Influence of Epoxidation on Physical Properties of SBR and its Interaction with Precipitated Silica

Transcription

1 PRÜFEN UND MESSEN TESTING AND MEASURING Epoxidation SBR Spectroscopical and thermo-analytical characterisation Filler-polymer interaction Dynamic properties Glass transition regime Precipitated silica The introduction of epoxy groups on the main chain of polydiene has emerged as a fruitful route to monitor polymer-filler interactions leading to useful changes of physical properties of the vulcanisates. The in situ epoxidation reaction was chosen to modify polybutadiene and poly(butadiene-co-styrene), due to the easy control of the reaction even for small degrees of epoxidation. The extent of modification of the polymer chain was quantitatively accompanied by means of FT-IR and 1H-NMR spectroscopy. The shifts of the Tg s to higher temperatures with the increase of the epoxy content on the polymer chain was monitored using the DSC analysis. From the temperature and amplitude dependency of the dynamic moduli it can be concluded, that the interaction of the epoxidised rubber matrix with silica is greatly improved even for small contents of epoxy groups in the rubber chains. Einfluss der Epoxidierung auf die physikalischen Eigenschaften von SBR und dessen Wechselwirkung mit gefällter Kieselsäure Epoxidierung SBR spektroskopische und thermische analytische Charakterisierung Füllstoff-Polymer Wechselwirkung dynamische Eigenschaften Glassübergang gefällte Kieselsäure Die Einführung von Epoxigruppen in die Hauptkette von Polydienen hat sich als zielführend erwiesen um die Polymer-Füllstoffwechselwirkung zu steuern und nützliche Ønderungen der physikalischen Eigenschaften der Produkte zu erzielen. Die in situ Epoxidierung wurde zur Modifizierung von Polybutadien und Poly-(butadienco-styrol) ausgewählt, da hiermit eine einfache Kontrolle der Reaktionsführung auch bei geringen Umsätzen möglich ist. Das Ausmaß der Modifizierung wurde mit Hilfe von FT-IR und 1H-NMR Spektroskopie quantifiziert. In Abhängigkeit von dem Gehalt an Epoxigruppen wurde unabhängig von der ursprünglichen Zusammensetzung der Polydiene dieselbe positive Verschiebung der Glassumwandlungstemperatur festgestellt. Die Ergebnisse temperaturabhängiger wie auch amplitudenabhängiger Untersuchungen der dynamisch-mechanischen Eigenschaften zeigen auch bei geringem Epoxidierungsgrad der Kautschuke eine intensive Wechselwirkung mit gefällter Kieselsäure. Influence of Epoxidation on Physical Properties of SBR and its Interaction with Precipitated Silica Polymer analogue chemical modifications have been studied for two main reasons. The most frequent one, the change of the polymer nature, can be achieved by modifications of the polymer backbone or the introduction of specific side groups as well. The extent of this changes can vary from slight modifications to quantitative ones. In the case of major modifications the properties of the polymer can be changed completely. Prominent examples are modifications of polydienes by introducing polar groups into the polymer backbone. This allows for adjustments of material properties like significant oil and solvent resistance [1], reduced gas permeability [2], better adhesion to ceramics, metal and textile [3, 4]. At a small degree of modification polydienes and polyolefines demonstrate, significant changes in their interaction potential with other polymers or with nano-scaled fillers [5]. Furthermore, such small modifications can improve the compatibility towards particular polymers or can lead, due to stronger polymer-filler interactions, to useful changes in the dynamic mechanical properties, which result in improved traction and wet-skid resistance [6]. Not as common, the task for polymeranalogue modifications is to bound useful chemical side groups onto the main chains in order to have chemically bonded antioxidants [7], softeners [8], cross- linking systems [9], and last but not least rubber-filler coupling agents [10]. The chemical modification on polyolefines exploits mainly the reactivity of double bonds. Therefore the most common reactions are epoxidation, chlorination maleination, carboxilation, sulfonation