Montmorillonite clay nanocomposites based on vinyl pyridine-styrene-butadiene terpolymer (VPR)/acrylonitrile-butadiene rubber (NBR) blend.
The polymer clay nanocomposites have gained attention of researchers due to some major advantages that nanocomposites have over conventional composite. They are, lighter weight due to low filler loading and improved properties (includes mechanical, thermal, optical, electrical, barrier, etc.) compared to conventional composites at very low loading of filler. Polymer clay nanocomposites can be prepared pared by solution blending or direct intercalation or sol/gel technique or in-situ polymerization or by latex blending.
Several publications in the field of polymer clay nano-composites highlight the research work carried out till date using both plastic and elastomeric matrices (1-10).
Because of easy availability of the different rubbers in latex form and swelling capability of the clay in the water, the mixing of the latex with the layered silicates (having high cation exchange capacity) followed by coprecipitation (coagulation) is a promising route to produce rubber nanocomposites. Wang et al. prepared natural rubber (NR)- montmorillonite (MMT) and chloroprene rubber (CR)-MMT clay nanocomposites by co-coagulating the rubber latex and the aqueous clay suspension (11). Zhang et al. prepared clay (natural clay fractionated from ben-tonite)-styrene-butadiene rubber (SBR) nanocomposites by mixing the SBR latex with a clay/water dispersion and coagulating the mixture (12), Wang et al. compared the mechanical properties of clay (fractionated bentonite)/ SBR nanocomposites prepared by the solution and latex blending techniques (13) and found that at equivalent clay loadings, the nanocomposites prepared by the latex route were better than those prepared by the solution blending technique. Varghese and Karger-Kocsis (14) described the preparation of natural rubber layered clay nanocomposites by latex blending technique.
However, most of the aforementioned publications used Na (sodium)-activated clay or organo-modified clay which is costly. The cost is mainly due to the refinement processes of the naturally occurring clay and preparation of the Na-activated clay, which is the precursor of the organo-modified clay (15).
This study represents a novel route to synthesize vinyl pyridine-styrene-butadiene lerpolymer (VP rubber)-mont-morillonite clay nanocomposite by latex blending technique. The pyridine moiety of the VP latex was transformed to pyridinium moiety by treating it with methyl iodide. Cation exchange reaction of the pyridinium ion with sodium montmorillonite clay helped to form exfoliated/intercalated nanocomposite after co-coagulation with dilute sulfuric acid. The master batch was further compounded with NBR rubber. Since the VP rubber is polar in nature, NBR was taken as a base polymer.
The vinyl pyridine-butadiene-styrene terpolymer latex (VP latex) with 27% bound vinyl pyridine, 10% styrene, 63% butadiene, and 40% solid content was supplied by Jubilant Organosys, Borada India. Acrylonitrile butadiene rubber (NBR, Perbunan NT 3430, bound acrylonitrile content is 34% and Mooney viscosity 30 (ML [1 + 4] at 100[degrees]C) was purchased from Lanxess, Mumbai, India.
The rubber compounding ingredients used in this work were of commercial grade, viz. zinc oxide, stearic acid, sulfur, and trtramethyl thiuram monosulfide (TMTM). Methyl iodide was purchased from SD Fine Chem, Mumbai, India. Sodium montmorillonite clay (Kunipia F, high purity sodium montmorillonite clay marketed by Kunimin Industries, Japan) was obtained from Kunimin Industries, Japan (CEC 115 meq/100 g).
Synthesis of the In-Situ Modified VP Latex Montmorillonite Clay Nanocomposite Master Batch
About 10 g of clay was dispersed in 1000 mL of water at 80[degrees]C with constant vigorous stirring for 2 hr, after which, 100 g of VP rubber latex was added in to the water-clay slurry and the stirring continued for another 2 hr. Heating was discontinued at this step. After 2 hr, 19 g of methyl iodide was added. Before adding methyl iodide, the temperature was kept well below 40[degrees]C to avoid its boiling (boiling range of methyl iodide is 40-43[degrees]C). The slurry was once again stirred for 2 hr. The latex clay slurry was coagulated by dilute sulfuric acid and was washed several times with water. The coagulated master batch was dried under vacuum at 70[degrees]C. The dried master batch was further milled on a laboratory two-roll mill from Santosh Industries, India. The calculated amount of clay in the master batch was 20%.
