Time-Of-Flight SIMS as a useful Technique for the Study of the Influence of Carbon Black in Natural Rubber Vulcanization

Transcription

1 PRUÈ FEN UND MESSEN TESTING AND MEASURING TOF-SIMS Vulcanization Mechanisms Carbon Black Accelerators The use of TOFSIMS technique to analyze the carbon black surface during the squalene vulcanization reaction has been demonstrated to be a useful technique to follow the sulfur vulcanization kinetics. The results obtained are in agreement with those using the classic techniques HPLC and GPC. The results show that the intermediate compounds are strongly adsorbed on the carbon black surface. Time-Of-Flight SIMS as a useful Technique for the Study of the Influence of Carbon Black in Natural Rubber Vulcanization S. Borro s, E. Vidal, N. AgulloÂ, Barcelona (Spain) and W. J. van Ooij, Cincinnati (USA) Time-of-Flight SIMS als nuètzliche Technik zur Untersuchung des Einflusses von Ruû bei der Vulkanisierung von Rohkautschuk TOF-SIMS Vulkanisierung Mechanismen Ruû Beschleuniger Die Verwendung der TOF-SIMS-Methode um die RuûoberflaÈ che waèhrend der Vulkanisierungsreaktion von Squalen zu analysieren erwies sich als nuè tzliche Technik um die Kinetik der Schwefelvulkanisation zu verfolgen. Die erhaltenen Resultate stimmen mit denen der klassischen Methoden aus HPLC- und GPC-Untersuchungen uè berein. Die Ergebnisse zeigen, daû die Zwischenverbindungen stark auf der RuûoberflaÈ che adsorbiert werden. Carbon black is a well-known filler extensively used in rubber industry to increase the strength and stiffness of rubber goods. Since its discovery, scientists have dedicated many efforts on the understanding of the reinforcing mechanism of carbon black and the improvement of its characteristics. However, the influence of this filler in the vulcanization reaction has not been studied thoroughly. Some authors suggest, by measuring the cross-link density of the vulcanized samples, that carbon black does not have a great influence in the vulcanization process [1]. However Koenig et al., using 13 C solid state measurements indicate that carbon black may affect the vulcanization mechanism. They suggested that the basic network structure formation is modified by cross-link desulfurization from poly to monosulfide structures [2, 3]. Furthermore, other studies with 13 C NMR and 1 H transversal relaxation solid state measurements were applied to carbon black filled natural rubber vulcanizates to determine the chemical crosslink density [4]. In this case, addition of carbon black seems to lead to a decrease of cross-link density with increasing filler content. The authors explained that it could be due to the adsorption of curatives on the filler surface, or to chain mobility reduction. Other research groups followed the vulcanization reaction using DSC and HPLC, and concluded that carbon black does not change the mechanism of the TMTD sulfur vulcanization of polyisoprene. They proposed that the role of carbon black consisted of reducing the scorch time. The increased rate of crosslink formation can be attributed to a catalytic effect on the formation of TMTD polysulfides [5]. Previous studies done by our research group on the effect of carbon black in vulcanization using the MCV (Model Compound Vulcanization) approach seemed to show that there is a strong interaction between the vulcanization intermediate compounds and the carbon black [6, 7]. The higher the carbon black surface activity, the less intermediate compounds are detected by HPLC. Thus it would be interesting to try to find if such compounds are really adsorbed on carbon black surface or not. For that purpose a suitable analytical technique to determine accurately if the intermediate compounds are adsorbed on carbon black surface would be required. TOF-SIMS is a surface analysis technique with high sensibility and it has been used in many research fields. For instance, static SIMS provides information on the molecular structure and on the orientation of polymers at interfaces. The mass spectra of polymers are very characteristic and allow fingerprinting as well as fractional group determination. This can be achieved under strictly static conditions and low dose [8]. Some authors have used static TOF- SIMS for the characterization of rubbermetal-interfaces for adhesion purposes. They used squalene as a model molecule of Natural Rubber, and study the interaction between squalene and steel by TOF- KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 12/

