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Tensile Stress Relaxation Studies of TiO2 and Nanosilica Filled Natural Rubber Composites A. P. Meera,† Sylve`re Said,‡ Yves Grohens,‡ A. S. Luyt,§ and Sabu Thomas*,† School of Chemical Sciences, Mahatma Gandhi UniVersity, Priyadarshini Hills P.O., Kottayam, Kerala, India 686 560, Laboratoire Polyme`res, Proprie´te´s aux Interfaces et Composites, Centre de Recherche, UniVersite de Bretagne Sud, Rue Saint Maude´, Lorient Cedex, France, and Department of Chemistry, UniVersity of the Free State (Qwaqwa Campus), PriVate Bag X13, Phuthaditjhaba, 9866, South Africa
Composites of natural rubber were prepared with TiO2 and nanosilica. The stress relaxation behavior of the composites under tension was studied with reference to the filler loading and strain level. It was observed that the rate of stress relaxation increases with increase in filler loading. The rate of stress relaxation was found to be higher for silica-filled NR compared to TiO2-filled NR. This is due to the high degree of agglomeration in silica compared to titanium dioxide filler. The effect of ageing on the stress decay was also investigated and the rate of stress relaxation was found to decrease after ageing. The experimental curves were fitted with the stretched Kohlrausch equation. From the fitting parameters, the relaxation time and the stretching exponent were estimated in order to understand the mechanism of the relaxation processes in the filled natural rubber composites. Introduction One of the most important phenomena in material science is the reinforcement of rubber by rigid particles such as carbon black, silica, aluminum oxide, clays (both unmodified and organically modified clays), calcium carbonate, aluminum, zinc, kaolin, red earth, carbon nanotubes etc.1-16 A variety of nonblack fillers are used in the rubber industry to improve and modify the physical properties of elastomeric materials. The addition of filler usually leads to the increase in modulus and to significant improvements in abrasion and tear resistance. Calcium carbonate, clay, precipitated silica, talc, and titanium dioxide are used in white sidewall compounds to impart desired physical properties and appearance.17 It has been well-known for decades that the addition of rigid filler particles, even in small amounts, to an elastomer strongly influences its response to mechanical behavior. Fillers are incorporated in rubbers mainly to increase the stiffness of the material, and to change the time dependent aspects of the material such as hysteresis, creep, and stress relaxation.18-21 Since creep and stress relaxation tests measure the dimensional stability of a material, and because the tests can be of a long duration such tests are of great practical importance. As far as the service performances of articles are concerned, the long-term properties such as stress relaxation, creep, and dynamic mechanical properties provide much information regarding the time dependent viscoelastic nature of polymers. Creep is the increase in deformation with time under a constant load, and stress relaxation is the decay of stress with time under conditions of constant deformation. The rates of the two processes can be related to each other if the shape of the forcedeflexion curve is known.22 Relaxation processes can be thought of in terms of the effect of thermal motion on the orientation of polymer molecules. Mullins and Tobin studied the stress-strain behavior in particulate filled rubber vulcanizates and proposed a phenomenological model for the stress-softening phenomenon known * To whom correspondence should be addressed. E-mail: sabut@ sancharnet.in;
[email protected]. † Mahatma Gandhi University. ‡ Universite de Bretagne Sud. § University of the Free State (Qwaqwa Campus).
