MONTMORILLONITE-REINFORCED NATURAL RUBBER NANOCOMPOSITES THROUGH EMULSION STABILIZATION–DESTABILIZATION METHOD
Abstract
A nanoplatelet of montmorillonite (MMT) was incorporated in the natural rubber (NR) matrix at a high loading content using a newly developed stabilization–destabilization process in the colloid states. Examination of the surface charge of the colloid drops by zeta potential measurement led to the identification of the stable and unstable conditions of the colloid mixture as well as each colloid system as a function of pH. The stabilized mixture of the MMT and NR colloids was subsequently destabilized by changing the pH of the mixture to achieve a well-dispersed and intercalated NR/MMT precipitate. The resulting NR/MMT nanocomposites provided an exfoliated MMT morphology up to 25 phr, and the mechanical properties and aging tolerance of the resulting nanocomposite vulcanizates were substantially enhanced by the incorporated MMT nanofillers. The developed stabilization–destabilization methodology ensured a highly loaded MMT rubber composite system, which could be used to obtain high performance NR/MMT nanocomposite systems with ease without using high-shear processing techniques.
INTRODUCTION
Rubber materials are widely used in the automobile industry, in such applications as vibration insulators, hoses, and seals, because of their high flexibility, three-dimensional design freedom, adjustable material properties, and low manufacturing cost. Because of recent trends in vehicle development, for example, rubber parts have been exposed to higher environmental temperatures due to the enhancement of the power-train performance and more compact engine room space. Also, the miniaturization of automotive parts for the sake of cost and weight reduction often cause the rubber materials to be exposed to more hostile load-bearing conditions. Moreover, the thermal stability and long-term durability required in the automobile industry are gradually increasing so that the product lifetime may ultimately reach a maintenance-free level.1
The mechanical strength and elastic modulus of rubber have been improved by introducing reinforcing fillers, such as carbon black and silica in general. Recently, nanoparticle-sized fillers have been reported to improve the elastic modulus, failure properties (tensile and tear strength), abrasion resistance, gas barrier properties, thermal stability, and electrical conductivity of their nanocomposite systems.2–5 In particular, nanoparticle-sized clays such as montmorillonite (MMT), hectorite, and saponite have received attention as excellent reinforcing fillers because they are cost effective and eco-friendly materials that can easily be obtained from nature.2 Those layered silicate clays have a large surface area and high surface reactivity with a strong negative charge, and, therefore, a small amount of nanoclay included in the rubber matrix may substantially enhance various physical and mechanical properties of nanocomposites.3,5 The structure of nanocomposites is classified as either intercalated or exfoliated based on the interlayer distance of the clay. In the intercalated structure, the interlayer distance is increased by the rubber of polymer chains placed between the silicate layers of the clay.6 Meanwhile, in the case of an exfoliated structure, the layered silicates are fully delaminated and lose their layered regularity in the matrix. The exfoliated structure is regarded as the ideal one for reinforcement because the exfoliated nanocomposites generally give greatly improved properties by the incorporation of a small amount of clay, say, <5 wt%.7 The melt intercalation method has been commonly used for the dispersion of layered silicates in the rubber matrix systems,6 where high-shear stress is applied by internal mixers or two-roll-mill mixers. However, in spite of the numerous studies on polymer/clay nanocomposites, good dispersion and desired exfoliation of clays is known to be difficult to achieve because of the large lateral dimensions of the layers, high intrinsic viscosity of matrices, and strong attractive forces between the layers arising from the electrostatic attraction and van der Waals force.5,8
