Some Contributions to the Understanding of the Accelerating System During Natural Rubber Vulcanization

Transcription

1 ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS NR vulcanization Accelerating system Squalene Sulfenamide CBS ZnO A method based on the modeling approach has been developed for the study of Natural Rubber vulcanization using two different accelerating systems. Squalene has been chosen as the model compound. The main reaction paths followed by sulfenamide (N-cyclohexylamine-2- benzothiazole, CBS) during the first stages of the process have been studied. In all cases, the active role of ZnO and the olefinic chain has been established. Some Contributions to the Understanding of the Accelerating System During Natural Rubber Vulcanization I. Behavior of Sulfenamide S. Borro s and N. AgulloÂ, Barcelona (Spain) BeitraÈge zum VerstaÈndnis des Beschleunigersystems waèhrend der Vulkanisation von NR I. Verhalten von Sulfenamid NR-Vulkanisation Beschleunigersystem Squalen Sulfenamid CBS ZnO Eine Methode auf der Grundlage der Modellvulkanisation wurde entwikkelt, um die NR-Vulkanisation mit zwei verschiedenen Beschleunigersystemen zu untersuchen. Squalen wurde als Modellkomponente ausgesucht. Die wichtigsten Reaktionswege des Sulfenamids CBS waè hrend des Angangsstadiums wurden verfolgt. In allen FaÈ llen wurde die aktive Rolle von ZnO und der aktiven Kette bestaè tigt. The complexity of sulfur accelerated vulcanization has given rise to many contradictory publications for the mechanism of the vulcanization process [1 ± 5]. There are two major problems scientists have to deal with when studying rubber vulcanization: the nature of rubber samples and the simultaneity of reactions. Despite these difficulties, there seems to be a general agreement with the proposed course for sulfur-accelerated vulcanization made by Morrison and Porter in 1984 [6] (Figure 1). Nevertheless, the exact reaction mechanisms that occur in each step of the scheme are not very clear yet. In order to simplify the study, a modeling approach has been traditionally used. The most well-known modeling approach for vulcanization is without any doubt the Model Compound Vulcanization (MCV), which consists of vulcanizing a low-molecular weight model for rubber, namely an olefin [7 ± 8]. Throughout the Figure 1. Proposed general course of sulfur-accelerated vulcanization KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 3/

2 literature, several model compounds can be found for the study of the vulcanization process of different types of rubbers [9 ± 11]. The choice of a model compound has been mainly done taking only into account the reactive site believed to play a principal role during the vulcanization, i. e. the allylic position. However, it is important not to forget that the process is really taking place between much larger molecules that contain more than one simple double bond. In an attempt to close the gap between model and real rubbers, in this paper a relatively novel model compound is presented for MCV studies, squalene. The usefulness of squalene for the study of NR vulcanization chemistry has been proved. This compound has been used in some studies focused on the adhesion between rubber and metal, and on the reversion problem NR presents. Recently it has been used as well for the identification of intermediates formed during sulfur-accelerated vulcanization of NR [12]. Usually, the vulcanization can be studied using two different approaches: * Characterization of the reaction attending to the fate of the ingredients of the vulcanizing systems and the formation of new intermediate compounds. * Investigation of network structures formed, i. e. number of crosslinks, number of sulfur atoms per crosslinks, accelerator moieties bound to the main chain and main chain modifications. The analytical strategy followed differs from one approach to the other, and so do the techniques applied. In this paper, due to the characteristics of the chosen model compound both aspects of the process have been taken into account. According to the before-mentioned proposed course for the rubber vulcanization, it is believed that during the first stages of the vulcanization the hydrocarbon backbone does not play a significant role, while ZnO is usually presented as being responsible for breaking down the initial organic accelerator [13]. In this paper it will be proved that the presence of the polymer is crucial for the vulcanization reactions to start. It will be shown as well, that the role of ZnO is highly dependent on the type of accelerator present in the initial vulcanizing system. Experimental Analytical strategy In order to be able to understand the importance of the role of every component in the accelerating system, the following analytical strategy has been used. All the mixtures have been characterized by several analytical techniques to cover both aspects of the process, the fading of the accelerator and the formation of crosslinks (Figure 2). First of all, a model mixture (reference mixture) which contains all the ingredients that are normally found in a vulcanizing system (accelerator/s, metal oxide and long chain fatty acid) has been characterized in order to establish the general course of the reaction. Afterwards, several experiments have been carried out on different mixtures containing determined components. This method allowed studying how the absence of each component affects the course of the reaction that has permitted to establish the exact role of each ingredient during the vulcanization. Model mixtures studied The vulcanization is carried out in a closed vessel at 140 8C. The model mixture is continuously stirred to assure its Figure 2. Analytical strategy Table 1. Composition of the mixture studied Mixture A Ingredients Amount (phr) Squalene 100 CBS 1,2 Sulfur 2 ZnO 5 Stearic acid 2 homogeneity. A nitrogen atmosphere is required in order to avoid oxidation of the double bonds of squalene. During the reaction, samples are taken from the mixture at previously established times. They are quickly cold quenched to stop the reaction. Since the work is done on a model mixture with squalene, liquid samples are handled, and complicated and long extraction stages are not required. The following vulcanizing system has been studied containing one single accelerator (N-cyclohexylamine-2-benzothiazole sulfenamide, CBS) (Table 1). High performance liquid chromatography HPLC experiments were conducted with a Hewlett Packard HP1050 chromatograph. An Ultrabase C-18 (5 lm, 150 4:6 mm) column was used with 132 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 3/2000

