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Mill processing and properties of rubber–clay nanocomposites
Materials Science and Engineering C 30 (2010) 590–596
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Mill processing and properties of rubber–clay nanocomposites M. Nadeem Qureshi a,⁎, H. Qammar b a b
MDI Products LLC, 1981 Newmark Cir., Vero Beach FL 32968, United States Department of Chemical and Biomolecular Engineering, University of Akron, Akron OH 44325-6236, United States
a r t i c l e
i n f o
Article history: Received 7 August 2009 Received in revised form 6 January 2010 Accepted 9 February 2010 Available online 14 February 2010 Keywords: Montmorillonite reinforcement Natural rubber nanoclay Roll mill processing Compare clay dispersion Latex dispersion
a b s t r a c t The dispersion of nanoscale composites in elastomers, which generally have higher molecular weight and viscosity as compared to plastics, is a challenge. Several techniques have been proposed for improvement of the dispersion of nanofillers in the polymers [1]. For example, the interaction of natural layered silicates can be improved by ion-exchange of hydrated cation with organic cations such as introducing bulky alkylammoniums to obtain larger interlayer spacing and provide the galleries for the polymer chain diffusion. The resultant swollen nanoclay was dried and dispersed in the polymer matrix by means of high shear mixers [2–3]. In this paper we describe the results from a new method of incorporating nanofillers into solid rubber by use of a conventional two-roll mill, which we call the modified mill method. The properties of the resultant material are compared with that of the material prepared by a latex method. We also test processability parameters, tensile behavior and crosslink density of carbon black composites prepared by the same two methods to provide a comparison between the nanocomposites. The rubber–clay nanocomposites prepared by the mill method are shown to have a fine dispersed phase structure and good reinforcement properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer layer silicate nanocomposites represent a rational alternative to conventional filled polymers for structural biological systems. With the use of well-dispersed nanofillers in polymer matrix, the resultant clay composites exhibit markedly improved properties when compared with pure polymers or conventional composites [1–4]. Although there has been considerable interest in forming nanocomposites as a means to improve the mechanical properties of polymers; simple incorporation of clay minerals into a polymer matrix, however, does not always result in significant improvements in the mechanical properties of the polymer. This is due to a lack of affinity between the layered silicate minerals and the organic polymers. Thus it has been proposed to use ionic interactions as a means of incorporating silicate minerals in an emulsion polymer. The interaction of natural layered silicates has been improved by ion-exchange of hydrated cation with organic cations such as by introducing bulky alkyl-ammoniums, this usually results in larger interlayer spacing and provides galleries for polymer chain diffusion. The most common techniques employed for preparation of polymeric nanocompsites are melt intercalation method [5], in-situ intercalation method [6,7], exfoliation–adsorption method [8–12], and the template synthesis method [13,14]. Among these the
⁎ Corresponding author. E-mail address: [email protected] (M.N. Qureshi). 0928-4931/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2010.02.008
