RHEOMETRIC AND DYNAMIC MECHANICAL ANALYSIS OF COMPLEX NATURAL RUBBER BASED COMPOSITES

1 Research Center for Environment and Materials, Macedonian Academy of Sciences and Arts, Krste Misirkov 2, 1000 Skopje, Republic of Macedonia 2 Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University, Rugjer Bošković 16, 1000 Skopje, Republic of Macedonia 3 Leibniz-Institut für Polymerforschung–Dresden, Hohe Straße 6, 01069 Dresden, Germany 4 Institut für Werkstoffwissenschaft, Technische Universität–Dresden, 01062 Dresden, Germany


INTRODUCTION
Natural rubber (NR) is used extensively in many different industrial and structural applications due to its outstanding mechanical properties: high resilience, high tensile and tear properties and good crack growth resistance.The mechanical properties of rubber as a soft material mostly depend on reinforcement by fillers which are an inevitable ingredient in rubber compounds.Colloidal fillers, like carbon black (CB) or silica, play an important role in this improvement of the mechanical properties of high performance rubber materials.However, due to their high structure and tendency to agglomerate, strong shear fields or filler modification are needed to ensure fine dispersion.Furthermore high quantities are needed to provide a sufficiently high level of reinforcement but on detriment of elasticity [1].
But not all desired properties of the filled rubber could be achieved by addition of low quantities of nano-particles.For instance, the energy dissipating capacity, a property that is significant in the design of structures and mechanical devices connected to problems involving mechanical resonance and fatigue, shaft whirl, heating and cyclic stress, is mainly governed by the filler-filler interactions and demands higher quantities of filler present in the rubber matrix.
Therefore, recently, an increasing interest is becoming evident for hybrid systems based on both nanofillers and conventional fillers [18][19][20][21].These works show significant enhancement of material dynamic-mechanical properties thanks to the use of the hybrid filler system.It was reported by Galimberti et al. [18,21] that the initial modulus values obtained with the hybrid CB-organo-montmorillonite (OMt) and CB-nanographite filler systems are much higher than those calculated through the simple addition of the two initial moduli of the composites containing only conventional fillers or only nanofillers.
In this study we performed thorough rheometric and dynamic mechanical analyses on complex NR based composites containing both nanoand conventional fillers.As a hybrid nano-filler a combination of 2 phr of multi-wall carbon nanotubes (MWCNT) and 16 phr of expanded organomontmorillonite (EOMt) was used.The optimization of this ratio was done in our previous study [22].The overall content of the conventional fillers in all composites was kept constant at 40 phr in which the content of the individual fillers CB and silica were varied form 0 phr to 40 phr.Our interest was to investigate how the different combinations of fillers influence the processability and crosslinking of the rubber compounds.The main goal was to explore the rubber-filler and filler-filler interactions in order to understand the mechanism of energy dissipation in these complex NR-based systems.Wang [23] suggested that breakdown and reformation of the filler network, accompanied by the release of the trapped rubber from the filler network, causes an energy dissipation process during dynamic deformation.This mechanism suggests that the Payne effect or strain dependence of dynam-ic modulus of rubber can serve as a measure of filler networking which originates from filler-filler interaction as well as polymer-filler interaction [23].
DMA strain sweep measurements, carried out by applying cyclic deformations in the tensile mode at two different frequencies, 2 Hz and 10 Hz, respectively, were used to determine the loss factor, tanδ and G′ (storage) modulus.This study is part of a wider project which purpose is obtaining complex rubber compound suitable for base seismic isolation and these frequencies, seemingly quite close to each other, lay in the range in which most of the earthquakes are dominant and that is 0-15 Hz [24].Finally, the CCA-model was implemented to understand more deeply the Payne effect, the strain dependency of the storage modulus, which is considered to be one of the mechanisms of energy dissipation.

Preparation of the hybrid rubber composites
The procedure of preparation of the expanded organo-montmorillonite (EOMt) is described in detail in ref. [22].For the compounds containing nanofillers, the MWCNT were firstly dispersed in ethanol in a ratio 1 : 20 by weight.This pre-dispersion is required regarding the safe manipulation of the MWCNT, but it also enhances their dispersion in the rubber [16].Then the batch composed of 100 phr NR and 10 phr MWCNT was mixed in an internal mixer (Haake Rheomix) at a fixed rotor speed of 60 rpm, at 90 °C for 20 min.Afterwards, this masterbatch was diluted with NR to a target of 2 phr MWCNT and was mixed together with 16 phr of EOMt, N330 and Ultrasil VN3 (for both fillers content was varied from 0 to 40 phr), ZnO, 6PPD and stearic acid in the internal mixer at 90 °C with 60 rpm rotor speed for 10 min.For the compound containing only conventional filler the NR was mixed together with 58 phr N330, the ZnO, 6PPD and stearic acid in the internal mixer at 90 °C with 60 rpm rotor speed for 10 min.In the last step of the preparation the vulcanization accelerators TBBS and CBS together with the sulfur were added in an open two roll mill (Polymix 110 L, size: 203 mm × 102 mm Servitec GmbH, Wustermark, Germany) at 90 °C.The milling was performed at 0.5 mm and later at 0.3 mm nip gap for 5 min.The formulations of NR compounds expressed as parts per hundred of rubber (phr) are shown in Table 1.The test samples were molded and cured to 2 mm tick plates by compression molding (150 kN) on an electrically heated hydraulic press at 150 °C.The rubber samples were vulcanized up to their respective optimum cure time (t90), previously determined with a vulcameter (Scarabaeus, Langgöns, Germany), and then stored for 24 h before the tests were performed.