as “Mullin’s Effect”.23 According to their observation, the filler network is composed of hard and soft regions. The hard regions contain a high volume fraction of filler and the soft regions contain a low volume fraction of filler. On applying mechanical stress to the sample, hard regions are converted into soft regions. A two-step relaxation mechanism was reported by Varghese et al.24 in the relaxation behavior of acetylated short sisal fiber reinforced natural rubber composites. The first relaxation step is due to the progressive failure of the polymer-fiber interface and the second step is due to the relaxation of the polymer molecules. Bhagawan et al. observed an increase in stress relaxation with increase in short jute fiber content in an NBR matrix.25 The stress relaxation behavior of natural rubber reinforced with coir fibers was studied by Geethamma et al.26 The introduction of particulate fillers in rubber generally results in an increase in relaxation rates. According to the theory of “strain amplification”,27 due to the inextensibility of the filler, the strain in the polymer phase of a filled material is greater than the overall strain. The rate of stress relaxation increases with increasing polymer extension, thus filled rubbers would be expected to show higher relaxation rates. In filled rubbers other reasons for the higher relaxation rates can be associated with the breakdown of filler-filler and polymer-filler networks. Kostov et al.28 studied the stress relaxation process in tetrafluroethylene-propylene rubbers at different temperatures and reported three relaxation processes in cross-linked elastomers which are associated with changes in the chemical network. The stress relaxation behavior of synthetic rubberorganoclay has been studied at different level of extensions by Privalko et al.29 Recently Tosaka et al. studied crystallization and stress relaxation behavior of highly stretched NR samples and its synthetic analogue using synchrotron X-ray.30 While filler particles are known to alter the important aspects of the macroscopic stress-strain behavior of elastomeric matrices, the mechanism by which the alterations occur is still a subject of debate. Although several studies have been conducted on the stress relaxation behavior of filled rubber composites, the role of filler structure, morphology, and filler networking on the relaxation behavior is not well explored and the relaxation mechanisms are not fully understood. The main objective of the present study is to
10.1021/ie801494s CCC: $40.75 2009 American Chemical Society Published on Web 03/09/2009
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Table 1. Characteristics of Titanium Dioxide properties
values
density oil absorption average particle diameter specific gravity pH
0.90 g /cm3 17 g /100 g pigment 190 nm 4.1 6.0-8.0
Table 2. Characteristics of Nanosilica properties
values
specific surface area (BET) average particle diameter carbon content moisture pH SiO2 content
160 ( 25 m2/g 12-13 nm 2.0-4.0 wt % e0.5 wt % g5.0 >99.8 wt %
Table 3. Formulation of Mixes ingredient
recipe, phr
rubber zinc oxide stearic acid TMQ a CBS b sulfur filler
100 5 1.5 1 0.6 2.5 variable
a N-Cyclohexyl-2-benzothiazyl-sulfenamide. b 2,2,4-trimethyl-1,2-dihydroquinoline, polymerized.
Figure 2. Stress relaxation curve for silica filled natural rubber. Table 4. The Negative Slopes of the Relaxation Curvesa samples
negative slopes
gum T10 T20 T30 T40 S5 S10 S20
0.0129 0.0126 0.034 0.0308 0.0401 0.0257 0.0406 0.0542
a Sample coding: T is given for TiO2-filled NR composites and S for silica-filled NR composites. The suffix indicates the filler loading.
Figure 1. Stress relaxation curve of TiO2-filled natural rubber.
investigate the relaxation behavior of filled natural rubber with special reference to particle size, filler morphology, filler loading, and strain level in order to understand the mechanism of the relaxation in vulcanized natural rubber. In this paper, the stress relaxation behavior of silica and TiO2-reinforced natural rubber has been investigated in detail. Experimental Section Materials. Natural rubber (NR) used for the study (ISNR-5) was procured from the Rubber Research Institute of India, Kottayam. The molecular weight of NR obtained by light scattering is Mn ) 2.68 × 105 g/mol and Mw ) 8.38 × 105 g/mol. All other ingredients used were of commercial grade and were kindly supplied by LANXESS. The filler titanium dioxide, TiO2 (KEMOX RC 800 PG), was supplied by Kerala Minerals and Metals Limited (KMML) Kollam, India. The surface modified nanosilica (AEROSIL R
Figure 3. Rate of stress decay for NR filled with 20 phr fillers.
8200) was obtained from Degussa, Germany. The characteristics of the fillers are given in Table 1 and Table 2, respectively. Preparation of the NR Composites. Formulation of composite mixes is shown in Table 3. Compounding was performed in an open two-roll mixing mill (laboratory size). NR was first masticated on the two-roll mill for about 2-3 min followed by the addition of the ingredients. Cure characteristics were analyzed using an Oscillating disk rheometer (Monsanto R-100) at a temperature of 150 °C. The composites were cured at their respective cure times in a hydraulic press under a pressure of about 120 bar at 150 °C. Aged samples were prepared by keeping the samples in a hot air oven at 70 °C for seven days.