The most popular nanoclay filler, the MMT nanoclay, is composed of stacked nanoplatelets with regular interlayer distance, and its interlayers are normally occupied by alkaline metal cations in nature such as sodium (Na+), potassium (K+), calcium (Ca+), and magnesium (Mg+). These metallic cations can be exchanged by organic alkyl ammonium salts with long chains, and this organic modification makes the clay surface organophillic to give organo-modified MMT (OMMT).9 Many studies have been reported on how to organically modify the pristine clay and how to compound rubber/clay nanocomposites using organo-modified clays.7–18 Generally, primary ammonium salts such as dodecylamine (laurylamine), tetradecylamine (myristylamine), hexadecylamine (cetylamine), and octadecylamine (stearylamine) have been used in organic modifications.7,10,11,13,15,16 Quaternary ammonium salts such as hexadecyltrimethyl ammonium bromide, octadecyltrimethyl ammonium chloride, and triethylene tetraammonium chloride have been applied as well.11–13 These organic modifications greatly increase the compatibility between the rubber and inorganic clay, but the fully exfoliated structures have seldom been obtained by the melt compounding method because of the high viscosity of rubber and strong attractive forces in between the layers.9–10
Compared with the melt process, the latex compounding method is a cost effective and eco-friendly method that does not require high-shear stress or hazardous solvents. Moreover, the viscosity of rubber latex is very low in the emulsion state, which is particularly advantageous in rubber compounding. The latex blending method has been applied to rubber/clay nanocomposites previously, but the exfoliated states of composites were not clearly proved to be obtained6,19–22 because the clay colloid and the rubber latex emulsion were simply mixed without considering the dispersion stability and the attractive forces between the rubber latex drops and clay particles. The mixture of the two colloids by a typical mixing process could be coagulated immediately after the mixing in the liquid state, which makes the exfoliation of MMT difficult because of the unstable colloid condition in pH.19 The organic modifiers containing amine or ammonium groups have been used for the clay/water colloids to modify the clay surfaces. A fully exfoliated structure could not be achieved in the nanocomposites, but a slight increment in the basal spacing, from 1.5 to 4.3 nm, was achieved.9
For the nanoclay particles to be well mixed and exfoliated, the solid-state compounding process should follow only when both rubber latex and aqueous clay colloid are prepared to be stable followed by mixing. We believe that the rubber and clay colloids should be mixed in the stable condition for the mixture of the two to be homogeneous, which could guarantee the MMT clays to be exfoliated in the rubber matrix. In this research, the stable and unstable conditions of the rubber latex emulsion and aqueous clay suspension were identified using the evaluation of the zeta potential. Then, the stable mixture was forced to coagulate by changing the pH condition from a stable to an unstable condition.23 The MMT clay was modified by dodecylamine in the stable suspension condition, as specified by the zeta potential, and successfully exfoliated in the natural rubber (NR) latex. The mechanical properties and thermal stability of the resulting MMT/NR nanocomposites were thoroughly investigated in this study.
EXPERIMENTAL
MATERIALS
Centrifuged NR latex with a 60% dry rubber content (stabilized with 0.64 wt% ammonia, pH 10.35) was purchased from Srijaroen Latex Company (Bangkok, Thailand). The solid NR, Standard Malaysian Rubber/Standard Thai Rubber Conventional Vulcanization 60 (Mooney viscosity, ML1′+4′ @ 100 °C = 60), was supplied by Chunjin-Chemtec Company (Seoul, South Korea). The pristine clay (sodium montmorillonite, Na+-MMT) has a cation exchange capacity (CEC) of 98 mequiv/100 g and minimum particle size of 45 μm, and the OMMT (Cloisite® 25A modified with dimethyl dehydrogenated tallow and 2-ethylhexyl quaternary ammonium) had a CEC of 95 mequiv/100 g (purchased from Southern Clay Products, Austin, TX). Dodecylamine with a minimum purity of 97% was supplied by Tokyo Chemical Industry Company (Tokyo). All other rubber ingredients were obtained from local rubber chemical suppliers including antioxidants of N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine (6PPD), polymerized-2,2,4-trimethyl-1,2-dihydroquinoline resin (TMQ), and paraffin wax; cure accelerators of tetra methyl thiuram disulfide (TMTD) and N-cyclohexyl-2-benzothiazole sulfonamide (CBS); activators of zinc oxide (ZnO) and stearic acid; and cure agent of sulfur.