last three methods require a water or solvent medium for clay dispersion and have limited application. In the melt intercalation nanocomposite preparation method, the layered silicate is mixed with the polymer matrix in the molten state. Under these conditions and if the layer surfaces are sufficiently compatible with the chosen polymer, the polymer can crawl into the interlayer space and form either an intercalated or an exfoliated nanocomposite. Burnside and Giannelis [5] have described the twostep preparation of silicon rubber-based nanocomposites. First, silanol-terminated poly(dimethylsiloxane) (PDMS, MW = 18,000) is melt blended at room temperature with dimethyl ditallowammonium-exchanged montmorillonite. Next, the silanol end groups are crosslinked with tetraethyl orthosilicate (TEOS) in the presence of the catalyst tin bis(2-ethylhexanoate) at room temperature. The authors claim an exfoliated structure confirmed by the absence of diffraction peaks in the XRD pattern and a TEM micrograph showing a molecular dispersion of the clay layers. Okada and co-workers [15,16] obtained a nitrile rubber (NBR)based nanocomposite in a two-step synthesis. They first modified a Na-montmorillonite through cation exchange with an amino-endcapped poly(butadiene-acrylonitrile) oligomer (MW = 3400) cationized by HCl in water. This modified clay was then melt blended on a conventional roll-mill with NTBR and the usual additives for vulcanization such as sulfur and ZnO were added to obtain vulcanized rubber sheets after compression molding at 160 °C for 15 min. Even if no direct and objective evidence of the nano structure is given, a large number of properties (gas permeability, enhanced mechanical
M.N. Qureshi, H. Qammar / Materials Science and Engineering C 30 (2010) 590–596
properties, etc.) tend to demonstrate that the behavior of these NBRbased composites is in the range of what is usually observed for nanocomposites. Researchers at Exxon Company invented a method of nanocomposite dispersion in emulsion polymerized latex and suggested improvement in the mechanical properties and gas impermeability [17]. Recently Wang et al. [4], prepared rubber–clay nanocomposites by two different methods namely the latex method and solution method and characterized these using TEM and XRD. The TEM showed clay had been dispersed to one or several layers. The XRD showed that the basal spacing in the clay was increased. It was evident that some macromolecules intercalated to the clay layer galleries. The clay layer could be uniformly dispersed in the rubber matrix on the nanometer level. Mechanical tests showed the nanocomposites had good mechanical properties. Some properties exceeded those of rubber reinforced with carbon black, so the clay layers could be used as an important reinforcing agent much like carbon black. The disadvantage of this method is that it requires aqueous phase dispersion and has less controllability. Therefore, a more direct, simple, and economic approach to preparing nanocomposites is highly desirable. We developed a new method, which we refer to as the modified mill method, of incorporating nanofillers into solid rubber by use of a conventional two-roll mill and air spray gun for dry filler particles coating on a freshly sheeted out rubber surface. The properties of the resultant material are compared with that of the material prepared by the above-mentioned latex method. We also tested processability parameters, tensile behavior and crosslink density of the carbon black composites prepared by the two methods to provide a comparison with those of the pretreated organophilic nanoclay. The rubber–clay nanocomposites prepared by the mill method are shown to have a fine dispersed phase structure as confirmed by TEM and good mechanical reinforcement properties. 2. Experimental 2.1. Materials Natural rubber latex was obtained from Firestone Synthetic Rubber and Latex Co: the solid content was 40%. The purified montmorillonite MMT was provided from Southern Clay Products, carbon black (HAF, N-330) was obtained from Columbian Chemicals Co., dodecylmethyl ammonium bromide (DMABr) was obtained from Aldrich Chemicals, and the remaining chemicals for curing formulation as shown in Table 1 were available from Akrochem Corporation and laboratory storage at the University of Akron. 2.1.1. Preparation of organophilic MMT The purified MMT clay is treated in our lab with DMABr by an accepted procedure [2,18]. First, 20 g of MMT was dispersed into 400 ml of water. Dodecylmethyl ammonium bromide (8.82 g) was then dissolved in a 200 ml water–ethanol (1:1) solution with vigorous stirring for 6 h at 60 °C. The precipitate was collected and re-dispersed
Table 1 Compound formulation. Ingredients
Filled
Rubber MMT/CB ZnO2 Stearic acid Processing oil TMTD Sulfur TBBS
100 Varyinga 3 1 2 0.2 2 1.5
a
CB; 5, 10, and 20 phr. MMT; 5 and 10 phr.