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Figure 4. SEM micrographs of the composites (a) T20 and (b) S20.
Figure 5. Effect of strain level for the pure NR gum.
Figure 7. Effect of strain level for NR filled with 20 phr silica.
Stress Relaxation Experiments. The stress relaxation measurements were carried out on a Hounsfield H5KS tensile machine (Hounsfield Test Equipment, Redhill, England). Rectangular samples (60 mm × 12 mm × 1.5 mm) were used for the test. The measurements were done at room temperature (25 °C). The test speed to attain the required strain was 100 mm/min. Decaying of stress was monitored for 90 min, and the stress monitoring was started immediately after the required strain was attained. The stress decay was monitored for a constant strain of 20% for all the measurements except for those with different strain levels. The measurements were done at least for five samples in each case to make sure that the results are reproducible. Results
Figure 6. Effect of strain level for NR filled with 40 phr TiO2.
Morphological Analysis. The morphology of the cryofractured composites were analyzed by scanning electron microscopy (JEOL 6400 WINSEM model (Jeol, Tokyo, Japan) at 5 KeV) and by atomic force microscopy done with the Park systems (XE-100 AFM) Park SYSTEMS, South Korea. The samples for SEM were prepared by first cryogenically fracturing them in liquid nitrogen. They were then gold plated to eliminate the problems of sample charging. The SEM micrographs were obtained using 5 KeV secondary electrons. The AFM samples were prepared by cryogenically fracturing them in liquid nitrogen, and the topographic AFM images were obtained.
Stress Relaxation: Effect of Filler Loading and the Nature of the Filler. Figure 1 and Figure 2 represent the stress relaxation plots, [(σt/σo) vs time, t, plot] for titanium dioxide (TiO2) and silica-filled natural rubber, respectively. The stress decay was monitored for a constant strain of 20%. In both the filled systems, the rate of stress relaxation was found to be increasing with filler loading and was the lowest for the unfilled rubber. This is quite evident from the increase in slopes (negative) of the relaxation curves with increase in filler loading given in Table 4. To evaluate the effect of the nature of the filler with special reference to the particle size, surface area and surface morphology on the relaxation behavior, the stress relaxation experiments were performed for 20 phr of the two fillers (Figure 3). A higher rate of stress decay was observed for silica-filled natural rubber (NR) than TiO2 filled system. The (negative) slopes of the composites were
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after ageing for both TiO2 and silica-filled rubber. This is clear from the decrease in slopes observed for the curve after ageing (Figure 8a and Figure 8b). Discussion
Figure 8. Effect of aging on the stress decay (a) T40 and (b) S20.
0.0340 and 0.0542 for TiO2 and silica filled-rubber, respectively. The scanning electron micrographs (SEM) are compared in order to evaluate the extent of agglomeration (Figure 4a,b). It can be seen that the extent of agglomeration is more in the case of silica filled NR than that in the TiO2 filled system. Effect of Strain Level. The effect of strain level on the relaxation behavior was studied at two different strain levels for the unfilled rubber and the composites with selected loading. Figure 5 represents the stress relaxation plot for the unfilled rubber at two different strain levels. The rate at which the initial strain attained was kept constant for all the measurements. It was observed that the increase of strain level from 20% to 200% does not have any effect on the rate of relaxation of the unfilled rubber. The slope values were 0.012 and 0.01, respectively. In the case of composites, no significant difference in slopes has been observed. For the composites the slopes of the curves for 20% and 200% strain were almost same as shown in Figure 6 and Figure 7. The change in slope in the case of the composites T40 and S20 is not substantial as clear from the figures. Effect of Ageing. The stress relaxation behavior of the aged and unaged samples was compared to evaluate the effect of ageing on the stress decay of the composites. Aged samples were prepared by keeping the samples in a hot air oven at 70 °C for 7 days. The experiment was carried out at room temperature at a strain level of 20%. The test was carried out for selected loading, for 40 phr TiO2, and for 20 phr silica. It was observed that the rate of stress decay decreases considerably