NR NANOCOMPOSITES PREPARATION
The composition of NR latex, MMT, and the other ingredients used in the compounding of the nanocomposites are presented in Table I. The Na+-MMT was dispersed in water (pH 10.5) in advance with continuous stirring using a homogenizer for 30 min at 800 rpm and 80 °C. In order to prepare the OMMT colloid, dodecylamine at a CEC of 1.5 was dissolved in the MMT colloid under vigorous stirring for 30 min at 800 rpm and 80 °C. In this study, a new compounding method, which we called the “stabilization–destabilization method,” was introduced by mixing the NR latex (167 phr, i.e., solid NR 100 phr), dodecylamine at a CEC of 1.5, and MMT colloid with mechanical agitation at 400 rpm at 80 °C for 30 min. Then, a solution consisting of 20 ml of HCl and 400 ml of distilled water was added to the NR/OMMT colloid in order to lower the pH and cocoagulate the NR/OMMT mixture. Afterward, the solidified NR/OMMT was rinsed until the pH value of the mixture reached about 7 and dried in an oven at 70 °C for 24 h.
The NR/MMT and NR/OMMT nanocomposites were prepared by either the traditional colloid mixing method or the developed stabilization–destabilization method followed by mixing in a two-roll mill (roll speed ratio 1:1.4) for 10 min, where the ingredients listed in Table I, viz., the vulcanizing agent, accelerators, activators, and antioxidants, were incorporated. For comparison with the latex compounding method, a traditional melt mixing method was used to prepare the NR composites with Na+-MMT and a commercialized OMMT (Cloisite 25A) fillers by using a two-roll mill. The curing was designed to comply with semiefficient vulcanization, that is, using a 0.8 to 2.5 ratio of sulfur to accelerators. The cure characteristics of the resulting rubber/MMT nanocomposites were monitored using a moving die rheometer (MDR, Myungjitech Co., Seoul, South Korea) at 160 °C for 720 s. All the nanocomposite test specimens were prepared by compression molding using a hot press (Korea Mtech Co., Gwangju, South Korea) at 160 °C for the vulcanization time determined from the MDR tests.
CHARACTERIZATION OF NANOCOMPOSITES
The zeta potentials of the NR latex and MMT colloids were measured by laser Doppler electrophoresis with a Zetasizer Nano-Z (Malvern Instruments Ltd., Worcestershire, U.K.) using a folded capillary cell (DTS 1060; Malvern Instruments). The calibration of the instrument was performed with DTS 1050 latex beads (zeta potential transfer standard, Malvern), where the freeze-dried microspheres were suspended in a buffer (Hepes 5 mM, pH 7.0, or ammonium acetate 5 mM, pH 6.0) and homogenized thoroughly before the measurement (0.5–1% w/v).
The cure characteristics of the uncured nanocomposites were determined using a MDR (Myungjitech Co.) at 160 °C for 720 s. X-ray diffraction (XRD) analyses of the MMT, OMMT, and rubber/clay nanocomposites were performed in an Xpert-Pro diffractometer (PANalytical B.V., Almelo, the Netherlands) operated at 30 kV and 30 mA using reflection mode at a scan rate of 1°/min with Cu Kα X-ray radiation (=1.54 Å). The basal spacing was calculated using the 2θ angles obtained from the XRD data. Prepared by a microtome in a liquid nitrogen environment, the NR/MMT nanocomposites were examined using a transmission electron microscope (TEM, Philips CM200 FEI Co., Hillsboro, OR) at an acceleration voltage of 120 kV.
The Shore A hardnesses of the vulcanizates were measured according to ISO 7619-1 using a HD-1110 type A durometer (Ueshima Seisakusho Co., Tokyo, Japan) and five different spots of the sample (over 6 mm in thickness) were measured for being averaged. Tensile tests were carried out according to ISO 37 (Rubber, vulcanized or thermoplastic, determination of tensile stress–strain properties). Dumbbell-shaped specimens of the nanocomposite vulcanizates (type 1A, 2 mm in thickness) were tested on a Z010 automatic universal testing machine (Zwick Roell AG, Ulm, Germany) at a speed of 500 mm/min. The tensile strength, elongation at break, and stress at strains of 100%, 200%, and 300% for five specimens were tested and averaged.
