible to zinc ions and some soluble zinc compounds have been classified as toxic to aquatic species. Since 29, April 2004, European Union has classified ZnO as dangerous for the environment and has legislated that its application in rubber technology be reduced and controlled [2]. The trend in rubber industry is therefore to reduce the zinc content, although to date no viable alternative for ZnO has been found. Technologies considered for reducing zinc levels include zinc oxide loading rationalization, the use of alternate zinc carboxylates [3,4] activators based on other metal centres [5,6,7] or new activators which either do not include heavy metals or use them sparingly[8,9]. In general these strategies have at best shown no clear advantage over conventional zinc activation or physical properties inherent to zinc stearate activated cure systems. Recently the Zn 2+ ions with accelerators are believed to be more reactive than the free accelerator and allow sulphur insertion to occur more rapidly [10]. The presence of ZnO also leads to a small increase in scorch and cure time [2]. ZnO can also be used as a cross linking agent, example of carboxylic rubbers [11,12] and elastomers containing halogen groups [13]. Moreover ZnO is sometimes applied as filler in elastomers [14]. There is also evident that ZnO has significant influence on physical properties of vulcanizates such as heat build up and abrasion resistance. The present study expands upon earlier work through a complete analysis of how sulphur cross link process is influenced in presence of zinc chelates for Indian Standard Natural Rubber (ISNR- 5). It is believed that zinc chelates can be dispersed more easily in elastomer matrix and that accessibility of zinc ions is higher. 2. Experimental All reagents and solvents for synthesis and analysis were Merck products and used as supplied. The natural rubber (ISNR-5, Mooney viscosity ML( o C-82) was supplied by Rubber Research Institute of India, Kottayam. The compounding ingradients, ZnO, stearic acid, naphthenic oil, TQ, HAF, TBBS and sulfur were commercial grade. 2.1 Synthesis of Ligands The ligands L 1, L 2 and L 3 are prepared by standard procedure of diazotization [15]. L 1 is synthesized by diazotising 4-aminoantipyrine and coupling with uracil [16]. L 2 by diazotising 2-amino-3- hydroxypyridine and coupling with 2-naphthol [17], and L 3 by diazotising 2-amino-5- chlorobenzophenone and coupling with resorcinol [18]. L 4 is prepared by refluxing an ethanolic solution of furfuraldehyde and 2-amino-5- chlorobenzophenone for 3-4 hrs [19]. Page 720

2 2.2. Preparation of complexes Zinc acetate and ligands were dissolved separately in methanol and the mixture was heated to boil for 5-7 hours forming a precipitate of zinc chelates viz, ZnL 1, ZnL 2, ZnL 3, ZnL Characterisation of complexes Based on the elemental analysis, metal estimation and mass spectra complexes are given stoichiometry as given in Table 1 Table 1 Compound Mol formula Mo Wt M% ZnL 1 [Zn(C 15 N 6 O 3 H 22 ) 2 (CH 3 COO) 2 ] ZnL 2 [Zn(C 15 N 3 O 2 H 11 ) 2 ] ZnL 3 [Zn(C 19 H 13 N 2 O 3 Cl) 2 (H 2 O) 2 ] ZnL 4 [Zn(C 18 H 12 N 2 ClO 2 ) 2 H 2 O] (CH 3 COO) Preparation and characterization of rubber compounds Rubber compounds with formulations given in Table 2 were prepared using an internal mixer Kointermix MK3 Francis shaw & Co. (Manchester) Ltd. The internal mixer prepare master batch and the compound is prepared in two roll mill. The samples were cured at 150oC until they developed a 90% increase of torque (measured using moving ie Rheometer). The tensile properties of vulcanizates were determined according to part 1- with a universal machine ZWICK The modulus 300% properties and elongation at break % of vulcanizates were determined according to the same method mentioned above. The hardness of vulcanizates measured as per standards using a shore A type urometer. Physical properties of the mixers studied and compared before and after ageing. Table 2. Composition of ISNR based rubber compounds Compound Control ISNR ZnO ZnL ZnL ZnL ZnL Stearic acid TQ HAF Naphthenic oil TBBS Sulphur ISNR 5-Indian standard Natural