Abstract
The most significant factor, which prescribes the enhancement of properties in rubber by the incorporation of nanoclay, is its distribution in the rubber matrix. The study deals with the utilisation of epoxidised natural rubber (ENR) as a polar compatibiliser to achieve better dispersion of nanoclay in non‐polar polymer matrix. Epoxidised natural rubber–nanoclay composites (EC) were prepared by solution mixing. The obtained nanocomposites were incorporated in ethylene propylene diene terpolymer (EPDM) with sulphur as a curing agent. The morphological studies proved the intercalation of nanoclay in ENR, and further incorporation of EC in EPDM matrix leads to exfoliation of nanoclay. Curing study demonstrated faster scorch time, cure time and increase in maximum torque for the nanoclay loaded EPDM compounds compared to pure one. Dynamic mechanical thermal analysis showed increase in storage modulus and lesser damping characteristics for the compounds containing nanoclay loading in EPDM matrix followed by substantial improvement in the overall mechanical properties.
Introduction
The chief aim of preparing polymer–nanoclay composites is to achieve a very high degree of dispersion of nanoclay aggregates in the polymer matrix, which can yield to very large surface areas. The better dispersion of nanoclay in the polymer matrix leads to remarkable enhancement in the overall properties. A lot of works have been done in clay filled nanocomposites for many thermoplastics and thermosetting polymers. However, the studies on rubber based nanocomposites constitute in lesser dimension.1
The achievement of better dispersion of nanoclay in the polymer matrix involves two main aspects. The primary aspect involves the compatibility between the polymer and nanoclay. The organically modified nanoclay which is polar, may not contribute to better dispersion upon direct incorporation in the non‐polar rubber like ethylene propylene diene terpolymer (EPDM). Hence a polar rubber, which is compatible with the matrix polymer, can be used as a compatibiliser for the better dispersion of nanoclay in the base non‐polar polymer matrix. Epoxidised natural rubber (ENR) obtained by epoxidation of 1,4‐polyisoprene, depicts higher glass transition temperature, increasing polarity and has a better compatibility with EPDM. Hence, ENR was chosen as a compatibiliser in this study. Arroyo et al., 2 Teh et al.3 and Varghese et al.4 carried out works using ENR as a compatibiliser for organoclay/natural rubber nanocomposites. From our laboratory, we have already reported the effect of nanoclay loading using ENR as a compatibiliser in various rubber matrixes such as natural rubber, 5 , 6 styrene butadiene rubber7 and nitrile butadiene rubber.8
The secondary aspect is the method used for the preparation of nanocomposites. In the present study, incorporation of nanoclay in ENR was done by solution mixing. The obtained ENR–nanoclay composites (EC) were incorporated in the EPDM matrix with sulphur as a curing agent. The changes obtained in the morphology, curing characteristics and mechanical properties have been analysed and compared to the control.
Materials
Ethylene propylene diene terpolymer was Royalene 502 (ethylene 63%, propylene 37%, ethylidene norbornene 4%).
Epoxidised natural rubber containing 50 mol.‐% epoxidic units was supplied by Agricultural Product Processing Research Institute, Zhanjiang, China.
Cloisite 20A, a natural montmorillonite modified with a quaternary ammonium salt with cation exchange capacity of 95 mequiv./100 g clay (Southern Clay, Inc., Gonzales, TX, USA), was used as a nanofiller in the preparation of the nanocomposites.
Other compounding ingredients such as sulphur, zinc oxide, stearic acid, N‐cyclohexyl‐2‐benzothiazyl sulphenamide and tetramethylthiuram disulphide were purchased from Bayer (M) Sdn Bhd Malaysia.
Methods
Solution mixing
Epoxidised natural rubber was dissolved in toluene. The rubber to solvent ratio was 1∶3, weight/volume. Vigorous stirring was done at room temperature, until the rubber dissolved completely in the solvent. Nanoclay (Cloisite 20A) containing 50 phr was separately dissolved in toluene and ultrasonicated for 10 min. The solution containing nanoclay was then mixed with the rubber solution and the whole mixture was ultrasonicated for 10 min followed by stirring. The obtained solution was then cast over in a plane glass plate and kept at room temperature for solvent evaporation. The resultant film was appeared to be transparent.
Compounding
The compounding formulation was portrayed in Table 1.
Formulation of rubber compounds
*Parts per hundred of rubber by weight.
†ENR–nanoclay composites.
‡N‐cyclohexyl‐2‐benzothiazyl sulphonamide.