The tear strength was measured by ISO 34-1 at a tension speed of 500 mm/min on a Z010 Universal Testing Machine taking the average of five specimens. The tension fatigue properties were measured by ISO 6943 using an FT-3103 constant elongation fatigue tester (Ueshima Seisakusho Co.). Five dumbbell-shaped specimens (Type 1A in ISO 37) were repeatedly loaded with sinusoidal deformation from 0 to 100% engineering strain in the tensile direction at a constant speed of 200 cycles/min (3.33 Hz), and the numbers of fatigue cycles reaching the complete rupture of specimens were averaged on a logarithmic scale.
The thermal-aging properties of the nanocomposite vulcanizates were tested according to ISO 188 method A. Dumbbell-shaped specimens (ISO 37) were aged in a cabinet type air-oven with ventilation for 1000 h at 85 °C. After the aging, the Shore A hardness, tensile strength, and elongation at break were measured and compared with those of the pristine specimens without aging. The resistance to ozone cracking was tested by ISO 1431-1 at a constant ozone concentration of 50 ± 5 parts per hundred million and a constant temperature of 40 ± 2 °C. Dynamic mechanical thermal analyses (DMTA) were carried out according to ISO 4664 using an Eplexor® 150 N dynamic mechanical thermal spectrometer (DMTS, Gabo Qualimeter Testanlagen GmbH, Ahlden, Germany) with rectangular specimens (length × width × thickness = 35 mm × 5 mm × 2 mm) in a tensile mode. The dynamic amplitude dependence of the storage modulus and loss factor (tan δ) at room temperature was measured with dynamic amplitudes of 0.2%, 1.0%, and 4.0% superposed on a static strain of 10.0% at a constant frequency of 25 Hz. The temperature dependence of the viscoelastic properties was measured in the temperature range from −80 to 100 °C at a heating rate 3 °C/min with dynamic amplitudes of 0.25% superposed on a static strain of 3.0% at a constant frequency of 10 Hz.
RESULTS AND DISCUSSION
COLLOID STABILITY BY ZETA POTENTIALS IN NR LATEX AND MMT COLLOID SYSTEMS
The zeta potential of the NR and the MMT colloids are shown in Figure 1. The NR colloid may be regarded as stable over pH 8, where the zeta potential for the electric double layer is lower than −30 mV. On the other hand, the OMMT, which is organically modified by dodecylamine, may be regarded as stable at pH values over 10, where its surface electric charge becomes less than −30 mV. Therefore, in this study, pH 10.5 was selected to ensure the stable and dispersed states for both MMT and NR colloid systems, where the electrostatic repulsive force makes the negatively charged MMT particles and NR drops repel each other. When these stable colloid systems are mixed in pH equal to or higher than 10.5, it is reasonable to consider that the MMT particles and NR drops are well distributed in the liquid, ensuring an exfoliated state of MMT in NR. In this state of mixture, the pH can be artificially lowered in order to destabilize the colloid mixture of NR and MMT, where the surface electric charge of NR becomes positive and the MMT remains negative (e.g., see pH at 3 in Figure 1), which subsequently makes the MMT particle and the NR drop attract each other to precipitate as a master-batch mixture.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88978
MMT/NR MIXTURE CHARACTERIZATION
Figure 2 shows the X-ray diffraction patterns for the pristine Na+-MMT, OMMT by dodecylamine, and NR/OMMT nanocomposite with various OMMT contents of 5, 10, 20, and 25 phr after vulcanization. For Na+-MMT, a diffraction peak appears at a diffraction angle of 2θ at 7.28° corresponding to the basal spacing at 1.22 nm. After being organically modified by dodecylamine, the interlayer spacing of OMMT expands to about 1.83 nm. For the OMMT composites containing 5, 10, 20, and 25 phr of OMMT, the X-ray diffraction pattern shows no clear peaks appearing in the range of our experiments except 5 phr. In this 5 phr composite sample, a small peak can be seen at around 2.0° corresponding to 4.4 nm of the basal spacing seemingly due to a small amount of intercalated MMT platelets included in the mixing process. Overall, there is no evidence of clear (100) basal spacing of MMT platelets demonstrating that the OMMT/NR nanocomposite systems are almost or completely exfoliated. It should be addressed that the exfoliated MMT content of 25 phr has never been achieved by any other processing methods like two-roll compounding processes, which usually employs very high mechanical shear stress.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88978