2 Table 1. Composition of the reaction mixture Ingredients Amount (phr) Squalene 100 Carbon black N DCBS 1.2 Sulfur 2 Zinc oxide 5 Stearic acid 2 SIMS. From those studies it has been concluded that this technique is useful for model compound vulcanization studies [9]. Bertrand and Weng [10] have also applied TOF-SIMS to the characterization of different types of carbon black. They concluded that this technique is valuable to detect the heteroelements present at the carbon black surface. TOF-SIMS showed high mass clusters intrinsically related to the CB structure. In this work, we studied the usefulness of TOF-SIMS to analyze the carbon black surface during the vulcanization reaction in a liquid rubber model system. The results obtained were compared with the accelerator and intermediate compounds formation determined by HPLC. Experimental The model compound chosen to simulate the behaviour of NR was squalene. Dicyclobenzothiazole sulfenamide (DCBS) was used as sulfenamide accelerator type. The ingredients of the reaction mixture are shown in Tab. 1. The vulcanization reaction was carried out in a vessel at 140 8C.The mixture was continuously stirred to assure its homogeneity. A nitrogen atmosphere was required to avoid oxidation of the double bonds of squalene. During the reaction, samples were taken at regular intervals between 2 and 60 minutes. They were cold quenched to stop the reaction. HPLC was used to follow the concentration of the accelerator during the vulcanization reaction. The behaviour of squalene was followed by GPC. For the HPLC analysis, 0.1 g of the cold quenched sample was extracted with 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. The injection volume was 20 ll. For the GPC analysis, samples taken from the reacting mixture were dissolved in THF, filtered and a 100 ll volume was injected. The chromatographic conditions are described in a previous paper [11, 12]. For the TOF-SIMS analysis, samples were taken out at regular intervals and the carbon black was rinsed gently (more than 10 times) with hexane prior the analysis by SIMS. A reaction without carbon black was carried out in order to compare the behaviour of the intermediate compounds with and without carbon black. TOF-SIMS analysis was done on a CAMECA ion-tof Model IV instrument equipped with a 25 kv liquid metal ion gun 69 Ga and a low energy high-density gas source for depth profiling. Results and discussion In Fig. 1, the observed intensities of DCBS and vulcanization intermediate compounds with and without carbon black (CB) is pretented. As it can be observed, the presence of carbon black affected slightly the DCBS lifetime (30 min with CB, 32 without CB). However the results obtained show that there was a strong interaction between carbon black and the vulcanization intermediate compounds. Fewer amounts of these compounds were detected when carbon black is added. This fact can be explained by the adsorption of the intermediate compounds on the carbon black surface. The same behaviour is observed for MBT, the final product of the accelerator decomposition. It can be concluded that the formation of intermediate compounds starts at about 25 min of reaction and ends around 10 min later. The analysis of the squalene concentration by GPC Fig. 2) confirms that crosslinked squalene appears after 30 min of reaction. Static TOF-SIMS experiments collecting both negative and positive ions were carried out to confirm that intermediate compounds are really adsorbed on CB surface. In Fig. 3 and 4 the TOF- SIMS negative ion spectra of carbon black after 2 min and 60 min reaction Fig. 1. Evolution of DCBS and vulcanization intermediate compounds with and without carbon black Fig. 2. Squalene analysis evolution by HPSEC 712 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 12/2000

3 Fig. 3. SIMS-TOF negative ion spectra of carbon black after 60 minutes Fig. 4. SIMS-TOF negative ion spectra of carbon black after 60 minutes time, respectively, are shown. There are clear differences between these two spectra. The signal corresponding to sulfur is much higher after 60 min. In this spectrum two intense signals corresponding to ions 134 and 166 also appear. These can also be identified as accelerator moieties (C 7 H 4 NS and C 7 H 4 NS 2 ). The ratios 32/13, 166/13 and 166/134 were chosen to follow the vulcanization kinetics. In Fig. 5 they are represented for different s. The sulfur signal increases during the reaction evolution, but between 20 and 30 min there seems to be a plateau. At the same time there is a maximum in the ratio 166/134. Ion 166 is the accelerator moiety with sulfur (Fig. 3 and 4) and it could be directly related to the formation of vulcanization intermediate com- Fig. 5. Evolution of 166/134, 32/13 and 166/13 TOF- SIMS ion ratios during vulcanization reaction KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 12/