§Tetramethylthiuram disulphide.
The compounds are prepared in open two‐roll mixing mill operated at room temperature. The speed ratio of the rotors was 1∶1·4. Vulcanisation of the rubber compounds was performed in the compression moulding machine at 150°C, as per the optimum cure time obtained from Monsanto Rheometer study.
Characterisation techniques
X‐ray diffraction
The clay gallery height was analysed using a Philips PANalytical X'pert PRO X‐ray diffractometer instrument with Cu Kα radiation. The generator voltage and wavelength was 40 kV and 0·154 nm at room temperature respectively. The crystallographic spacing (d) of the nanoclays was calculated using Bragg's law. The range of 2θ scanning of X‐ray intensity employed was 1·5–10° at a scanning rate of 2° min−1.
High resolution transmission electron microscopy (HRTEM)
The morphology of nanoclay dispersion in ENR and EPDM matrix were observed through a high resolution transmission electron microscope (JEOL 2100). Sample preparation was done in a Leica Ultra cut UCT ultramicrotome equipped with a diamond knife. The thickness of the ultra thin specimens was ∼80 nm and the temperature of the samples was maintained at −50°C with the help of liquid nitrogen. These samples were then placed on the copper grid.
Cure characteristics
The cure characteristics of the compounds were studied in the Monsanto Rheometer R‐100 testing instrument operated at 150°C with a 3° arc over a period of 60 min.
Dynamic mechanical thermal analysis
The storage modulus (E′) and loss tangent (tan δ) of the compounds are analysed using a TA Instrument DMA 2980 model in tension mode. The samples are subjected to a sinusoidal displacement of 0·1% strain at a frequency of 1 Hz between the temperatures ranging from −80 to 40°C at a heating rate of 3°C min−1.
Mechanical testing
Dumbbell and crescent shaped specimens were cut from the moulded slabs for tensile and tear tests. The testing was carried out using a Universal tensile testing machine, Hounsfield HS 10KS model operated at room temperature at an extension speed of 500 mm min−1 with an initial gauge length of 25 mm. The values were recorded directly from the digital display at the end of each test.
Scanning electron microscopy
The morphology of tensile fractured surface was viewed through a scanning electron microscope (SEM, VEGA TESCAN). Gold coating was done under vacuum conditions to prevent electrostatic charge while examining.
Results and discussion
X‐ray diffraction
The X‐ray diffraction patterns of pure nanoclay, EC, EP/5EC and EP/10EC are shown in Fig. 1.
X‐ray diffraction patterns of pure nanoclay (Cloisite 20A), EC, EP/5EC and EP/10EC
The nanoclay (Cloisite 20A) showed an intense peak around 2θ = 3·144°, corresponding to the basal spacing 2·82 nm (d001). Incorporation of nanoclay in ENR (EC) pattern depicted the d001 main diffraction peak at 2θ = 2·31° corresponding to the basal spacing of 3·88 nm (d001). Hence, the main peak of the nanoclay had been shifted towards to the lower angle upon incorporation in ENR. Thus the basal spacing was increased due to the penetration of rubber chains in between the clay platelets proving the formation of an intercalated structure. In addition, along with the main peak, two accompanying peaks arose at 4·63 and 6·9° corresponding to the basal spacing 1·93 and 1·29 nm. This may be due to some re‐aggregation of nanoclay layers. However, the peak diminished relative to the main peak assuring the presence of an intercalated structure.9 The peaks found in EC pattern completely disappeared when 5 and 10 phr of EC was incorporated in the EPDM matrixes. This proved that the intercalated nanoclay in ENR becomes exfoliated upon incorporation in the EPDM matrix.
HRTEM analysis
Figure 2 shows HRTEM images of the nanocomposites for EC, EP/5EC and EP/10EC respectively.
Images (TEM) of a EC, b EP/5EC and c EP/10EC
Bunching of nanoclays was identified in the EC image, the dark lines found represent the silicate layers. This proved that the majority of nanoclay platelets are intercalated upon incorporation in ENR. Low loading of ENR–nanoclay in EPDM (EP/5EC) showed better dispersion of nanoclay platelets in the bulk EPDM matrix compared to the one that contains higher nanoclay loading in EPDM (EP/10EC). The nanoclay platelets may be partially exfoliated in both the compounds (EP/5EC and EP/10EC), but the agglomeration of nanoclay was found to be minimum for the compound EP/5EC that contains lower nanoclay loading.