Characterization
Curing studies were performed using Scarabaeus SIS-V50, a rubber processing analyzer (Scarabaeus, Langgöns, Germany) in the isothermal time sweep mode at 150°C for 60 min.
Amplitude sweep measurements of the dynamic mechanical analysis were performed on rectangular samples with dimensions 10 mm × 35 mm using an Eplexor 2000 N (NETZSCH GABO Instruments GmbH, Ahlden, Germany) in the tension mode at room temperature, at a frequency of 2 Hz, a static tensile load of 60% pre-strain was applied and then the samples were oscillated to a dynamic load from 0% to 40% strain, and at a frequency of 10 Hz, a static tensile load of 60% pre-strain was applied and then the samples were oscillated to a dynamic load from 0% to 50% strain.

RESULTS AND DISCUSSION
The rheometric curves and rheometric characteristics of the gum compound and the com-pounds with hybrid nano-and conventional fillers are shown in Figure 1 and Table 2, respectively.It is evident that the presence of fillers (both hybrid and conventional) results in an increase of the maximum rheometric torque as compared to the gum.
The compound containing 40 phr of reinforcing CB and the hybrid nanofiller (2 phr MWCNT and 16 phr EOMt) showed the highest maximum torque value and then it decreases with the decrease of the CB and increase of silica content.The lowering of the torque in the filled rubber compounds is a common fact when non-reinforcing clay, calcium carbonate and silica without silane coupling agent, such as in this case, are used as fillers [25].The influence of the different fillers on the curing parameters is quite complex.The scorch time reduces significantly when 40 phr CB and the hybrid nanofiller are added to the gum compound, from 6.54 minutes to 1.74 minutes.For both nanofillers such an effect is already reported [1,26,27].At present there are no clear explanations how MWCNT interfere with the cure chemistry, but it is plausible to think that the catalyst employed for the synthesis of MWCNT which remained trapped inside the carbon material may affect the vulcanization [27].On the other hand, the accelerating effect of the organoclay is essentially attributed to the presence of an amine modifier inserted into the clay galleries, since it is well known that amine groups activate the functional groups of the accelerants, giving rise to a synergic effect that leads to a faster and more effective vulcanization reaction [1,27].The same trend of changes can be observed in the optimum cure time (given by t90).What is interesting is its significant reduction when silica filler is introduced into the compound.The introduction of silica also markedly influences the curing kinetics of the compounds, which can be interpreted in terms of the cure rate index (CRI).CRI is a parameter proportional to the average slope of the cure curve (100/t90-t2) in the curing step region.The higher the value of CRI, the faster is the curing process.It is apparent that the curing rate is significantly increased when silica is added to the com-pound compared to the gum and the compound containing the hybrid nanofillers and only CB.This influence of silica on the curing parameters is in agreement with other studies which reported similar phenomena [28,29].This higher viscosity could be also observed as higher values of minimum torque determined from rheometer cure curves (Figure 1).In order to explore the rubber-filler and filler-filler interactions in these complex NR-based composites DMA strain sweep measurements were performed at two different frequencies 2 Hz and 10 Hz.It is well known that the storage modulus for the filled rubbers depends on deformation amplitude, besides on frequency and temperature.This nonlinear behavior is the so called Payne effect [30,31] and is explained with the existence of a fillerfiller network in the polymer matrix, above the filler percolation threshold.The strain dependence of the storage modulus (E') and the loss factor (tanδ) at 2 Hz and 10 Hz are reported in Figure 2a and b and in Figure 3a and b, respectively, for the NRgum compound and the NR composites containing conventional and hybrid nanofillers.There is a pronounced non-linear strain dependence of the storage modulus (Figures 2a and 3a) for all composites, especially for those containing high CB content (NR-h-CB40-S0, NR-h-CB30-S10).It is interesting to note that there is a more pronounced difference between the storage modulus of the compounds containing a high amount of CB at higher frequen-cy of 10 Hz, while for the compounds with a high silica content this difference is more pronounced at lower frequency of 2 Hz.This is connected to the structure of the filler networks and the way they respond to a cyclic dynamic strain.Tanδ (a measure of the dissipation energy) follows this trend, namely, a remarkable increase for all composites, especially for those containing high amounts of CB, compared to the NR-gum.