591
for 1 h. The solution was then filtered and freeze-dried to yield an organophilic layered silicates OMMT. 2.2. Processing 2.2.1. Latex method Organophilic MMT was dispersed in water with vigorous stirring (5% dispersed) before the latex was added and mixed for a period of time. It was coagulated in potassium chloride solution, washed with water until reaching a pH close to 7. It was then dried and designated as RML. In a similar way, carbon black (5% dispersed) was mixed with latex, coagulated and dried. This is designated as RCL. 2.2.2. Mill method Dried OMMT was ground and the particle size fraction of 10–20 μg was collected. The sieved particles were spread on the surface of raw rubber using a continuous milling process on an open two-roll mill equipped with a spray gun. The speed of the two frictional rolls was set to a ratio of 25:35 rpm, the temperature of the mill is controlled by means of cold water running inside the rolls, however, shearing of polymer at the nip can increase temperature of polymer to several degrees higher than room temperature. In our procedure, temperature of the polymer was maintained in the range of 60–80 °C. For this purpose, coagulated and dried solid polymer was initially warmed by passing it through the rolls. As it became sufficiently soft to adhere to the front, slower moving roll, it formed a continuous bank at the nip of the two rolls. At this stage, dried OMMT (organofiller) were sprayed on the rolling rubber surface by means of a pneumatic spray gun. Continuing rolling action elongated and compacted the stock to create a fresh polymer surface that is ready to be impregnated by the particulate OMMT layers. The spray coating of OMMT ensures fine dispersion and the nip action between the rolls further thins down and embeds the skin layer. This procedure ensures a homogenous dispersion with reduced chances of agglomeration or secondary structure formation during mixing. Milling continued in this way for at least 6–10 min in order to obtain good homogenous dispersion. The non-adhering particles falling off the surface of the polymer at the front roll are carefully collected and fed back to the spray gun or manually spread at the nip of the mill. Master batches prepared by the above mill method are designated as RMM. Similarly, other batches using carbon black were prepared in the same way and these are designated as RCM. The filler phr is adjusted using density measurements. 2.2.3. Sample preparation Master batches prepared from the latex method and the mill method were mixed with other vulcanization ingredients in a Brabender static mixer at 70 °C and 60 rpm according to the formulation shown in Table 1. 2.2.4. Characterization Cure kinetic curves were obtained from a Monsanto oscillating disk rheometer (ODR) at a temperature of 140 °C and analyzed according to ASTM method D2084. The storage modulus of curative free master batches was obtained at 35 °C in the frequency range between 0.1 and 30 Hz by using APA 2000. Mechanical tests of the vulcanized samples were performed according to prescribed ASTM methods; Mooney viscosity — ASTM D1646; wear rating — D231-90; hardness (Shore A) — ASTM D2240; Rebound — D2638-96; stress– strain — D638-98. The crosslink density (ve) was determined by means of the Florry–Rehner Eq. (1); details of this method can be found elsewhere [19]. νe =
ρ − lnð1−Vro Þ + Vro + χðVro Þ2 n o = Mc V ðV Þ1 = 3 − 2Vro s
ro
f
ð1Þ
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M.N. Qureshi, H. Qammar / Materials Science and Engineering C 30 (2010) 590–596
where Vs is the molar volume of the solvent (Vs = 87.5 cm3/mol for toluene); Vro is the rubber volume fraction in swollen gel; ρ is the density of rubber; Mc is the molecular weight between crosslinks; χ is the interaction parameter taken as 0.34 [20]; and f is the functionality of crosslinkages (f = 4). 3. Results and discussion 3.1. Properties of unvulcanized rubbers Prior to any reinforcing investigation, it is necessary to validate the micromorphology of the nanocomposites. A powerful tool to observe structures of nanometer range is the transmission electron microscopy, TEM. Two micrographs of RMM filled with 5 wt.% clay are presented in Fig. 1. The TEM image in Fig. 1(a) shows a nanocomposite prepared by the modified mill method and in Fig. 1(b) a nanocomposite prepared by the latex method. The dark entities can be clearly assigned to organosilicate layers as described by others [21,22]. Both preparation methods lead to partial exfoliated morphology, that is several separated silicate sheets can be observed along with large
silicate particles. However, more prominent dispersion of silicate layers is obtained in the modified mill method. This proves that our attempt for physical facilitation of organosilicate OMMT dispersion is improved since it is a more homogenous dispersion. The characteristic curves of the crosslinking reaction are analyzed for properties such as time of scorch (t2), time of 90% cure (t90) and optimum torque (T90). These values are summarized in Table 2. The scorch time represents the delay of the crosslinking reaction at a given temperature. It is interesting to note that at 5 phr loading of the fillers, the stock prepared by the modified mill method have longer scorch times than those of the stock prepared thorough the latex method in both rubber–OMMT (RM) and rubber–carbon black (RC) types of composite systems. The opposite trend is found at 10 phr loading of fillers. This shows that