Similarly, a master batch of VP rubber and clay was prepared without adding methyl iodide. Another master batch was prepared without clay. This may be treated as methyl iodide modified VP rubber.
Two-stage mixing of the compounds was carried out in a Brabender Plasticorder, model PL 2000-3 (Brabender, Germany) having a chamber volume of 80 [cm.sup.3], cam rotors, ram pressure weight of 5 kg, and batch weight of 70 g.
In first stage, mixing was done at 90[degrees] C at a rotor speed 60 rpm. Initially, NBR and VP rubbers were masticated for 60 s followed by the addition of clay or nanocomposites master batch. It was mixed for additional 9 min. In second stage mixing, the temperature control unit (TCU) was kept at 60[degrees]C at a rotor speed 60 rpm. The compound (mixed at first stage) was initially masticated for 60 s. Zinc oxide, stearic acid, sulfur, and accelerator were added and mixed for 4 min. The batches were sheeted out in the laboratory two-roll mill.
Compound formulations are reported in Table 1.
TABLE 1. Compound formulation. Material (phr) R Rl El E2 Acrylonitrile butadiene rubber (NBR) 80 80 80 80 Coagulated VP rubber 20 20 20 20 Unmodified Clay (Kunipia-F) 5 - - - Clay from clay-VP rubber master batch - - 8 - Clay from clay-methyl iodide modified - - - 5 VP rubber master batch Zinc oxide (ZnO) 3 3 3 3 Stearic acid 1 1 1 1 TMTM 3.5 j.3 3.5 3.5 Sulfur (S) 1.5 1.5 1.5 1.5
Compound "R" (regular compound) was the control compound with unmodified montmorillonite clay added directly (not from any master batch) during mixing. Compound Rl was unfilled compound with no filler. In case of compound "El" (experimental compound 1), montmorillonite clay was added from the master batch (clay/VP rubber) in which VP rubber was not modified with methyl iodide. In compound "E2" (experimental compound 2), clay was added from the master batch in which methyl iodide was added to modify the pyridine moiety of the VP rubber. Addition of 5 phi of clay in the compound (from a master of Clay/VP rubber containing 20% of clay) gave raise ~20 phr of VP rubber in the formulation. In the formulation, all the compounds had the same amount of VP rubber (i.e., 20 phr). Thus, the effect of the VP rubber was nullified.
Characterization of the VP RuhheriVP-NBR Rubber Montmorillonite Clay Nanocomposites
The Fourier transform infrared spectroscopy of the samples in the form of thin films (~100-/[much less than]n thick) was carried out on a System 2000 FTIR of Perkin Elmer, Norwalk, USA with a scan range of 400-4000 [cm.sup.-1] at a resolution of 4 c[m.sup.-1].
Wide angle X-ray diffraction measurements were carried out in a Philips 1710 X-ray diffractometer using a scan rate of 0.5[degrees] min ' with Cu K[alpha] target at 40 kV and 25 mA (wavelength = 0.154 nm) with 20 scan range from 2[degrees] to 10[degrees].
[FIGURE 1 OMITTED]
For transmission electron microscopy measurements, 100 nm sections were microtomed at -- 120[degrees]C using Ultracut E ultramicrotome (Reichert and Jung) with a diamond knife. Measurements were carried out with a Philips CM200 TEM at an acceleration voltage of 120 kV.
Energy dispersive X-ray spectrophotometry study was carried out with the help of Quantax 200 with X-FIash liquid nitrogen free detector from Bruker, Germany for the elemental mapping of the samples.
The rheometric properties were determined in a moving die rheometer (MDR 2000E) from Alpha Technologies, Akron, USA at 160[degrees]C for 30 min keeping the rotor arc at 0.5[degrees] in accordance with ASTM D 5289.
Curing of tensile slabs was done using a compression molding technique in an electrically heated curing press from Hind Hydraulics, New Delhi, India at 160[degrees]C for 15 min. The tensile samples were died out in accordance with ASTM D412 Type C die.
The stress-strain properties were determined using a universal testing machine, Zwick UTM 1445 from Zwick, Ulm, Germany in accordance with ASTM D412. The hardness was determined in a Shore A durometer from Prolific Industries, New Delhi, India in accordance with ASTM D2240.
The tear properties were measured according to ASTM D624. The Mooney viscosity was measured using MV2000E from Alfa Technologies, Akron, USA in accordance with ISO 289-1.