NR-gum NR-h-CB40-S0 NR-h-CB30-S10 NR-h-CB20-S20 NR-h-CB10-S30 NR-h-CB0-S40
To understand deeply the Payne effect and to describe this strain-dependency of the storage modulus, we will use the cluster-cluster aggregation (CCA) model.The storage modulus, well above the percolation threshold, can be approximated by a function of the Havriliak-Negami type [32]: Here, γc is the strain amplitude, where half of the clusters are broken, m being an empirical exponent and τ ≈3.6 is the elasticity exponent of percolation [33].
The theoretical models for filled rubber are developed for shear modes but we attempted to use this model to predict the dynamic behavior in tension mode.
Figure 4 and Figure 5 show the experimental results of the strain-dependency of the modulus E' for the different NR based composites containing different combinations of fillers and as seen already with this approximation a fairly good description of the Payne effect is possible.The solid lines correspond to fitted curves according to Eq. ( 1).
The fitting parameters ΔE'0, γc, E'∞, and m are summarized in Table 3.It becomes obvious that their values are affected by the type of the fillers present in the NR matrix but also by the frequency, except for the empirical Kraus-parameter m.The composites containing more CB exhibit a more pronounced Payne effect, which is reflected in the highest value of ΔE'0 for the NR-h-CB40-S0 composite.Addition of silica reduces the ΔE'0 value which is lowest for NR-h-CB0-S40 composite.The same trend is observed for both frequencies with the difference that for the higher frequency of 10 Hz the Payne effect is more pronounced than for the frequency of 2 Hz.A similar behavior is observed for the large strain E'∞ values that are dominated by the pronounced hydrodynamic amplification of structured particles like CB. E'∞ is the highest for the NR-h-CB40-S0 composite and then its value decreases with increase of silica content.On the other hand, these structured filler particles like CB, lead to a lower stability of the filler network, as seen by the lower γc-values for the composites containing more CB (NR-h-CB40-S0 and NR-h-CB30-S10).This value of γc (the strain amplitude, where half of the clusters are broken) increases when silica is more present in the NR matrix.The largest γc-values of 12.91% and 13.6% (for 2 Hz and 10 Hz respectively) are found for the NR-h-CB0-S40 composite, suggesting the highest stability, which is due to poor compatibility of silica filler to hydrocarbon rubber, its polar character and the ability to form hydrogen bonds [34].The empirical Kraus-parameter m, that considers the power law dependency of the number of surviving clusters on the applied strain, shows very small variation with the type of applied fillers and also negligible variations with frequency [32].

CONCLUSION
Based on the above work, devoted to thorough rheometric and dynamical mechanical analyses of complex NR based composites, the following conclusions can be drawn.
The rheometric studies showed the highest maximum torque value for the composite containing 40 phr CB and then it decreased with decrease of CB and increase of the silica content.The scorch time and optimum cure time reduced significantly when 40 phr CB and the hybrid nanofiller were added to the gum compound and further decreased with addition of silica, suggesting a better processability of these rubber composites and faster curing process.
The dynamic mechanical analysis showed a pronounced non-linear strain dependence of the storage modulus and a remarkable increase of the loss factor tanδ for all composites, especially for those containing high CB content, compared to the NR-gum.The influence of the frequency on the storage modulus was as follows: a more pronounced difference between the storage modulus of the composites containing high CB content at higher frequency of 10 Hz, and opposite to this, a more pronounced difference for the composites containing high silica content at lower frequency of 2 Hz.This might be connected to the structure of the filler networks and the way they respond to a cyclic dynamic strain.
The CCA-model gave a fairly good description of the Payne effect.From the calculated parameters ΔE'0, γc and E'∞ it is obvious that their values are affected by the type of the fillers present in the NR matrix but also by the frequency.The composites containing more CB exhibit more pronounced Payne effect, a trend that is observed for both frequencies with that difference that for the higher frequency of 10 Hz the Payne effect is more pronounced.On the other hand, CB as structured filler particles, lead to a lower stability of the filler network, as seen by the lower γc-values for the composites containing more CB (NR-h-CB40-S0 and NR-h-CB30-S10).This value of γc increases when silica is more present in the NR matrix, suggesting a higher stability, which is due to poor compatibility of silica filler to hydrocarbon rubber, its polar character and the ability to form hydrogen bonds.

Figure 1 .
Figure 1.Rheometric curves if the gum compound and the compounds with hybrid nano-and conventional fillers

Figure 2 .
Figure 2. Strain dependence of E' (a) and tan δ (b) for the NR-gum and NR composites at 2 Hz

Figure 3 .
Figure 3. Strain dependence of E' (a) and tanδ (b) for the NR-gum and NR composites at 10 Hz

Figure 4 .
Figure 4. Plot of storage modulus vs. strain amplitude for the different NR based composites at 2 Hz

Figure 5 .
Figure 5. Plot of storage modulus vs. strain amplitude for the different NR based composites at 10 Hz

Table 1 .
Formulation of the NR compounds * Mass of the ingredients was taken in parts per hundred of rubber (phr).The ingredients which amounts were kept constant in all compounds were ZnO 5 phr, stearic acid 2 phr, 6PPD 1 phr, CBS 1.5 phr, TBBS 0.2 phr and sulfur 1.5 phr.

Table 2 .
Curing characteristics of the NR based compounds