Rubber, TQ- Polymerized 2,2.4-trimethyl-1.2- dihydroquinoline, HAF High Abrasion Furnace(Carbon Black), TBBS - N-t-butyl benzothiazole-2-sulfenamid 5. Results and iscussion The efficiency of zinc chelates as compared to the conventional ZnO/stearic acid activator system studied on ISNR-5. The influence of zinc chelates on cure characteristics of rubber compounds was estimated based on rheometer measurements. The optimum cure time of ISNR-5 with different activators are done and is given in Table 3.Cure rate depends on the availability of zinc ions in its chelates. If zinc ion in the cure system is less, cure rate will be smaller. Among the zinc chelates studied ZnL 2 shows good and ZnL 3 shows least cure characteristics. Increase in cure rate may be due to better availability of zinc ions. Zinc ions trapped in crystal system are not used for cross linking due to their limited accessibility. Based on ZnO lattice parameters [20] the amount of Zinc ions available in a single crystal is estimated to be 1.8 mmole/g of ZnO. Therefore the calculated amount Zn 2+ available for interactions with sulphur and accelerator should be more than 1.8 mmole/g to function it as a better activator than ZnO. Moreover Zn 2+ ions are strongly connected to the crystal structure in order ZnL 3 > ZnL 4 > ZnL 1 > ZnL 2 and are less likely to form intermediate active sulfurating agents that can be observed during vulcanization. For vulcanization rate the effective activators comparatively seems to be ZnL 2 > ZnL 1 > ZnL 4 > ZnL 3. The lower efficiencies of ZnL 3 and ZnL 4 may be due to high stability of these complexes. In case of ZnL 3 and ZnL 4 the sterric hindrance of benzophenone group, which reduces the availability of Zinc ions to form the zinc-accelerator complex, may also cause lower efficiency of these complexes in vulcanization. According to Heideman etal [21] different reactivities of zinc complexes result from their different basicity. The alkaline character of activator accelerates sulphur vulcanization of rubber. ZnL 2 and ZnL 1 possess alkaline character to show enhanced activity as compared to ZnL 4 and ZnL 3. However considering the structure and acid-base properties of zinc chelates, we conclude that the main reasons for their different reactivity in cross linking process are differences in complex stability and availability of zinc ions in zinc complex molecule. The lower torque that was observed for compounds with zinc chelates had no deterimental effect on physical properties of obtained vulcanizates. Cure characteristics of ZnL 1, ZnL 2, ZnL 3, ZnL 4 and with standard ZnO as control are given in figure 1&2 Page 721

3 of stress in cross linked sample can be more effective as a result of stress relaxation. The propagation of microcracks within these areas could be hindered and microcracks could rise only under higher stresses and deformations. As a result materials materials tensile strength and elongation at break increases. It can be concluded that elastomer network obtained using Zinc chelates are more heterogeneous than those formed with ZnO as the activator. The ZnL 1 and ZnL 2 shows more homogeneous character compared to ZnL 3 and ZnL 4.The tensile strength of the zinc chelates and ZnO activator as control are given in Table.3 Table 3.Processing and physical properties of mixes before ageing Figure: 1 Figure Cross linking density of vulcanizates and the distribution of cross links in elastomer network Zinc chelates as activators in sulphur vulcanization of ISNR had a detrimental effect on the cross linking density. From the cure characteristic property of different Zinc chelates, its found that ZnL 1 and ZnL 2 are effective than ZnL 3 and ZnL 4. The less cure characteristics property of ZnL 3 and ZnL 4 can be due to decrease in cross linking density. Only ZnL 1 and ZnL 2 chelates allowed to achieve cross linking efficiency comparable to ZnO. Apart from cross linking density, the distribution of cross links and presence of defects in the elastomer network have a great importance as far as the mechanical properties of vulcanizates are concerned [22]. Not only the number of cross