etc [2, 12 14]. For the epoxidation of polydienes, peracids play an important role [16]. They can be employed as such or be generated in situ by the reaction of the corresponding organic acid with hydrogen peroxide [2, 12, 15, 16]. Especially the in situ generation of performic acid appears to be one of the most convenient routes to perform epoxidations. Reinforcement of elastomers by nano-scaled fillers generally requires strong physical interactions between polymer chain segments and the filler surface [17, 18]. In some cases chemically bonding of the polymer on the filler surface, i.e. by coupling agents [19, 20], supports the reinforcing effects. These interactions are affecting the most important physical properties and predominantly the dynamical-mechanical properties. While the modulus of unfilled compounds does not change upon increasing strain amplitude, it decreases significantly for the filled rubber [21]. The non-linear strain amplitude dependency of G, (Payne effect), is explained by the breakdown of the filler network if the strain is continuously increased during periodic deformations [22, 23]. By improving the filler-polymer interaction and therefore, the state of filler dispersion the strain amplitude dependency becomes significantly less pronounced. The present work is aimed to emphasis the influence of epoxidation of polydienes on the interaction with precipitated silica, taking mainly in account the dynamic-mechanical properties in the glass transition region and the strain amplitude dependency at normal temperature. Experimental Material The rubbers used were prepared by means of anionic polymerisation. The sample characteristics are shown in Tab. 1. T. L. A. C. Rocha, R. H. Schuster, Hannover, M. M. Jacobi and D. Samios, Porto Alegre (Brazil) Corresponding author: T. L. A. C. Rocha DIK e.v. Eupener Str. 33, Hannover Tel.: 05 11/ Fax: 05 11/ KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004

2 Tab. 1. Rubber Mw Mn/Mw Styrene Vinyl 1,4-cis 1,4-trans SBR SBR , SBR SBR , BR , Epoxidation For the epoxidation hydrogen peroxide (Merck, 30 %) was titrated against KMnO 4 and used at concentration of 30 %. All other reagents such as formic acid (98 % Fluka), toluene (Fluka) were used without further purification. The epoxidation reactions were carried out as followes. The rubber was dissolved in toluene (1 g polymer/15 ml of solvent). Hydrogen peroxide was used in excess to the polymers double bond content (H 2 O 2 /C¼C 1,5/1) and formic acid was added at a ratio of the reactant mol (H 2 O 2 / HCOOH 2/1) [15] to generate in situ performic acid. The reaction temperature was stepwise varied from 50 8C down to 5 8C. The reaction was performed in a double-neck round-bottomed flask using a mechanical stirrer, an addition funnel, a thermometer and a thermal bath. At the end of the reaction, the mixture was neutralized with a (5 % w/v) Na 2 CO 3 solution. After removing the aqueous phase, the organic phase was washed with distilled water and then coagulated in ethanol and dried under vacuum to a constant weight. From each of rubbers indicated in Tab. 1, epoxidised samples were prepared in a wide range of epoxidation degree varying from 7 until 75 mol %. The epoxidised samples used in this work are abbreviated showing first the name of the rubber followed (in brackets) by the epoxidation content, i.e. SBR-2(ep7) is the abbreviation for the rubber SBR-2 with 7 mol % of the double bonds epoxidised. Sample Preparation SBR-2(ep7) and silica (VN3,Degussa AG) were mixed in an internal mixer (Thermo Haake PolyLab System) fitted with a 50 ml mixing chamber. The silanisation reaction was carried out in three mixing stages with TESPT(Si69) as coupling agent. Characterisation The epoxidation degree of the modified rubber was analysed by means of 1H- NMR spectroscopy in a DRX 500 MHz (Bruker). SBR samples were dissolved in CCl 4 with a D 2 O capillary using a concentration of the 7 20 mg polymer/ml solvent. The Fourier-Transform-Infra-Red spectroscopy (FT-IR) was carried out in a FT-IR Nicolet Nexus on films cast from a chloroform solution on KBr plates. The glass transition temperature was recorded by DSC (polymer laboratory thermal equipment -DSC 2920 Modulated DSC, TA Instruments) at a heating rate of 10 8C/min. Samples were cured using a sulphur curing system (S/CBS 1.6/2.5 phr) at 160 8C at the rheometer optimum. The dynamic-mechanical properties of the vulcanised rubbers were analysed using strip samples considering the torsion modulus using a Dynamic Mechanical Analyser (Ares 3A5 Rheometric Scientific) in a temperature sweep experiment ( 100 until þ 100 8C) at a heating rate of 1 8C/min, frequency of 1 Hz and deformation amplitude of 1 %. The amplitude dependency of the storage modulus and the loss modulus was determined using samples with a double sandwich geometry in shear on an Elastomer- Testing-System ( MTS Systems Corp) at room temperature considering the region of the force from 2 N up to 5 kn. Results and discussion Material characterisation Spectroscopy Besides the overall degree of epoxidation achieved during the reaction a major concern was focused on the selective reactivity of the butadiene microstructure units. Both tasks were followed by FT-IR and 1 H-NMR spectroscopy respectively for epoxidised rubbers. Using the out of plane vibration band of the (-hydrogen atom of phenyl ring at 699 cm 1 as a reference, the changes of characteristic vibration bands of butadiene units and the appearance of new bands representative for oxirane rings (epoxy groups) were analysed (Fig. 1). At 760 cm 1, 911 cm 1 and 960 cm 1, the characteristic stretching vibration bands of 1,4-cis butadiene, vinyl and 1,4-trans-butadiene units respectively, clearly appear in the spectrum. As a result of the epoxidation reaction in the spectrum of SBR- 2(ep30) new bands arise in the spectrum. The one at 740 cm 1 is attributed to the stretching vibration of the 1,4-cis epoxy group and the other at 890 cm 1 corresponds to the out of plane vibration of the H atoms of oxirane ring. The characteristic stretching vibration band of the 1,4- trans epoxy group appears at 760 cm 1, and overlaps with the stretching vibration band of the 1,4-cis butadiene-units. Comparing the spectra of the original SBR-2 with the epoxidised product SBR- 2(ep30), it is observed that the vibration band of the 1,4-trans-butadiene units at 960 cm 1 practically disappears in the spectrum. This underlines the nearly quan- Fig. 1. Infrared spectra of the SBR-2 and SBR-2(ep30) KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/

3 The characterisation of the thermal behaviour was focused on the impact of the degree of epoxidation on the glass transition temperature. The results are summarised in Fig. 3. The unmodified rubbers show an increase of the Tg-values, which depends greatly on the increasing content in styrene and especially vinyl units in the rubbers. Due to these side groups the average chain cross-section increases and at the same time the chain mobility decreases. Due to the higher reactivity of 1,4-cis- and 1,4-trans-butadiene units the epoxidation reaction leads primarily to a main-chain modification of the rubbers leaving the side vinyl groups unreacted. The statistical distribution of epoxy-groups results finally in a linear increase in Tg with the degree of epoxidation. This result demonstrates the reduced translational and rotational degree of freedom of the modified repeating units, wich is caused by the increased interand intra-molecular interactions of the epoxy-groups. For the BR and all types of SBR chosen (except for SBR-3), the effect of epoxidation results in the same incremental increase of the glass transition temperature, which is ca C/mol % epoxy-groups. This similarity in the thermal behaviour indicates for the statistically replacing of double bonds by epoxygroups rather than the formation of small blocks of epoxidised segments. As a consequence a similar increase of the characteristic chain relaxation time takes place in all rubbers. Fig H-NMR spectra of the SBR-2, SBR-2(ep18) and SBR-2(ep34). The epoxidation reaction of the butadiene rubbers was monitored by 1 H-NMR spectroscopy through the change in the peaks area titative epoxidation and the high reactivity of the 1,4-trans-butadiene units. In contrast to this, the characteristic