The process of stress relaxation takes place due to both physical and chemical changes in the rubber matrix. The initial (reversible) change is caused by relaxation of the polymer chains and fillers. Long-term (irreversible) changes occur due to chemical reactions over time. Let us briefly discuss the effect of filler loading on the relaxation behavior of the vulcanized rubber. In the case of filled rubbers, the desorption of polymer chains from the filler surface occurs and the dewetting starts or propagates during the time of observation and the rate increases greatly. In filled rubbers, the higher relaxation rates can also be associated with the breakdown of filler-filler and weak polymer-filler networks. It was already reported31 that the extent of such breakdown varies with the nature of the filler. The relaxation rate increased with increasing TiO2 loading. This is quite evident from the negative slopes of the unfilled rubber and the composites given in Table 4. For the unfilled rubber, the slope was 0.0129 and it increased to 0.0401 for T40. This is due to the progressive breakdown of the filler aggregates. The same trend of behavior was observed by Derham21 in carbon-black-filled rubber. At low loading, the chances of forming aggregates are comparatively less and hence good dispersion of the filler is achieved. So the rate of stress relaxation is almost comparable with that of the unfilled rubber. This is shown in a schematic representation (Figure 9). But at higher loading, due to the breakdown of the aggregates, the relaxation becomes faster. In the case of highly filled system, the polymer chains get physically adsorbed on the filler surface and there are multiple points of attachments of polymer chains at the filler surface as shown in the schematic. With the breakdown of the aggregates, the polymer chains get desorbed from the filler surface and hence the relaxation rate increases. A similar behavior has been observed for the silica-filled rubber as well. In the silica filled system, the negative slopes of the curves for the composites (S5, S10, and S20) were 0.0257, 0.0406, and 0.0542, respectively. The increase in slope is drastic in the case of silica-filled rubber. This is explained by the fine particle size of silica (12-13 nm) and high surface area (160 ( 25 m2/g). Because of the very small particle size and high surface area, silica particles tend to form aggregates even at low filler concentrations. These primary aggregates form weak secondary associations known as agglomerates. So in a silica-filled system, multiple points of attachments at the filler surface exist even at low filler concentrations due to the very high specific surface area. Now let us discuss the effect of the nature of filler on the relaxation rate. For that, the stress relaxation plot for 20 phr of the two fillers was compared. A higher rate of stress decay was observed for natural rubber filled with silica compared to TiO2filled system. This is evident from the slopes given in Table 4 which were 0.034 and 0.054, respectively. This means at 20 phr loading, the extent of formation of aggregates in the rubber matrix is different for the two fillers. TiO2 being a microparticle (0.19 µm) has got less tendency to form aggregates. Moreover, silica particles have available surface hydroxyl groups which leads to weak hydrogen bonding and the forming of associations, whereas TiO2 particles do not possess any active hydroxyl groups on their surface. This is one of the reasons why TiO2 has less tendency to form aggregates. This is shown in the
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Figure 9. Schematic representation of unfilled and filled rubber.
Figure 10. Schematic representation of silica- and TiO2-filled rubber.
Figure 12. Time dependence of the normalized stress for silica-filled NR. The dotted lines represent the curve fits. Table 5. Fitting Parameters of Equation 1a
T10 T20 T30 T40 S5 S10 S20 Figure 11. Time dependence of the normalized stress for TiO2-filled NR. The dotted lines represent the curve fits.