In order to obtain better insight into the MMT dispersion, the TEM images of thin cuts of the nanocomposites are presented in Figure 3. The dark and slightly curved lines are the cross-sections of single or multiple MMT layers. Figure 3a, in which a commercial OMMT (Cloisite 25A) was mixed with NR by the melt compounding method, shows that the MMT platelets are not completely exfoliated with some MMT tactoids composed of over 10 lamella. Figure 3b,c shows that the nanocomposite specimens compounded in the colloid states are better than the melt mixing one in light of MMT dispersion and exfoliation. Counting the number of lamella of the MMT stacks, the colloidal-mixing composites in Figure 3b,c are composed of ca. 5 lamella, which is less than the melt mixing one.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88978
CURE CHARACTERISTICS OF NANOCOMPOSITES
Figure 4 presents the cure characteristics of the various rubber compound systems measured using an MDR. Comparing Figure 4a,b, the melt mixing NR composite with commercial OMMT (Cloisite 25A) gives 15% higher maximum torque and 25% shorter curing time than the pristine NR. The incorporated OMMT nanoparticle-sized platelets increase the elastic modulus of the nanocomposite vulcanizate. The cure initiation and speed seems to be affected by the alkyl ammonium used for the organic modification and acidic chemicals used for the coagulation of MMT. As seen in Figure 4c,d, the maximum torques of the nanocomposites mixed with OMMT by the latex stabilization–destabilization method are 20% to 25% higher than that of the pristine NR without MMT, which is further higher than the specimen in Figure 4b. The overall behaviors in Figure 4c,d are similar, and the maximum torque of the nanocomposite compounded by the proposed latex stabilization–destabilization method is 5% higher, which can be ascribed to the better exfoliation and binding with rubber. Figure 4e shows the cure characteristics of the nanocomposite incorporated with pristine Na+-MMT using the latex method, showing the slowest curing speed among all the compound systems or pristine NR. Conclusively, the OMMT nanoplatelets incorporated in NR accelerate the curing reaction rate seemingly because of the organic modifier of alkyl ammonium and/or the catalytic effect of the negatively charged MMT nanoplatelets.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88978
MECHANICAL PROPERTIES OF NR/MMT NANOCOMPOSITES
Table II summarizes the mechanical properties of the various nanocomposite vulcanizates: (a) pristine NR without MMT, (b) melt compounded NR containing commercial Cloisite 25A OMMT, (c) NR/OMMT nanocomposite compounded by the traditional latex method, (d) NR/OMMT nanocomposite compounded by the stabilization–destabilization latex method, and finally (e) NR/Na+-MMT nanocomposite compounded by the latex method. Overall, the Shore A hardness of the nanocomposites filled with 5 phr of MMT or OMMT is increased by 3 to 8 Hs, and the elastic modulus of the MMT reinforced nanocomposites is drastically increased by up to 147%. Especially, the Shore A hardness of the nanocomposite filled with 5 phr of OMMT prepared by the stabilization–destabilization latex method shows 8 Hs, which cannot be achieved by any other filler systems, for example, carbon blacks or silica, which usually gives an increase of 1 Hs per 2 to 3 phr content of those fillers. The elongations at break of the MMT-filled rubber compounds are observed to be at almost the same level as that of the unfilled NR compound, that is, the high flexibility of NR is maintained after it is reinforced by nanoparticle-sized clays. However, the tensile strengths of the nanoclay-filled compounds are increased by 20%, 28%, 57%, and 54% for compounds b, c, d, and e, respectively, due to an effective reinforcement by the high surface area of the exfoliated MMT.