stretching vibration band of the vinyl unit at 911 cm 1 remains nearly unchanged, considering the sensibility of the method, indicating that those units practically do not react until 30 mol% of epoxidation. This is confirming the results obtained in previous studies for the epoxidation of SBR and BR [15, 16]. In addition, by employing 1 H-NMR-spectroscopy complementary information was derived from the change of the peaks area (Fig. 2). The 1 H-NMR-spectrum of the original SBR demonstrates a broad signal at 7.0 ppm, which denotes the five hydrogen atoms of the phenyl ring. The characteristic signals of the hydrogen of 1,4-cis and 1,4-trans butadiene units appear at 5.4 ppm, the vinyl units become evident at 4.9 ppm. In addition, in the region from 1.0 until 2.2 ppm the overlap of the signals of terminal CH and CH 2 units can be seen. In comparison to the reference, the spectra of SBR-2(ep18) and SBR-2(ep34) shows two new signals at 2.45 and 2.7 ppm, which can be attributed to the cis- and trans-epoxy groups respectively. As the degree of epoxidation increases the area of both epoxy group signals increases while the area of the signals for 1,4-cis butadiene and 1,4-trans butadiene units decreases. However, the area of the signal of the vinyl remains practically unchanged. This is once again confirming the reduced reactivity of the double bonds in 1,2-units, as observed earlier for the epoxidation of BR [16]. Thermoanalysis Dynamic-mechanical properties Strain amplitude dependency of G 0 The influence of the main-chain epoxidation of SBR on rubber-filler interaction is well expressed through the non-linear viscoelastic behavior of the filled compounds. This reflects generally the state of the filler network and its breakdown induced by increasing the strain amplitude during a dynamic-mechanical test. The vulcanisate obtained from the reference mix, containing mainly untreated SBR-2 and precipitated silica, shows a four times larger storage modulus at low strain amplitudes, G 0 0, if compared with the test system SBR-2(ep7)/silica. The reduction in G 0 0 demonstrates a weaker filler network, due to an energetically more favourable interaction between the epoxy groups of the SBR and the silanol groups on the silica surface. Exactly this leads to a more efficient filler dispersion and a significant reduction of the Payne effect (Fig. 4). The modification of the silica surface activity by in situ silanisation, has been used to reduce the polarity of the silica. As expected, a reduction of G 0 0 and of the entire Payne effect is obtained by in situ silanisation of the precipitated silica. Analysing the strain amplitude dependency of the SBR-2/ silica/silane compound, it can be noticed that silanisation leads to a decrease of 2.5 times of the G 0 0 if compared to the value for the reference compound. However it is worth to mention, that (i) this reduction of the Payne effect is considerably smaller than that obtained with SBR-2(ep7) and non-silanised silica and (ii) the value of G 0 0 obtained for SBR-2(ep7) and silanised silica is comparable to that value for SBR-2/silica/silane. As far as vulcanisates from SBR-2(ep7) are concerned, the in situ silanisation of the precipitated silica leads to a slight increase of G 0 0, indicating that the filler-polymer interaction is impaired by the modification of the filler surface activity. Since the silica surface became partly hydrophobic by the in situ silanisation the polar epoxy groups are not able to exhibit a similar extensive interaction as in the case of the non-silanised silica. Thus, the fillerfiller interaction becomes more intensive and G 0 0 is larger. 658 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004