3 an UV detector set at 254 nm, and the mobile phase flow was 1 ml/min. The experiments were carried out using a gradient elution with acetonitrile, water, and ammonium acetate buffer (5 mmol/l). Data processing was carried out using the Gyncosoft Chromatographie Datensystem version 4.12 software. 0,1 g of the cold quenched sample was dissolved in acetonitrile for 5 min in an ultrasonic bath at room temperature. Before injection, samples were filtered with a 0,45 lm Nylon filter in order to remove insoluble particles that would damage the column. The injection volume was 20 ll. The disappearance of the CBS occurs at the onset of the crosslink formation (Figure 3). This suggests that the reason for the cure delay period has to do with more rapid reaction of the intermediate compounds with the initial vulcanizing agents rather than with the reaction of intermediate compounds with the unsaturated hydrocarbon to yield crosslinks. The chemistry of sulfenamides, 2-mercaptobenzothiazole (MBT) has a catalytic effect on the CBS decomposition [14]. This catalytic effect was detected when the reference mixture A (see Table 1) was heated in the presence of a small amount of MBT (Figure 4). The addition of MBT to the formulation provokes a considerable shortening of the scorch time. It seems logical to think then that CBS dissociation requires a certain amount of free MBT to start. Therefore, the formation of MBT during the first stages of the vulcanization reaction is very important for the onset of the process. However, there is still a lot of controversy on how this first amount of free MBT is formed. The different hypotheses postulated concerning this first step of the vulcanization have been studied focusing on the induction period for CBS dissociation. Some authors support the hypothesis of the dissociation of sulfenamide accelerators via thermal decomposition [15]. Such a reaction, however, exhibited an induction period of about 135 min (Figure 5), which is obviously too long to consider thermal decomposition as the unique reaction path for the dissociation of CBS. Concerning this first step of the vulcanization process, other authors emphasize the role of ZnO in this dissociation. If that is true, then taking out ZnO from the reference mixture A, should not result in changed CBS concentrations. The absence of ZnO in the reacting mixture resulted in a more delayed onset of the CBS dissociation (35 min) compared to the one observed with the reference mixture A (25 min) (Figure 6). The GPC results Gel permeation chromatography GPC experiments were carried out in a Hewlett Packard HP1090, using a Waters HR 0,5 Styragel (7,8 300 mm) column with a UV detector set at 275 nm. The mobile phase used was tetrahydrofuran (THF, HPLC quality) at a flow of 1 ml/ min. A Hewlett Packard HP 3D Chemstation was the software used for data adquisition. Samples taken from the reacting mixture were dissolved in THF at a concentration of 0,1 wt.-%, and the injected volume was 100 ll. Results and discussion Figure 3. Mixture A vulcanization at 140 8C: (~) CBS concentration vs. reaction time; (*) Crosslinked squalene vs. reaction time Figure 4. Comparison between the CBS concentration decrease in mixture A (~) and in the mixture with added free MBT, 0,2 phr (~) during vulcanization at 140 8C KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 3/

4 Figure 5. CBS concentration when heated at 140 8C in an inert solvent (CBS, 1,2 phr; sulfur, 2 phr; stearic acid, 2 phr) Figure 6. CBS concentration during vulcanization at 140 8C (~) mixture A (~) as A but without ZnO showed that, in this case, the crosslink formation occurred as well with the same delay. That fact confirms that CBS concentration decrease occurs when the crosslink formation starts. According to these results, this second hypotheses proposed turned out to be incomplete as well to explain the global course of the vulcanization process. There is a 5±10 min delay in the onset of the dissociation, but it did occur despite the fact that there was no ZnO in the mixture, and it was accompanied by crosslink formation. So there should exist still another parameter that is taking part in the reaction. A second modification was done on the reference mixture A, this time taking out the model molecule and carrying out the reaction in an inert solvent (octane). It has been observed that under those conditions the induction period for CBS dissociation is 85 min (Figure 7). The induction periods obtained for the 3 mixtures obtained are 1. Reference mixture A 25 min 2. Reference mixture A without ZnO 35 min 3. Reference mixture A without olefin 85 min the larger difference between the last and the former two seems to point out that the presence of the hydrocarbon model molecule could be influencing somehow the dissociation of the CBS. A third modification has been done on mixture A, this time taking out both the model molecule and ZnO. It has been observed that despite prolonging the reaction time for up to 120 min, no dissociation occurs at all. The following can be concluded from all these variations of the induction periods for CBS dissociation: both postulated reaction paths to obtain a small amount of the MBT required for the onset of CBS dissociation might be probable depending on the type of elastomer vulcanized. However, there is a third parameter that turned out to be the most favored one. It has been proved that in order to obtain the mentioned free MBT, an interaction between the accelerator and the model compound must occur (Figure 8). However, the product formed according to the above scheme is known to be non-reactive since it is a mono-sulfidic rubber-bound compound. The group of McGill proposed a first sulfuration of the accelerator-giving rise to compounds of type BtSS x NR 2 [16]. These sulfurated products were already mentioned in the earlier works of Campbell and Wise [17]. Considering this, the initial rubberbound compounds formed from CBS would be polysulfidic. These are known to be crosslink precursors with neighboring chains releasing MBT. The consumption of sulfur by the direct sulfuration of the accelerator is negligible and no significant decrease was observed. Only a very small amount of R 2 NS x SBt is required. Once the MBT is generated, its catalytic effect on CBS dissociation is much faster than the direct sulfuration of the accelerator, turning out to be the major reaction path within this period, until there is no more CBS left in the reacting mixture (Figure 9). However, throughout the work done in this paper, no CBP (sulfurated accelera- 134 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 3/2000