As also summarized in Table II, the fracture toughness of the nanoclay-filled nanocomposites is evaluated by measuring the tear strength and tensile fatigue life of the vulcanizate. The tear strengths of the nanoclay-filled compounds are higher than that of the unfilled NR rubber: 14%, 19%, 38%, and 66% increment for compounds b, c, d, and e, respectively. It can be seen that the MMT reinforcement of the rubber vulcanizates is very effective in enhancing tear resistance, that is, retarding the crack initiation and propagation at the point of high stress concentration during a single monotonic large tensile deformation. The fatigue lives of the MMT-filled NR vulcanizates under repetitive tensile deformation are decreased by −62%, −28%, and −25% for compounds b, c, and e, respectively, in comparison with the unfilled NR compound, seemingly due to the high probability of crack initiation at the rubber/clay interface. Note that the NR with the commercial OMMT made by melt compounding method b exhibits the worst fatigue properties. This result demonstrates that the latex compounding method is more effective than the melt compounding method in light of crack initiation and propagation as a result of enhanced bonding and excellent dispersion achieved by the liquid-state mixing process. In particular, nanocomposite d compounded by the proposed stabilization–destabilization latex method exhibits fatigue life even longer than the unfilled NR compounds by 2% because the fully exfoliated structure of MMT and effective boning with NR can be achieved by the proposed stabilization–destabilization method.
In the case of compound e incorporated by Na+-MMT by the latex method, a similar level of Shore A hardness and tensile properties were obtained compared with compound d; moreover, a higher value of the elastic modulus and higher tear strength were observed than those of the other OMMT-filled compounds. Direct bonding between the NR molecules and Na+-MMT nanoplatelets seems to be more effective than the one via organic modifiers such as an alkyl ammonium. However, the direct bonding may be effective in elastic reinforcement but ineffective in the improvement of the fatigue resistance, since the fatigue life of compound d is 36% higher than that of NR/MMT compound e. Conclusively, the NR/OMMT nanocomposite compounded by the proposed latex method shows substantially improved mechanical properties that can be achieved by the newly developed latex compounding method based on the colloid stabilization–destabilization methodology.
THERMAL-AGING RESISTANCE OF NR/MMT NANOCOMPOSITES
The mechanical properties of the NR/MMT nanocomposite rubber vulcanizates are presented in Table III after thermal aging at 85 °C for 1000 h. The tensile strength and elongation properties of NR/MMT rubber vulcanizates in samples b through e show higher deterioration of properties than the unfilled NR system (sample a), seemingly because the interface between the MMT platelets and NR matrix is damaged more significantly by thermal aging. The Shore A hardness of the NR/MMT rubber vulcanizates, however, is not as damaged as the unfilled NR system by thermal aging. It seems that the ultimate failure properties, like strength or elongation, of the MMT-incorporated composites are damaged more significantly by thermal aging, but the Shore A hardness or stiffness properties are likely to be sustained through thermal aging.
It is interesting to compare samples f and g in Table III, both of which are prepared through the roll-mill melt process of MMT/NR composites. Sample f, which is composed of NR with unmodified MMT, exhibits much higher values in Shore A hardness, tensile strength, and elongation at break than sample e, which is composed of the same composition as sample f without the step of melt processing. In sample g, the NR/OMMT master batch containing 20 phr of OMMT was prepared by the proposed stabilization–destabilization process followed by a roll-mill melt compounding process adding 300 phr of NR solid in order to meet the MMT content at 5 phr in the resulting NR/OMMT compound. Comparing sample g with samples c and d in Table III, it can be seen that sample g exhibits the best tolerance characteristics in thermal aging. Therefore, the MMT content in sample g should be the same as other samples in Table III. As can be seen in Table III, the tensile strength and Shore A hardness of sample g are superior to the other samples. In conclusion, the best aging-resistant properties can be achieved by using a high-content MMT master batch followed by a melt mixing process. In the case of the compounding of the master batch, we believe that the thermal-aging resistances of the NR/MMT nanocomposites are substantially improved to give the best thermal-aging stability seemingly due to the barrier effect and tortuous path provided by the large-surface clay platelets in the nanocomposite system.24 We also believe that the melt compounding process enhances the NR/MMT interface and wetting characteristics to give robust bonding and excellent dispersion.