4 Fig. 3. Influence of the epoxidation content on the Tg of the modified rubbers Fig. 4. Influence of the silica modification on the interaction of matrixfiller of SBR-2 and SBR-2(ep7) This underlines that the G 0 0 is very sensitive to changes in the degree of filler-polymer interaction, which can be caused either by functional groups of the polymer or by modifications of the filler surface activity. The dynamic-mechanical behaviour can be taylor-made if blends of original and epoxidised SBR are used (Fig. 5a). It is interesting to see that as the content of SBR- 2(ep7), in the blend increases the value of G 0 0 decreases greatly. Just 30 parts of SBR-2(ep7) which is equivalent to an overall content of 2 mol % epoxy groups present in the blend, lead to a decrease of ca. 40 % in the value of G 0 0. Taking into account the region of large deformations, there is a trend to a small increase in the G 0 as the epoxidation content increases in the compound. Taking into consideration the loss modulus, G 00, a decrease of the maximum value by ca. 35 % is observed as the SBR-2(ep7) content increases, which underlines again the more efficient filler-rubber interaction (Fig. 5b). The relatively strong filler-rubber interaction between epoxidised SBR and precipitated silica can be attributed to the formation of hydrogen bonds or more than that to chemical bonds formed after a proton catalysed opening of the oxirane ring and formation of covalent bonds. The hypothetical reaction scheme is shown in Fig. 6. The following discussion starts off by presenting the influence of precipitated silica on the dynamic mechanical moduli (storage modulus G 0 and loss modulus G 00 ) of unmodified SBR and SBR-2(ep7) in a temperature sweep at constant deformation amplitude and frequency. As discussed in an earlier publication [24] useful information concerning the rubber-filler interaction can be derived from the temperature dependency of the loss modulus of a filled systems in the glass transition regime. The area of the damping maximum indicates the energy dissipated per unit volume of a sample during the transition from the rigid glassy state into the flexible rubbery state. If filler-polymer interactions are present in the system a given fraction of the polymer chains are immobilised at the filler-matrix interface. Therefore, the energy dissipated during the glass transition increases above the level of the unfilled sam- Rubber-Filler Interaction (glass transition regime) Fig. 5. Influence of the epoxidation content on the compound contained 60 phr of silica as filler KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/

5 Fig. 6. Proposed mechanism of filler-matrix interaction of an epoxidised rubber and silica Fig. 7. Analysis of the interaction between rubber matrix, SBR-2 and b) SBR-2(ep7), with 60 phr of silica (VN3) considering de changes on loss modulus (G 00 ) in a temperature sweep with an amplitude of deformation of 0.1 Fig. 8. Relative increase in the G 00 maximum value with the filler content Fig. 9. Influence of the filler polarity on the filler-rubber interaction to SBR-2(ep7) ple, due to the contributions of the attached polymer chains. To this increase are contributing both, the volume fraction of the immobilised polymer layer and the strength of the filler-matrix interaction. Consequently, by monitoring the increase of the amplitude (or the area) of the damping maximum in the glass transition regime at least qualitative information about the filler-matrix interaction can be deduced. The changes in the damping behaviour caused by 60 phr silica in SBR and SBR- 2(ep7) respectively became evident, when compared to the corresponding values of the unfilled vulcanisates (Fig. 7). Considering the unfilled systems it can be seen that even 7 mol % of epoxy groups in the polymer chain lead to a considerable increase shift in both the glass transition temperature and the G 00 maximum value. This result underlines the thermo-analytical data concerning the changes of the chain dynamics (see Fig. 3). The incorporation of precipitated silica into both polymers leads to slight changes in the glass transition temperature, but is causing significant increases of the G 00 maximum values. A more quantitative evaluation of the data reveals that the relative increase in the G 00 maximum value is more pronounced for SBR-2(ep7)/silica than for SBR/silica if the unfilled compound is taken as a reference (Fig. 8). Keeping in mind that hydrodynamic reinforcing effects play the same role in both systems, the higher G 00 maximum values in SBR-2(ep7)/silica can be attributed to the specific fillerrubber