4 pounds. That would explain the fact that the accelerator moiety directly related with the intermediate compounds (166) reaches a maximum in this period of time and the ratio S/C (32/13) remains almost constant. Regarding the ratio 166/ 13, an increase in the slope of the curve is also observed between 20 and 30 min indicating preferred adsorption of the intermediate compounds at this time. These results are coincident with ones obtained by HPLC analysis: intermediate compounds were detected by HPLC (25 ± 35 min) just at the same reaction time as the 166/134 ratio reached the maximum. This fact seems to confirm that the intermediate compounds are strongly adsorbed by carbon black. In Figs. 6 and 7 the TOF-SIMS positive ion spectra of carbon black are shown after 2 and 60 min, respectively. The SIMS pattern observed in the 2 minute-reaction spectra is very similar to the one published for pure squalene [9]. Fragment 69 (C 5 H 9 ) was formed in an extremely high yield, and it can be identified as CH 3 CH ˆ CH CH 2 CH 2. This ion could be interpreted as the monomeric unit of squalene. It seems that at the beginning of the reaction squalene is strongly adsorbed in carbon black surface, in the same way as bound rubber in real rubber formulations. This hydrocarbon layer prevents the interaction of carbon black with accelerator in the first stages of the vulcanization, and consequently the scorch time does not change. Fragments 41 and 55 confirm the hydrocarbon origin of the spectra. The ends group of squalene can easily form them. Ion 15 (CH 3 ) can be used as reference for the hydrocarbon level on the carbon black surface. The spectra after 60 min of reaction show some interesting differences. First of all fragment 69 is no longer the most abundant signal. This effect could imply that at the end of the vulcanization the amount of squalene adsorbed on carbon black decreases. These results suggest that, after crosslinking by sulfur, squalene has less affinity to carbon black. On the other hand, an intense fragment appears at 182 that it can be identified as the diclyclohexyl amine accelerator fragment. The ratio 182/15 was selected to follow the vulcanization kinetics. In Fig. 8 this ratio is shown as a function of the reaction time. It can be seen that the ratio 182/15 is almost zero during the first 25 minutes of the reaction. This result is in agreement with the corresponding HPLC analysis and negative ion SIMS: the accelerator breaks down giving rise to an amine and this only can occur when the accel- Fig. 6. SIMS-TOF positive ion spectra of carbon black after 2 minutes Fig. 7. SIMS-TOF positive ion spectra of carbon black after 60 minutes 714 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 12/2000

5 References erator concentration decreases (HPLC analysis) and the ion of m=z ˆ 166 increases (negative ion SIMS). During the intermediate compounds formation the 182/15 remains constant at a low level. Fig. 8. Evolution of 182/15 TOF-SIMS ratio during vulcanization reaction This results can indicate that, during this stage of the reaction, the intermediate compounds ( 166 m=z peak in Fig. 5) are preferentially adsorbed on the carbon black surface. [1] S. Wolff, Rubber Chem. Thechnol. 69 (1996) 325. [2] M. Mori and J.L. Koenig, Rubber Chem. Thechnol. 68 (1995) 559. [3] M. Mori and J.L. Koenig, Rubber Chem. Technol 70 (1997). [4] G. Simon and A. Wutzler, DIC Agenwande Makromoleculare Chemie 202/203 (1992) 283. [5] W.J. McGill and S.R. Shelver, J. Appl. Polym. Sci. 58 (1995) [6] S.Borro s, Worldwide Amazon Rubber Conference, Manaus (Brazil), [7] A. Pont, Final Project Work, Ramon Llull University (2000) Unpublished work. [8] W.J. van Ooij and M. Nahmias, Surface characterization of rubber by Secondary Ion Mass Spectrometry. [9] W.J. van Ooij and V. Rangarajan, Interfaces between natural rubber and metals. Part I. Study of the interaction between squalene and steel using XPS and SIMS. [10] P. Bertrand and L.T Weng, Rubber Chemistry and Technology 72 (1999) 384. [11] N. Agullo, S. Borro s et al., K.G.K. 52 (1999) 438. [12] S. Borro s and N. Agullo, K.G.K. 53 (2000) 131. Corresponding author Salvador Borro s Seccio de CieÁ ncia de Materials Institut Quimic de SarriaÁ Universitat Ramon Llull Barcelona Catalonia Spain KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 12/