Cure rale index (CRI) was measured according to ASTM D5289. The following formula was used for the CRI in the study;
Swelling index of the cured samples was measured using the following formula in accordance with ASTM D3616. Dimethyl formamide (DMF) was taken as solvent.
Swelling Index = Swollen weight/Initial weight
Rebound resilience and compression set of the samples were carried out according to ISO 4662 and ASTM D395, respectively.
Bound rubber content was also measured by using the following formula
Bound rubber = ([M.sub.B] - [M.sub.F] - [M.sub.D])/([M.sub.B]X100)
where [M.sub.B] = weight of the uncured mix before immersing, [M.sub.F] = weight of the filler in the uncured mix, and [M.sub.D] =3 weight of the rubber dissolve in the solvent. The solvent used in this study was dimethyl formamide (DMF).
The physical properties are represented in Table 3.
TABLE 3. Physical property of the compounds. Parameter R Rl El E2 I00%mod (Mpa) 1.5 1.3 1.5 1.8 300%mod (Mpa) -- 3.8 3.9 6.2 TS (Mpa) 2.7 3.9 4.1 6.4 EB (%) 224 301 312 364 Hardness (S) 55 54 56 59 Swelling index 2.20 2.30 2.13 1.90 Tear (N m[m.sup.-1]) 13.8 12.9 18.2 24.2 Mooney (ML 1+4, 100 [degrees]C) 35.5 35.0 38.9 42.8 Rebound resilience (%) 50 49 50 50.2 Compression set (%) at 105C/24 hr 24.3 24.0 28.6 31.6 Bound rubber content (%) 15.6 - 19.2 26.8
RESULTS AND DISCUSSION
The FTIR spectra of VP rubber and methyl iodide modified VP rubber are represented in Fig. l.The samples were dissolved in DMF and a thin film was cast on the sodium chloride disc. The spectra of both the samples were identical, except for an additional peak exhibited by modified VP rubber at 1470 c[m.sup.-1] which was attributed to the presence of pyridinium salt (16), (17). This clearly indicated the modification of the pyridine moiety of the VP rubber. The FTIR spectra of compounds El and E2 are represented in Fig. 2. Both the spectra were identical except a small peak at 1471 c[m.sup.-1] in compound E2. The peak was attributed to the presence of pyridinium ion in compound E2. The intensity of the peak was low due to low concentration of the pyridinium ion in the matrix.
Dispersion Morphology of Clay Nanocomposite
[FIGURE 2 OMITTED]
WAXD Study. Figure 3 shows the X-ray diffraction patterns of Kunipia F, compound R, El, and E2, respectively. Kunipia F clay shows a characteristic diffraction peak at 20 = 7.0[degrees] corresponding to an inter-gallery distance of 1.26 nm. Compound R did not exhibit any shift in the diffraction peak and thus the inter-gallery distance in compound R remained in the same position at 1.26 nm. Thus the addition of kunipia F clay in the NBR-VP rubber matrix formed a conventional composite at a microscopic level, where polymer was not intercalated into the silicates galleries. In the case of compound El, the XRD diffraction peak was ~6.6[degrees] corresponding to a layer spacing of 1.34 nm. Little increase in the inter layer spacing indicating a very low level of intercalation. Blending of latex and clay slurry probably helped to form some intercalated morphology in the matrix. In compound E2, a clear shift in the XRD diffraction peak was observed. Compound E2 showed a characteristic diffraction peak at 2 [theta] = 3.4[degrees] corresponding to an inter-gallery distance of 2.61 nm. This is clearly indicating intercalated-exfoliated nature of the composites. Methyl iodide modification helped to form nanocomposites.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
TEM Study. The TEM micrographs of compounds R, El, and E2 are shown in Figs. 4-6, respectively. The micrographs are typical of two-phase system of NBR/VP rubber with occluded nanoclay. The images were captured at different magnifications to understand the nature of the clay aggregates/particles and to measure the size. Capturing the TEM images at the same magnification would have missed some of the information. There were number of large agglomerates in compound R (see Fig. 4) ranging from 1 to 10 [micro]m. This was due to the formation of conventional composites with microscopic distribution of clay in the matrix. In case of compound El (see Fig. 5), the size of the agglomerates was reduced and were ranging from 0.5 to 1.5 [micro]m. During latex stage mixing, the distribution of the clay particles was relatively better and some intercalation was observed in compound El. This was further supported by the fact that the bound rubber content of compound R, El, and E2 was 15.6, 19.2, and 26.8%, respectively. Addition of clay from master batch increased