Cure characteristics
The cure characteristics of the rubber compounds are shown in Table 2.
Cure characteristics of rubber compounds
The minimum and maximum torque value showed a tremendous increase for the nanoclay filled compounds compared to the control. The compounds EP/5EC and EP/10EC depicted 33 and 67% increase in minimum torque, 20 and 33% enhancement in maximum torque value compared to that of pure EPDM. The maximum torque depends on both the extent of crosslinking and reinforcement by the filler particles in the polymer matrix. Hence the higher reinforcing efficiency of the nanofiller in the EPDM matrix contributes to the enhancement in maximum torque.
The nanoclay incorporated compounds showed faster scorch and cure time than that of pure one. This may be due to the presence of ammonium groups in the organically modified nanoclay. The possible formation of a Zn complex in which sulphur and ammonium modifier participate may contribute to the increase in rate of cure. Hence, the formation of Zn–sulphur–ammonium complex in the nanocomposites is dependent on the ammonium concentration in the nanoclay. The greater the ammonium concentration, the shorter the scorch time and cure time.10
Dynamic mechanical thermal properties
The dynamic storage modulus (E′) and loss factor (tan δ) of the rubber compounds are shown in Fig. 3a and b respectively.
a storage modulus and b tan δ of EPDM, EP/5EC and EP/10EC
The compounds containing nanoclay composites showed drastic increase in storage modulus. At 25°C the compounds EP/5EC and EP/10EC showed 89 and 342% increase in storage modulus values compared to that of the control. This may be because of good dispersion of nanoclay that formed exfoliated platelets in the non‐polar EPDM matrix due to the presence of a polar compatibiliser.
The compounds with gradual increase in nanoclay loading showed consecutive decrease in tan δ peak compared to pure EPDM. This may be due to the reinforcing tendency of the nanofiller in the rubber matrix. Reduced chain mobility owing to physical and chemical adsorption of the EPDM molecules on the filler surface causes height reduction of tan δ peak during dynamic mechanical deformation.11 The decrease in tan δ peak proved minimum heat build‐up as a result of lesser damping characteristics for the compounds containing nanoclay composites.
Mechanical properties
The mechanical properties of the compounds are shown in Fig. 4. The nanoclay filled compounds showed tremendous increase in the mechanical properties compared to the control. The compounds EP/5EC and EP/10EC showed 82 and 100% increase in tensile strength, 146 and 167% increase in tear strength, 44 and 82% increase in modulus value compared to pure EPDM. The percentage elongation at break showed an increase for the compound EP/5EC and with increasing nanoclay loading for the compound EP/10EC it was found to be similar to pure ones. Lower loading of nanofiller may contribute to higher reinforcing efficiency of the filler in the matrix, due to its exfoliation. With increasing nanofiller loading, it may reduce the reinforcing efficiency of the filler due to some agglomeration of the nanoclay platelets. The exfoliation of the nanoclay in the EPDM matrix and better interaction between the filler and rubber may be responsible for the improvement in the mechanical properties.
a tensile, elongation at break of EPDM, EP/5EC and EP/10EC and b 100% modulus and tear strength of EPDM, EP/5EC, EP/10EC
Scanning electron microscopy
The SEM images of the tensile fractured surfaces are shown in Fig. 5.
Images (SEM) of a EPDM, b EP/5EC and c EP/10EC
Compounds EP/5EC and EP/10EC depicted highly rough and tortuous path of fracture compared to pure EPDM. This may be due to better interaction between the nanofiller and the matrix. The exfoliated nanoclay platelets in the EPDM matrix alter the crack path along their length depending on their orientation in the matrix. Hence, it forms more resistance to crack propagation as a result of higher tensile strength.
Conclusion
The morphological studies proved the intercalation of nanoclay in ENR and exfoliation of nanoclay, when EC was incorporated in the EPDM matrix. The curing study showed shorter scorch time, cure time and increase in maximum torque for the nanoclay filled compounds compared to pure EPDM. Dynamic mechanical thermal analysis results showed tremendous improvements in storage modulus and decrease in tan δ value consequently upon gradual increase in nanoclay loading in the EPDM compounds, and this corresponds to the higher reinforcing efficiency of the nanofiller in the matrix. Furthermore, these nanoclay filled compounds showed better enhancement in mechanical properties. Scanning electron microscopy images of fractured surfaces displayed that increase in roughness and tortuous path for the nanoclay filled EPDM compounds may be due to the better interactions between the filler and matrix.