links, but also their distribution and spatial arrangements contribute to network mechanical performance. From a mechanical point of view the heterogeneous distribution of cross links in the elastomer network is supposed to have a detrimental effect on mechanical properties of vulcanizates. ensely cross linked domains of the network concentrate stresses under external strain and initiate breaking of sample. Therefore vulcanizates with homogeneous structure exhibit higher tensile strength [23]. Vilgis and Henrich [24] Postulated that heteroginity of the network strongly affects the vulcanizates behavior under high strain and therefore such mechanical properties as tensile strength and elongation at break. In less cross linked part of network the distribution Test parameter Tensile strength Modulus 300% Elongation at break, % Hardness, Shore A Method Control Having established the effect of zinc chelates on the vulcanization of rubber compounds, we examine the mechanical properties of vulcanizates. Results are given in Table.4. Table 4. Processing and physical properties of mixes after ageing Test parameter Optimum cure time t 90 at 150 o C t 90, (m:s) Tensile strength Modulus 300% Elongation at break, % Hardness, Shore A Method Contr ol :55 6:21 5:11 4:01 12: It follows that applying zinc chelates have no considerable effect on tensile strength of vulcanizates compared to using conventional ZnO. The stress at relative elongation of 300% also decreased which may be a result of cross links density decrease. Vulcanizates containing zinc chelates have lower Page 722

4 elongation at break corresponding to their lower elasticity. Tensile strength of ZnL 2 is comparable to control. This zinc chelates contain higher Zn 2+ ion content and have the best availability of these ion as well. The explanation for deterioration of mechanical properties for ZnL 3 and ZnL 4 may be due to difficult availability of zinc ions in chelates due to sterric hindrance. Low tensile strength and elongation at break could be a result of network homogeneity. This observation confirms that elastomer network heterogeneity improves the physical property of vulcanizates. The dispersion of zinc chelates in elastomer matrix has a great importance as far as activation of sulphur vulcanization is concerned. For the Zinc chelates to be more effective the particles need to be distributed in elastomer homogenously and surface energy of zinc chelates minimized to prevent agglomeration in elastomer. 6. Conclusion In order to find routes to reduce ZnO levels in rubber compounds, this paper describes an attempt to study the substitution of conventional activator ZnO by zinc chelates. Real rubber and model compound systems are employed to investigate the effect of zinc complexes on vulcanization process. By means of model compound vulcanizations information about the alkaline character, sterric hindrance in chelate structure, cross linking nature, nature of surface, dispersion property of zinc chelates in elastomer are studied. From the cure characteristics and various obtained data its found that ZnL 1 and ZnL 2 on further structural modification its activity can be enhanced to the desired level. The differences in activity of ZnL 1, ZnL 2, ZnL 3 and ZnL 4 complexes may be related to the stability of chelates. The complexes with stronger coordination bonds may have lower tendency to form intermediates, active sulfurating vulcanization compounds: Zinc accelerator complexes. Furthermore, the activity of zinc complexes can be affected by basicity of ligands or sterric hindrance caused by substituents. It is anticipated that a significant reduction of zinc content can be achieved by optimization of the zinc complex levels. Research concerning the influence of the ligands in the zinc complex is considered worthwhile to possibly achieve even better systems. Acknowledgements We are thankful to the epartment of Chemistry, University College, Trivandrum for instrumental facilities under FIST ST and STIC, Cochin University and Technology for analytical facilities. We are also thankful to Rubber Institute of India for providing us necessary instrumentation facilities. References 1. Li Z.H; Zhang J; Chen S.J; Effects of carbon blacks with various structures on vulcanization of filled ethylene propylene-diene rubber, Express Polym. Lett., 2008 ; 2 : Chapman A; Johnson;. The role of zinc in the vulacanisation of styrene-butadiene rubbers. KautshukGummiKunstst., 2005 ; 58 : Heideman G; atta R.N; Noordermeer J.W.M; Van Baarle B. Influence of zinc oxide during different stages of sulfur vulcanization,.rubber. Chem. Technol, 2005 ; 78 : Struktol rubber hand book, Activators online edition. 