5 Conclusions Figure 7. CBS concentration during vulcanization at 140 8C (CBS, 1,2 phr; sulfur, 2 phr; ZnO, 5 phr; stearic acid, 2 phr) Squalene has been proved to be a suitable model compound to study the vulcanization process. In this paper, the MCV approach has given more insight in the first stages of the natural rubber vulcanization using a sulfenamide type accelerator. Small amounts of MBT are required for the formation of the active sulfurating agents. It has been proved that this compound is formed from the direct interaction between the accelerator and the double bonds of the hydrocarbon. The role of the ZnO usually described as a catalyst for the breaking down of the accelerator has been proved not to be the most favored. Similarly, the thermal decomposition of the sulfenamide is neglected to be the cause of the active sulfurating agents formation. tor) was detected in any stage of the reaction. Some authors claim that such compounds are formed very rapidly in the earlier stages of the vulcanization and that they react readily, but this could not explain the fact that no decomposition at all was detected when CBS was heated in the presence of sulfur presented previously. On the other hand (Figure 10), some evidence was found to support the direct interaction between accelerator and the olefin proposed in Figure 8. The mass spectrometry detection [12] allowed the identification of certain compounds whose presence during the reaction could be explained by such an interaction. References [1] P.W. Allen, D. Banard, B. Saville, Chem. Ind. Britain 6 (1970) 382. [2] M.M. Coleman, J.R. Shelton, J.L. Koenig, Ind. Eng.Chem., Prod. Res. Develop. 13 (1974) 154. [3] A.Y. Coran, Chem. Techn. 13 (1983) 106. [4] C.J. Hahn, M. Runk, M. Schollmeyer, U. Theimer, E. Walter, Kautsch. Gummi Kunstst. 51 (1998) 206 [5] M.R. Krejsa, J.L. Koenig, Elastomer Technology Handbook, Chap. 11, CRC Press, New York, (1993). Figure 8. Interaction between the accelerator and the hydrocarbon Figure 9. Interaction between the accelerator and the hydrocarbon previous to the sulfuration of the former (CBP stands for polysulfide cyclohexylamine-2-benzothiazole sulfenamide) KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 3/

6 Figure 10. Schematic approach to the direct interaction between the CBS and the hydrocarbon [6] M.J. Morrison, M. Porter, Rubber Chem. Technol. 57 (1984) 63. [7] H.E. Westlake, Chemical Reviews 39 (1946) 219. [8] P.J. Nieuwenhuizen, et al., Rubber Chem. Technol. 79 (1997) 120. [9] F.K. Lautenschlaeger, K. Edwards, Rubber Chem. Technol. 53 (1980) 27. [10] J.H.M. van der Berg, et al., Rubber Chem. Technol. 57 (1984) 265. [11] P. Versloot, et al., Rubber Chem. Technol. 70 (1997) 106. [12] S. Borro s, Kautsch. Gummi Kunstst., in press, (1999). [13] J.A. McCleverty, Studies Inorg. Chem. 5 (1984) 311. [14] J.L. Craine, M. Raban, Rubber Chem. Technol. 62 (1989) 67. [15] G. KyselaÁ Kautsch. Gummi Kunstst. 49 (1996) 678. [16] R.H. Campbell, R.W. Wise, Rubber Chem. Technol. 37 (1964) 635. [17] M.H.S. Gradwell, W.J. McGill, J. Appl. Polym. Sci. 61 (1996) The authors Prof. Dr. S. Borro s is Head of the Material Science Laboratory at the Institut Quimic de Sarria of the Universitat Ramon Llull, Bacelona, Spain. Ms. Dr. N. Agullo is a former PhD student in the research group of S. Borro s; she is now working in a pharmaceutical company. Correspondong author Dr. S. Borro s, Institut Quimic de SarriaÁ, Materials Science Laboratory, Via Augusta 390, E Barcelona 136 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 3/2000