Influence of compounding and mixing on filler dispersion and curing behavior of silica compounds
Jungmin Jin is a PhD student in the research group Elastomer Technology and Engineering (ETE). His supervisors are dr. W.K. Dierkes and prof.dr. A. Blume from the Faculty of Engineering Technology (ET).
The main objectives of this study were:
- to investigate the physicochemical mechanism underlying the marching modulus phenomenon during the vulcanization of silica filled SBR/BR compound;
- to evaluate the influence of mixing parameters, mixing protocols, compound formulation characteristics and mixing equipment;
- to provide possible guidelines for a tailored mixing process and compound in order to achieve a better silica dispersion and rubber reinforcement.
In order to achieve the goals, the fundamental knowledge and actual practices to obtain a higher degree of silica dispersion and rubber reinforcement were reviewed concerning materials, mixing process parameters and mixing equipment in the beginning of this work: Chapter 2. Additionally, earlier studies relevant for the main topic of the present study – the cause of marching modulus of silica/silane compounds – were investigated as a starting point of the present study. Aspects looked at are:
- filler-filler interaction;
- filler-polymer coupling via silane;
- formation of additional crosslinks by released free sulfur from the silane.
With the knowledge from Chapter 2, the origin of the marching modulus phenomenon of silica compounds and its effect on rubber reinforcement were investigated in Chapter 3. Several series of silica filled tire tread compounds were mixed in order to clarify the origin of marching modulus in Chapter 3.1. Various silane coupling agents with different sulfur ranks and functionalities were used in the different mixing series varying the silanization temperature in order to obtain:
- different degrees of filler-polymer coupling;
- different amounts of sulfur donated by the silanes;
- different degrees of silanization.
Commonly the marching modulus phenomenon was understood as a result of further formation of sulfur crosslinks, therefore the influence of crosslink density on marching modulus was also investigated.
The outcome of this study answers the first research question and confirmed that three factors: intensity of filler-filler interaction, silica Filler Flocculation Rate (FFR) and filler-polymer Coupling Rate (CR), turned out to be the major influencing factors on the marching modulus phenomenon of silica compounds, in order of decreasing importance. All three factors are related to each other (Figure 3.22) and are the consequence of the degree of silanization of the silica during mixing. A higher degree of silanization not only lowers the filler-filler interaction but also leads to formation of more chemically bound rubber, thus resulting in a lower FFR level. The formation of chemically bound rubber provides an additional reduction of the filler-filler interaction as well as the flocculation of silica particles. Additionally, a higher amount of chemically bound rubber indicates that there is a lower concentration of free sulfur – additionally introduced or released from the silane – as well as of remaining silane, resulting in a lower CR value. Finally, a higher degree of silanization will lead to a lower MMI. However, the total crosslink density was not a major influencing factor for the marching modulus phenomenon.
In Chapter 3.2, the effect of marching modulus on the consistency as well as on the level the of the mechanical properties and reinforcement of the silica filled rubber were investigated. The mechanical properties of the compounds in Chapter 3.1 were evaluated and compared to each other for three different curing times: calculated cure time, 20 minutes and 30 minutes. The expectation was a large alteration of tensile properties for the compounds having a higher Marching Modulus Intensity (MMI). However, it turned out that the MMI does not give any information about mechanical property variation along with the vulcanization time, as the crosslink density of the compounds were very similar regardless of the cure time. The level of tensile properties of silica filled rubbers are in first instance roughly determined by the total crosslink density, which includes not only polymer-polymer sulfur crosslinks, but also filler-polymer coupling established by the silane. The degree of micro-dispersion turned out to be another key factor for rubber reinforcement, as the level of the moduli at 100% of strain (M100) and the reinforcement index (M200/M100) were affected by the filler-filler interaction measured as Payne effect.
Chapter 4 gives answers to a part of the second (evaluation of the influence of mixing parameters and protocols) and third objective (guidelines for a tailored mixing process). Based on the outcome of Chapter 3, the mixing parameters and protocols which can influence the degree of silanization were investigated in Chapter 4, in order to provide guidelines for minimizing the marching modulus of silica compound.