schematic picture (Figure 10). This observation was also supported by SEM micrographs (Figure 4). To discuss the effect of strain level on the relaxation rate of the unfilled rubber, relaxation curves at 20% and 200% were compared. The slopes were found to be almost the same (0.012 and 0.01, respectively). This is in agreement with the earlier reported observation that the slope of the stress relaxation curve of unfilled rubber is independent of strain levels below the threshold strain, which causes the phenomenon of strain-induced crystallization.32,33 At higher extensions, with increase in the ordering of molecules, crystallites are formed. The polymer chains in the crystallites are oriented in the direction of the applied force and they behave as ordinary crystal. This means that under the influence of an external field, vibrations of atoms occur in rubber about their equilibrium positions. This causes the stress to relax rapidly. This phenomenon is known as the strain-induced crystallization. However this occurs only beyond a threshold value of strain. The change in slope in the case of the composites T40 and S20 is not substantial. This may be because the strain level of 200% may be too low for crystallization to occur. The effect of ageing on the relaxation behavior was studied by comparing the rate of stress decay of the composites before and
σ∞/σo
σ1/σo
β
10-3 τ/sec
χ2/DoF
R2
0.96 0.89 0.86 0.83 0.84 0.83 0.82
0.04 0.13 0.17 0.20 0.18 0.18 0.22
0.41 0.38 0.28 0.31 0.32 0.31 0.33
0.06 0.04 0.15 0.17 0.07 0.07 0.03
3.1917 × 10-6 1 × 10-5 5.5787 × 10-6 2 × 10-5 5.4352 × 10-6 2 × 10-5 2 × 10-5
0.986 0.993 0.997 0.994 0.993 0.993 0.996
a Sample coding: T is given for TiO2-filled NR composites and S for silica-filled NR composites. The suffix indicates the filler loading.
after aging. During heat ageing several changes in the network structure can occur which include scission of the main chains and of the cross-links, the formation of more cross-links of the same type as those already present or of a different type, which may be immune to further scission. During ageing a lot of irreversible permanent changes can also occur. The rate of attack of atmospheric oxygen is increased by a rise in temperature. Hence an aged sample is not expected to relax more than the unaged one when the stressed sample is allowed to relax with time. This is evident from the slopes of the curves corresponding to the unaged and aged samples, which are 0.04 and 0.015, respectively, for T40 sample and 0.054 and 0.04, respectively, for S20. The decrease in the slope of the relaxation curve for the aged sample indicates a decrease in entropy and relaxation rate of the sample. The susceptibility to oxidative deterioration of the vulcanizates was confirmed during the stress relaxation at elevated temperatures.34 Modeling of the Stress Relaxation Behavior. To explain the observed behavior, the experimental relaxation curves of the composites were fitted to the stretched exponential Kohlrausch equation35 given by σt /σo ) σ∞ /σo + (σ1 /σo) exp[(t/τ)β]
(1)
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Figure 13. AFM topographic images of composites (a) T30, (b) T40, and (c) S10.
where σ∞/σo, σ1/σo,, τ, and β are the fitting parameters. Here τ is the relaxation time, β is a parameter known as the stretching parameter (0 < β e 1), σ1/σo is the transient stress and σ∞/σo is known as the limiting stress. The experimental curves fitted to the above equation are shown in Figure 11 and Figure 12 for TiO2 and silica-filled natural rubber, respectively. The dotted lines represent the obtained curve fits. The best-fit values of the parameters of eq 1 are given in Table 5. From the fitting parameters, the relaxation time, τ, and the stretching exponent, β, were estimated. The values of χ2/DoF and R2 to assess the quality of fits are given in the Table 5. The characteristic relaxation time, τ, and the transient stress, σ1/σo, values were found to be increasing with increase in TiO2 loading as can be seen from the table. The low values of the limiting (quasi-equilibrium) stress σ∞/σo combined with longer relaxation times for T30 and T40 compared to T10 and T20 suggest a mechanism of structural relaxation involving large-scale displacements of isolated clusters of filler within the rubber matrix. This is supported by AFM observations (Figure 13). The shorter relaxation time, τ, lower transient stress, σ1/σo, and higher values of σ∞/σo for low loading of TiO2 indicate that smaller scale structural rearrangement occurs in these systems during stress relaxation. At the same time, a substantial decrease in relaxation times has been
observed with increase in silica content. This is quite evident from the stress relaxation curves as well. The reduction in relaxation times estimated with increase in silica content indicates a greater extent of filler-network breakdown at higher loading. The degree of agglomeration can be clearly seen, as for example, for S10, in the AFM image (Figure 13c). The shorter relaxation time estimated for S20 compared to T20 also supports the experimental observation. Now let us comment on the practical application of stress relaxation studies. There are mainly two instances where stress relaxation is of direct practical interest. First, when a mechanical connection is attained by a friction fit, the stress responsible for the connection is not supposed to relax significantly. Hence stress relaxation is important in rubbers to measure their efficiency as seals or gaskets. If a tightening stress has relaxed too much, the seal may start to leak. However in other cases where an unfavorable stress distribution results from the fabrication of a part, it is desirable to relax such stress. Many products in service carry loads for far longer times up to a few years. For many applications such as bridge bearings, rubber must perform consistently over a long period. Hence the studies of stress relaxation behavior is important in many applications.