the bound rubber content (compound El) in comparison to direct addition of clay (compound R). This was also supported by WAXD data. TEM micrograph of compound E2 clearly (see Fig. 6) pointed out the distribution of the clay in nano-meter range. A mixed morphology was observed in compound E2. Some of the clay particles were totally exfoliated and some of them were intercalated (thickness range 20-30 nm). The findings were well correlated with WAXD results. There was some difference in contrast in the TEM images. The dark lines or images were the clay particles. Gray section was the polymeric phase and the while portion was probably due to improper microtoming, which allowed the beam to pass through.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
SEM-EDS Study. Figure 7 compares the Al mapping of compounds R, El, and E2, respectively. It was evident that there was significant amount of bigger aggregation of the clay in compound R. This was duo to poor distribution of the clay in compound R. Relatively better distribution of the clay particles was found in compound El. Still, there were a number of aggregates of clay particles. However, the size of the clay aggregates was relatively small in comparison to compound R. This was due to the latex stage mixing of the clay with VP rubber latex. In compound E2, no such aggregates were found. Nano-scopic distribution of clay particles led to such observations in compound E2.
The aluminum mapping was well supported by the TEM and WAXD findings.
Bound Rubber Content of the Master Batch. The objective of this study was to understand the degree of dispersion of the clay in the vinyl pyridine-styrene-butadiene terpolymer rubber (VP rubber) matrix before the mechanical mixing with NBR rubber, since the contribution of the shearing stress inherent in the later step might over-ride the effects of the clay treatment. It was found that the bound rubber content of the clay-VP rubber master batch (with out methyl iodide modification) was ^30%. It was further found that the bound rubber content of the clay-VP rubber master batch (with methyl iodide modification) was ~85%. This was clearly indicating that the methyl iodide modification helped to increase the clay rubber interaction by means of cation exchange reaction with the sodium ion of the clay.
Rheometric Property. It was found that the extent of curing (given by the torque differences or [DELTA]Torque values) was highest in compound E2 followed by El and R/Rl, respectively (Table 2). Better rubber to filler interaction was responsible for the torque difference. As supported by WAXD and TEM, exfoliation and intercalation of the rubber in the clay generated better rubber to filler interaction in compound E2. There was no such interaction in compound R. As a result, compound R exhibited lowest ATorque value after curing.
TABLE 2. Rheometric property of the compounds. Parameter R Rl El E2 Maximum torque 11.1 11.0 11.7 12.8 ([T.sub.max]), (dN-m) Minimum torque 0.4 0.4 0.5 0.6 ([T.sub.min]), (dN-m) [DELTA]Torque = 10.7 10.6 11.2 12.2 [T.sub.min] - [T.sub.min] (dN-m) TS02 (min) 3.7 3.5 3.1 2.6 TC90 (min) 7.5 7.4 6.5 5.2 Cure rale index 26.3 25.6 29.4 38.5 (CRI)
Scorch safety (TS02) of compound E2 was found to be lowest, followed by El and R, respectively. Also, the cure rate index was highest in compound E2 followed by El and R, respectively. It has been reported that the quaternary amine present in the organo clay act as an effective vulcanizing agent (16). In the present study, there was no quaternary amine in the matrix. It was the pyridinium ion of the VP rubber which acted as intercalating/exfoliating agent. Chakraborty et al. (18) reported that the pyridine moiety of the VP rubber can act as secondary accelerator in the vulcanization process. Moreover, it has also been reported that the quaternary amine-clay nanocomposites are more effective vulcanizing agent than the amine itself (16), (19). The availability of the pyridinium ion was highest in compound E2 which reduced the TS02 value and increased CR1 value.
Compound R and El had same formulation. However, compound El contained master batch prepared by latex stage blending. The latex blending helped to distribute the pyridine moiety properly in to the matrix which in turn helped to produce relatively low TS02 and high CRI value in comparison to compound R.
Rheometric property of compound Rl was comparable with that of compound R.
It is important to mention here that the samples were cured for 15 min at 160[degrees]C.