In Chapter 4.1, silanization temperature and time turned out to be the crucial factors for the degree of silanization, FFR, CR as well as the amount of bound rubber. MMI was significantly reduced with increasing silanization temperature and time, as FFR and CR decrease due to a higher amount of total and chemically bound rubber. Therefore, applying an appropriate silanization temperature and time is crucial for silica/silane filled rubber mixing in order to obtain a higher quality of the compound: when silica is not properly silanized, the curing process cannot repair that. Additionally, a better processability in the last processing stage of tire manufacturing, the vulcanization step, can be obtained by minimizing the marching modulus phenomenon.
The results of Chapter 4.2 showed that the presence of ZnO – more precise, zinc stearate – influences the marching modulus behavior of silica filled rubber, but its concentration in the early mixing step had no effect at all. When ZnO reacts with stearic acid to form zinc stearate which can be adsorbed onto the silica surface, the two long hydrocarbon groups (C18) are capable of blocking the access of the silane to the silica surface. Zinc stearate might provide a positive effect on reducing filler-filler interaction; however, as soon as ZnO is added in the early stage the amount of bound rubber is reduced and resulting in a higher MMI.
The concentration of 1,3-diphenylguanidine (DPG) in the early or later mixing stage does influence the efficiency of the silanization as well as the filler-polymer coupling reaction via the silane coupling agent: Chapter 4.3. However, neither the presence nor concentration of DPG in the early mixing step turned out as an influencing factor for the marching modulus behavior of silica filled rubber. The concentration of DPG in the early mixing step influences the curing behavior of the compound as well as the crosslink density variation of the vulcanizates along with the curing time: the higher the DPG concentration in the early mixing step, the slower the curing rate. Besides, it reduces the crosslink density variation along with increasing the curing time.
However, the results of the mixing trials discussed in Chapter 4.2 and 4.3 did not fully confirm the well-known role of ZnO (or zinc stearate) and DPG in silica compounding, therefore a model compound study was performed in Chapter 4.4. The direct reaction possibility between bis-(triethoxysilylpropyl) tetrasulfide (TESPT) and the mentioned ingredients was analyzed using High Pressure Liquid Chromatography (HPLC), Liquid Chromatography-Mass Spectrometry (LC-MS) and 13C-Nuclear Magnetic Resonance (13C-NMR). The outcome of this part confirmed another functionality of DPG in silica/TESPT filled rubber, which was proposed by Mihara: DPG can react with TESPT, and both are thus partly consumed during the mixing process. However, ZnO or zinc stearate did not affect the release of sulfur from the TESPT molecule.
The other parts of objective 2 (evaluation of the influence of the compound formulation) and objective 3 (guidelines for compound design) are addressed in Chapter 5. The effect of the polymer and silica characteristics on marching modulus of a silica filled tire tread compound were investigated in this Chapter. Compounds were mixed with different polymer blending ratios (Chapter 5.1) and using two types of silica having different dispersibility (Chapter 5.2) and their properties were further analyzed.
Based on the results of Chapter 5.1, the SBR/BR blending ratio turned out to be a strong influencing factor for the marching modulus as well as for the other curing characteristics. Silica tends to be localized preferably in the SBR phase or at the interphase of SBR and BR domains due to the physical affinities of these rubbers towards silica. When the volume of the SBR decreases, the population of silica in SBR increases leading to faster silica flocculation (higher FFR) due to the short distance between the silica clusters. However, when the incorporation capacity of SBR reaches its maximum due to a lower SBR volume, the remaining silica is distributed in a higher volume of BR therefore the flocculation again slows down (lower FFR). Consequently, the MMI varies due to the silica flocculation rate varying with the SBR/BR blending ratio. Besides MMI, other curing characteristics such as maximum vulcanization rate are influenced by the total amount of 1,2- and 1,4-structured double bonds in the polymer. The 1,4-double bonds in SBR and BR are more reactive in sulfur curing than the 1,2-double bonds. Due to this difference in reactivity of these types of double bonds, faster vulcanization is observed for the compounds having a higher amount of 1,4-double bonds.