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Conclusions The stress relaxation behavior of natural rubber and its composites reinforced with TiO2 and nanosilica was investigated. The patterns of stress relaxation behavior were found to be qualitatively similar in both systems. The rate of stress relaxation was found to increase with increase in filler loading for both the filled systems. This is due to the breakdown of filler-filler and weak polymer-filler networks during the course of the relaxation process. The rate of stress decay was compared for 20 phr TiO2- and silica-filled NR. The silica-filled system showed a higher rate of stress relaxation compared to TiO2filled NR. This is due to the large scale filler cluster break-up in the silica-filled system. This was also supported by the estimated relaxation times. The effect of strain level on the relaxation behavior has also been investigated. It was observed that the increase of strain level from 20 to 200% does not have any effect on the rate of relaxation of the unfilled rubber and the composites. From this, it was concluded that a strain level of 200% was not sufficient for the strain-induced crystallization to occur. The effect of ageing on the relaxation behavior was also studied. From the decrease in slope of the stress relaxation curve after ageing it is understood that some irreversible changes like chain scission or cross-link formation take place during ageing. The experimental relaxation curves of the composites were fitted to the stretched exponential Kohlrausch equation and the relaxation times were estimated in order to understand the mechanism of relaxation processes in filled natural rubber composites. Finally, it is important to add that stress relaxation experiments are very important to understand the long-term dynamic behavior of rubber products in many applications. Acknowledgment We gratefully acknowledge Kerala Minerals and Metals Limited (KMML), Kollam, Kerala, for providing TiO2 and LANXESS Deutschland GmbH, Germany, for supplying the rubber chemicals for the research work. We thankfully acknowledge Park Systems, South Korea for the AFM studies. Thanks are due to DST, India, for financial support. Note Added after Print Publication: The title of the paper in the version of this paper that was published in print in Vol. 48, Issue 7, pp 3410-3416 and online March 9, 2009 was incorrect; because of a production error, two words were omitted by mistake. The current version, with the correct title, was reposted to the Web April 2, 2009. A formal Addition/Correction appears in Vol. 48, Issue 9. Literature Cited (1) Kraus, G., Ed. Reinforcement of Elastomers, Interscience: New York, 1965. (2) Blow, C. M. Rubber Technology and Manufacture; Butterworths: London, 1971. (3) Wagner, M. P. In Rubber Technology; Morton, M., Ed.; Reinhold: New York, 1987. (4) Bokobza, L. Some new developments in rubber reinforcement. Compos. Interfaces 2006, 13, 345–54. (5) Anuar, J.; Mariatti, M.; Ismail, H. Properties of aluminium and zinc filled natural rubber. Compos. Polym. Plast. Technol. Eng. 2007, 46, 667–74. (6) Osabohien, E.; Egboh, S. H. O. An investigation on the reinforcing potential of red earth as filler for natural rubber compounds. J. Appl. Polym. Sci. 2007, 105, 515–20. (7) Da Costa, H. M.; Ramos, V. D.; Visconte, L. L. Y.; Furtado, C. R. G. Design and analysis of single factor experiments: Analysis of variance of the effect of rice husk ash and commercial fillers in NR compounds. Polym. Bull. 2007, 58, 597–610.
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ReceiVed for reView October 3, 2008 ReVised manuscript receiVed January 9, 2009 Accepted February 6, 2009 IE801494S