interactions. A further point is achieved by comparing the dynamic-mechanical effects caused by silanised silica and the pure silica, discussed before (Fig. 9). At the same filler loading the compound containing silanised silica shows a lower G 00 maximum value (by ca. 70 MPa) than the one with precipitated silica. This result can be understood easily if the decrease in the filler surface activity caused by the silanisation reaction is considered. As a consequence of the silanisation the physical interaction of the less polar filler surface with the epoxidised SBR or the cross-section for the chemical bonding became reduced. Both mechanisms are causing a decrease in the volume fraction of the strongly immobilised polymer chains on the filler surface. Therefore, less mechanically energy is dissipated during the glass transition process in the systems containing in situ silanised silica. Conclusion The Tg s of the SBR and BR increase with the increase of epoxidation content. As the polymer chains are epoxidised there is a stronger interaction between the chains leading to a decrease on the chains mobility. The stronger interaction between the pure to silica and the epoxidised rubber occurs 660 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004

6 due the higher polarity character of the epoxidised rubber. It has also been shown that the use of an epoxidised SBR with just 7 mol % of epoxidation groups is enough to reduce de value of the G 0 in ca. 4 times, indicating a better interaction between rubber matrix and filler. References [1] S. Roy, B. R. Gupta, and S. K. De, in Elastomer Technology Handbook; edited by N. P. Cheremisinoff (1993) 635. [2] C. S. L. Baker, I. R. Gelling and R. Newell, Rubber Chem. Technol. 58 (1985) 67. [3] X. Wang, H. Zhang, Z. Wang and B. Jiang, Polymer 38 (1997) [4] S. Gnecco, A. Pooley and M. Krause, Polymer 37 (1996) 609. [5] A. K. Bhowmick and H. L. Stephens, Handbook of Elastomers, Dekker, New York (2001). [6] R. Comander, Global Rubber 2000 Sarnia (Canada). [7] A. G. Margaritis, N. K. Kalfoglou, Polym.J. 24 (1988) [8] S. M. Wang, R. C. C. Tsiang, J. Polym. Sci.(A) 34 (1996) [9] O. Nuyken, R. Wedner and R. H. Schuster, Makromol. Chem. 190 (1989) 459. [10] J. Hofstetter PhD Thesis University of Strassbourg (1997). [11] D. Zuchowska, Polymer 21 (1980) 514. [12] J. Sheng, X. L. Lu and K. D. Yao, J. Macromol. Sci.- Chem. A27(2) (1990) 167. [13] A. Mokrini and J. L. Acosta, Polymer 42 (2001) 9. [14] B. M. Badran, M. H. El-Souckary and Sh. Aggour, Elastomerics April (1980) 40. [15] M. M. Jacobi, T. L. a. C. Rocha, R. H. Schuster, C. P. Neto and C. G. Schneider, Kautsch. Gummi Kunstst. 55(11) (2002) 590. [16] M. M. Jacobi, C. K. Santin, M. E. Vigânico and R. H. Schuster, Kautsch. Gummi Kunstst. 57(3) (2004) 82. [17] G. Kraus, Reinforcement of Elastomers, Wiley- Interscience, New York (1965). [18] J.-B. Donnet, Bansal and M.-J. Wang (Eds.), Carbon Black Science and Technology, Marcel Dekker Inc. New York (1993). [19] U. Görl, A. Hunsche, A. Müller and H. G. Koban, Rubber Chem. Technol. 70 (1997) 608. [20] J. Fröhlich and H. D. Lugisland, Rubber World 28 (2001) 244. [21] A. R. Payne, J. Polym. Sci. 6 (1962) 57. [22] A. I. Medalia, Rubber Chem. Technol. 59 (1986) 432. [23] M.-J. Wang, Rubber Chem. Technol. 72 (1999) 430. [24] J. Ziegler, R. H. Schuster, Kautsch. Gummi Kunstst. 56(4) (2003) 159. The authors Dr. R. H. Schuster is head of the Deutsches Institut für Kautschuktechnologie e.v. (DIK). Dr. T. L. A. C. Rocha is Post Doc at DIK. Dr. Marly Maldaner Jacobi is teacher of the Chemistry Department at the Federal University of Rio Grande do Sul in Porto Alegre, Brazil. Dr. Dimitrios Samios is head of the Chemistry Department at the Federal University of Rio Grande do Sul in Porto Alegre, Brazil. Acknowledgements The authors wish to thank the Conselho Nacional de Pesquisa (CNPq), Coordenação de AperfeiÓoamento de Pessoal (CA- PES) and Deutscher Akademischer Austauschdiente (DAAD) for the financial support, Bayer AG and Degussa AG for the material for the study and Prof. J. Hahn (University of Cologne, Germany) for the 1 H-NMR measurements. KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/