Physical Property. The tensile properties of the compounds are compared in Table 3. The data represented in the table was the mean of five measurements. The mechanical properties, viz., 100 and 300% modulus, tensile strength, elongation at break, and tear strength were maximum for compound E2. The addition of clay from VP master batch (with out methyl iodide modification) in compound El resulted in an increase of --52% in tensile strength and --39% in the elongation at break (EB) over R. It was found that compound R failed to cross the 300% elongation. Hardness values of compound El and R were comparable. Lower swelling index, higher mooney viscosity and tear strength in compound El was due to better rubber to filler interaction. Rebound resilience was found to be same for all the compounds. In compound E2, the corresponding increases in the unaged 100% modulus, tensile strength, EB, and tear strength were --20, 137, 63, and 75%, respectively over compound R. The intercalation and/or exfoliation of the NBR and VP rubber chains in the clay were probably responsible for the increase in the properties. Both El and E2 contained the same phr of clay and VP rubber but the sole difference between them was the VP moiety of the VP rubber which was modified by methyl iodide in E2. This modification resulted an increase in the 100% modulus, 300% modulus, tensile strength, EB and tear strength were --20, 30, 55, 20, and 33%, respectively over compound El. The TEM micrograph (see Fig. 6) corroborated partial exfoliation and intercalation of the clay platelets in E2 and this was responsible for the increase in mechanical properties. Mooney viscosity of compound E2 was highest due to better rubber to filler interaction followed by El and R. Compression set of compound E2 was highest followed by El and R. Higher compression set was probably due to irreversible slippage of the rubber chains from the clay surface. Same observation was reported by Wang et al. (20). Although, compound E2 had highest torque and modulus, still it exhibited higher compression set property. Not only higher compression set, compound E2 also exhibited higher elongation at break also. Mousa and Karger-Kocsis (21) speculated that the unexpected high elongation in SBR nanocomposites is likely due to the encapsulation of individual clay layers and tactoids in a more cross linked rubber fraction than the bulk itself. As a consequence, the overall deformability corresponds to that of a less cross linked rubber.
Physical property of compound Rl was comparable with compound El. Thus, there was no noticeable improvement in the physical property due to addition of unmodified clay in the matrix.
VP rubber is a random copolymer of vinyl pyridine, butadiene, and styrene. The vinyl pyridine moiety of VP rubber was the targeted moiety of modification. Vinyl pyridine readily reacts with methyl iodide to from methyl pyridinium iodide (22). Cation exchange reaction of the pyridinium ion and sodium montmorillonite clay occurs during latex blending. Since the methyl iodide is a liquid and is insoluble in water, prolonged vigorous stirring at room temperature (<40[degrees]C) is required to facilitate cation exchange reaction. A pictorial presentation of the reaction is given in Fig. 8.
[FIGURE 8 OMITTED]
The above study represented an effective way to prepare organo-clay nanocomposites via a single stage latex blending technique. The coagulated nanocomposite master batch can be compounded with nitrile rubber. WAXD study indicated an increase in basal spacing from 1.26 to 2.61 nm. This was well supported by TEM and EDS findings. Around 140% improvement in tensile strength was found in nanocomposites over control compound. This method of co-coagulation of VP rubber latex and montmorillonite clay is very promising from the industrial viewpoint due to the simplicity of the preparation technique and environmental friendliness.
The authors thank HASETRI and JK Tyre management for kind permission to publish this work. Authors are also thankful to Kunimin Industry, Japan, for providing clay samples free of cost.
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Correspondence to: Samar Bandyopadhyay; e-mail; sbanerjee[congruent to]ktp.jkmail.com
Sugata Chakraborty,1 Saptarshi Kar,1 Saikat Dasgupta, (1) Rabindra Mukhopadhyay, (1) Narendra P.S. Chauhan, (2) Suresh C. Ameta, (2) Samar Bandyopadhyay (3)
(1) Hari Shankar Singhania Elastomer and Tyre Research Institute (HASETRI), Jaykaygram, PO Tyre Factory, Rajsamand 313 342, Rajasthan, India
(2) Department of Polymer Science, Mohanlal Sukhadia University, Udaipur 313 001, Rajasthan, India
(3) R&D Centre, JK Tyre, Jaykaygram, PO Tyre Factory, Rajsamand, Rajasthan, India
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|Author:||Chakraborty, Sugata; Kar, Saptarshi; Dasgupta, Saikat; Mukhopadhyay, Rabindra; Chauhan, Narendra P.S|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2011|
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