DYNAMIC VISCOELASTIC PROPERTIES OF NANOCOMPOSITES
Figure 5 presents the ratio of the dynamic (Ed) to static (Es) moduli, Ed/Es, and loss factor, tan δ, of the nanocomposite compounds measured in the DMTA. Generally, the dynamic-to-static amplitude ratio and the loss factor are increased when fillers like carbon black or silica are incorporated, since the dissipation of the elastic deformation energy and corresponding hardening are induced by the friction at the filler–filler and rubber–filler interfaces.25 The dynamic modulus of the carbon black–filled rubber gradually is increased, and the loss factor first increases and then decreases as the dynamic amplitude decreases approximately below a dynamic strain of around 5%; the resulting nonlinear viscoelastic behavior is known as the Payne effect.25 The turn-over amplitude of the loss factor of the silica-filled vulcanizates is known to be around 50%, which is higher than that of the carbon black ones because the silica filler is generally modified by the silane coupling agent to induce chemical bonding and compatibility.25 The nanocomposites in this study exhibit a similar behavior in their dynamic properties except sample c. In the case of nanocomposite c prepared by the typical latex method, a sufficient amount of organic modifiers may be bound to the MMT surface, and this modification may prevent interfacial friction. The incomplete chemical bonding between the rubber and fillers in the other compounds seems to result in the interfacial friction and the parasitic hardening of the vulcanizates. However, the turn-over amplitudes of the loss factor might be higher than that of the carbon black rubber system (around 5%), since the loss factor increases with a similar rate up to a dynamic amplitude of 5%. It is thought that the chemical bonds between the rubber molecules and MMT or OMMT are also tighter than between the rubber molecules and carbon black because of the ions on the MMT interlayer surfaces.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88978
Figure 6 and Table IV present the temperature dependency of the viscoelastic loss factors of the rubber compounds. The glass transition temperatures (Tg) are slightly affected by the nanoclay fillers. When the rubber is filled with carbon black, the maximum loss factor at Tg is decreased.25 The maximum loss factors of samples b, d, and e are lower than those of the unfilled rubber, and that of sample d prepared by the proposed latex stabilization–destabilization method shows the lowest value. The highest reinforcing effect in sample d is likely a result of the fully exfoliated structures of MMT and efficient chemical bonds providing the lowest decrease in the maximum loss factor. However, the maximum loss factor of compound c is higher than that of the unfilled rubber because of the chemical bonds as previously mentioned. Unusually, the Na+-MMT-filled nanocomposite shows a decrease of 1.5 °C in its glass transition temperature, seemingly because the low degree of chemical bonds between the pristine Na+-MMT and NR makes the composite flexible at low temperatures.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88978
Remarkable enhancements of the Shore A hardness, elastic modulus, tensile strength, and fatigue fracture toughness were achieved from the NR/MMT nanocomposites compounded by the proposed latex method. The ability to produce the NR/MMT composites at high loading contents without using high-shear mixing is a significant step toward meeting the increased needs for mass production of high performance NR composite systems.
CONCLUSIONS
A novel latex compounding method combining stabilization–destabilization and the organic modification method in a single process was proposed to prepare high performance NR/OMMT nanocomposites. In order to identify the stability of the NR latex and Na+-MMT suspended in emulsion form, the Zeta potential was measured from pH 3 up to pH 10. The NR latex and MMT water were mixed at pH 10.5, at which both were stable, to produce a stable colloid mixture. The alkyl ammonium (dodecylamine) was directly added to the suspended NR/MMT mixture to modify the interfacial surface, and then the coagulated mixtures of NR and OMMT were obtained by artificially lowering the pH below 4 as the destabilization condition. When the XRD patterns and TEM images were analyzed to verify their dispersion and structure, the NR/OMMT nanocomposites were observed to have completely exfoliated structures up to an OMMT content of 25 phr.
Zeta potential of NR latex and OMMT colloid systems plotted as a function of pH.
XRD patterns of Na+-MMT powder, OMMT (dodecylamine) powder, and their NR/OMMT nanocomposites at various OMMT contents.
TEM micrographs of NR/OMMT nanocomposites: (a) 5 phr Cloisite 25A OMMT by melt compounding, (b) Cloisite 25A OMMT by traditional latex compounding, and (c) OMMT by the proposed stabilization–destabilization compounding methods.
Cure characteristics of various rubber compounds (extracted for 300 s @ 160 °C).
Dynamic-to-static modulus ratio and loss factor as a function of dynamic amplitude for various NR/MMT nanocomposite systems.
Temperature dependence of loss factor (extracted from −60 to 0 °C) for various NR/MMT nanocomposite systems.
Contributor Notes