The dispersibility of silica turned out as an influencing factor for the degree of macro-dispersion, but showed no effect on the marching modulus behavior of silica filled rubber: Chapter 5.2. Compared to the Highly-Dispersible (HD) silica, ConVentional (CV) silica showed poor macro-dispersion. However, due to the fact that the amount of chemically bound rubber, CR and FFR are strongly affected by the degree of silanization, HD and CV silica showed similar values in these properties when the compounds are mixed at the same silanization temperature and time. It is interesting to note that there was no relation between the degree of micro- and macro-dispersion of silica, and this phenomenon is further evaluated in Chapter 7.
The influence of mixing time and equipment (the remaining part of the second objective) on MMI and silica dispersion is evaluated in Chapter 6. Besides, a guideline for silica compound mixing equipment, which is related to the third objective, was prepared. In this chapter, compound mixing is performed using plant scale mixers in order to check, if the basic mechanisms observed in the previous chapters on laboratory scale are valid regardless of the mixer scale. The silanization time, temperature as well as the rotor types were selected as the main variables to obtain different degrees of silanization and micro-dispersion of silica. The result of this work confirmed, that the basic mechanisms for the marching modulus phenomenon of silica compounds still apply regardless of the mixer scale. The mixing parameters which lead to a proper degree of silanization – silanization time and temperature – reduce the FFR and CR of the compound and thus reduce the MMI.
The intermeshing rotor is generally recommended for silica filled rubber mixing, but the rotor geometry turned out as a minor factor on MMI. However, a higher degree of micro- and macro-dispersion of silica filler can be achieved by using an intermeshing rotor.
Chapter 7 dealt with the investigation of the relationship of micro- and macro-dispersion of silica in rubber and its effect on reinforcement. No dependency could be found between these two types of dispersion. The results of this study confirm, that micro- and macro-dispersion processes of silica happen simultaneously, but independently. Based on Scanning Electron Microscope (SEM) analysis, a silica dispersion model which can explain the simultaneous process of micro- and macro-dispersion is suggested: the stretched natto food model (Figure 7.14). When mixing at a lower temperature and without silane, macro-dispersion can be enhanced due to a high compound viscosity. However, at higher temperatures and with silane, macro-dispersion can also be improved depending on the degree of silanization. The presence of the silane enhances the degree of micro-dispersion, even more when the compound viscosity becomes lower at a higher temperature. As was seen in Figure 3.8, the penetration of polymer chains into the voids of silica clusters is a main factor for the micro-dispersion of silica. A higher mixing temperature will lead to an enhanced mobility of the polymer chains, therefore a higher degree of polymer penetration into the voids of silica clusters is possible. Additionally, the degree of silanization is strongly influenced by the mixing temperature as was seen in the previous chapters.
Regarding the material, the structure of the silica has not much effect on the micro-dispersion of silica, but strongly influences the degree of macro-dispersion due to the degree of packing of the silica clusters.
The degree of micro-dispersion turned out as a key factor for silica filled rubber reinforcement. A first indication was that micro-dispersion showed a good correlation with the reinforcement index (M200/M100), whereas macro-dispersion did not. A higher level of reinforcement indices was obtained for the compounds showing lower Payne effect values regardless of the degree of macro-dispersion. Secondly, a higher degree of micro-dispersion leads to enhanced reinforcement when silane is present. All in all, in case of silica-filled rubber compounds, the degree of silanization is crucial for both, macro- and micro-dispersion as well as rubber reinforcement.
Overall, this thesis confirmed the importance of the mixing process for tire manufacturing. The last stage in tire manufacturing – vulcanization – is still influenced by the first process – mixing – even though there are several steps in-between.
In a nutshell, the degree of silanization turned out a key factor for:
- reducing marching modulus;
- a higher degree of silica dispersion in terms of micro- and macro- scale;
- a higher reinforcement effect of for a silica filled rubber compound.