5. Rakhmatullina A.P; Akhmed R.A; Yanova A.G; Liakumovich A.G; Portnoi T.B; Mokhnatkina E.G; Yasov R.S;.Int.Polm. Sci.Tecnol ; 2004 ; 31(12) : Heideman G; atta R.N; Noordermeer J.W; Van Baarle ;. Tire Technology Int; 2006 ; Versloot P; Rubber chem. Tecnol, 1994 ; 67 : Heideman G; atta R..N; Noordermeer J.W.; Van Baarle B; Influence of zinc oxide during different stages of sulfur vulcanization.rubber. Chem. Technol, 2004 ; 77 : Heideman G; atta R.N; Noordermeer J.W.M; Van Baarle B; Influence of zinc oxide during different stages of sulfur vulcanization., Rubber. Chem. Technol, 2006 ; 79 : Coran A.Y;.Chemistry of the vulcanization and protection of elastomers. A review of the achievements. Journal of Applied Polymer Science 2003 ; 87 : Kruger F.W.H; Mc Gill J. A; SC study of curative interactions. The interaction of ZnO, sulfur and stearic acid. Journal of Applied Polymer Science 1991 ; 42 : Frit Zsche J; as A; Jurk R; stockelhuber K.W; Henrich G; Kluppel M; Relaxation dynamics of carboxylated nitrile rubber filled with oraganomodifiednanoclay, Express Polymer Letters, 2008 ; 2 : Sae-Oui P; Sirisinha C; Hatthapanit K.; Effect of blend ratio on aging, oil and ozone resistance of silica filled chloroprene rubber/ natural rubber blends.,express Polymer Letters,2007 ; 1 : Ohm R.F; Rubber chemicals in Kirk-Othmer encyclopedia of chemical Technology, John Wiley and sons, New York. 1997; 21: Sadasivan V; Alaudeen M; Synthesis and crystal structure of the zinc(ii) complex of5-(2,3- dimethyl-1-phenyl-3-pyrazolin-5-one-4- ylhydrazono)hexahydropyrimidine-2-thioxo- Page 723

5 4,5,6-trione,.Indian J. Chem 2007 ; 46A : Vidya V;G, Meena S.S; Pramod Bhatt; Sadasivan V; Mini S; Spectroscopic studies on Fe(II) and Fe(III) complexes of 5-aryl azo substituted lhpyrimidine-2,4-dione.,aip Conference Proceedings 2013 ; 1536 : Vidya V.G; Meena S.S; Pramod Bhatt; Sadasivan V; Mini S; Synthesis and spectral study of new azo dye and its iron complexes derived from 2-naphthol and 2-amino-3- hydroxypyridine, AIP Conference Proceedings 2014 ; 1620 : Mini S; Meena S.S; Pramod Bhatt; Sadasivan ;,Vidya V.G; Synthesis and characterization of Fe (III) complex of an azo dye derived from (2- amino-5-chlorophenyl) phenyl methanone; AIP Conference Proceedings 2013 ; 1536 : Yin J; Liu Z; Liu G; Wang HXS; Zhu T; Liu J.M; The epitaxial growth of wurtzitezno films on LiNbO 3 substrates, Journal of Crystal Growth,2000 ; 220 : Heideman G; atta R.N; Noordermeer J.W.M; Van Baarle ;. Influence of zinc oxide during different stages of sulfur vulcanization,.rubber. Chem. Technol,2005 ; 78 : Shah G.B; The effect of bimodality on tensile properties of filled silicone networks,express Polymer Letters 2008 ; 2 : Hernandez L.G; iaz A.R; Benito Gonzales J.L;,Orossa I.F; Fernandez A.M;.Effects of the structure and cross link distribution on the physical properties of a natural rubber network., KautschuckGummiKunststoffe,1992 ; 45 : Vilgis Y.T.A; Heinrich G; New aspects in rubber elasticity. A challenge for theoretical physics and applied material sciences., KautschuckGummiKunststoffe,1992 ; 45 : Zabosrki M;Przybyszewska M; Nanoparticle zinc oxide applied for crosslinking of butadiene rubbers in the 8 th European symposium on polymer blends and Eurofillers,,Burges, Belgium, 2005 ; 116. Page 724

Use of nano zinc oxide in natural rubber

Use of nano zinc oxide in natural rubber VÉÇàxÇàá 4.1 Introduction 4.2 Experimental 4.3 Results and discussion 4.4 Conclusions 4.5 References 4.1 Introduction Natural rubber (NR) is a high molecular weight polymer of isoprene (2-methyl-1,3-butadiene).