the filler loading has some influence on the delayed action of the sulfonamide (TBBS) chemical agent. At lower filler loading, the modified mill method shows more effectiveness of the delayed action agent in the experimental system of our study, however, more investigation is needed for better understanding of inherent interactions of RM composites with different scorch time controlling agents. Further RM composites are faster curing than RC composites, as can be seen from both t2 and t90 values in Table 2. It is well known that carbon black reduces the induction time and increases the rate of crosslinking reaction in rubbers [23]. Many studies have investigated the mechanism for this behavior. While the actual cause for such a rate acceleration effect of carbon black is still open to debate, generally the participation of the surface functional groups of carbon black in the crosslinking reaction is suspected for this trend [24]. Sometimes the physical properties of carbon black may also play a role in rate acceleration [25]. It has been shown that thermal conductivity of black filled compounds is generally higher than that of the gum rubber and it increases with carbon black loading [26]. Since conductivity has a cumulative effect in a composite, such a trend in black filled compounds is related to the higher conductivity of carbon black. We note that the thermal conductivity of the OMMT is higher than that of carbon black. Therefore higher heat conduction in RM type composites can be considered as one of the reasons for the higher rate of vulcanization in such systems. Further the improved interaction between rubber and organosilicates and their surface functionality may also provide some means of promoting the crosslinking reaction. Earlier, Okada et al. [15,16] showed an increase in cure reaction with improved interaction of rubber–organoclay composite. They related slow vulcanizing reaction in the untreated clay–rubber composites to the lack of interaction and reaction inhibition effects on the mineral surface. The values of optimum cure can be related to the viscosity and molecular weight between the crosslinks of the compounds. As seen in Table 2, T90 values for both RML and RCL composites are higher than those for the composites prepared by the modified mill method. It is obvious that rubber undergoes higher shear breakdown at the mill and retains lower bulk viscosity values for the stock prepared by the modified mill method as compared to the samples prepared by the latex method. This can be seen by comparing the Mooney viscosity of these
Table 2 Curing characteristics and Mooney viscosity of compounds.
Fig. 1. TEM micrographs of 5 wt.% filled nancomposites prepared by mill method (a) and by latex method (b).
Compound
Scorch time, min t2
Cure time, min t90
Optimum torque, lb-in T90
Mooney viscosity at 100 °C
Gum RML5 RML10 RMM5 RMM10 RCL5 RCL10 RCM5 RCM10
13.2 10.1 9.8 11.7 9.5 11.9 11.2 12.6 10.7
21.2 14.4 13.9 16.3 14.7 15.1 14.3 17.4 15.8
63.3 69.7 72.8 69.1 71.4 78.3 82.9 76.5 81.3
32.3 37.5 42.7 36.3 38.8 41.2 43.6 38.2 39.7
M.N. Qureshi, H. Qammar / Materials Science and Engineering C 30 (2010) 590–596
Fig. 2. Dynamic storage modulus at 30°C of rubber-filler composites prepared by mill method.
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Fig. 4. Comparison of dynamic storage modulus of rubber-filler composites prepared by both latex and mill methods.
the figures that in general, the values of G′ in both types of filler systems are larger in the latex method than those for the modified mill method. Further, the viscoelastic properties of unvulcanized RM composites are superior to those of the RC composites. The storage modulus of RML10, containing 10 phr of OMMT filler, is higher than that of the rubber containing 20 phr of carbon black as shown in Fig. 4.
rubber compounds as shown in Table 2. The lower viscosity of RM stocks provides ease of processing and thus this new method of OMMT incorporation into rubber can improve processability of nanocomposites. The dynamic storage modulus (G′) is a measure of the energy (elastic) stored and recovered in cyclic deformation; therefore, in a filled system it gives a direct measure of dispersion and interaction. Clearly for higher interaction of well-dispersed fillers, higher values of G′ are desired. On the other hand, G′ is also related to the molecular weight of the primary chain of polymer. The results are compared for both of the processing methods and presented in Figs. 2 and 3. It can be seen from
In RC composite systems, the higher values of T90 are related to the larger crosslink density of the final vulcanizates. As shown in Fig. 5,
Fig. 3. Dynamic storage modulus of rubber-filler composites prepared by latex method.
Fig. 5. Crosslink density of rubber-filler composites.
3.2. Properties of vulcanized rubber
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Fig. 8. Modulus of the rubber-filler composites prepared by latex method. Fig. 6. Tensile strength of rubber-filler composites.
the RC composite has higher values of crosslink density than those of RM composites in both processing methods involved in this study. A simple explanation for this is that in RC composites an uneven distribution of curatives takes place due to the formation of the carbon rich phase and carbon free phase. Since the immobilized rubber in the carbon rich phase is formed before curatives are added, it is deficient in curatives concentration, while the concentration of curatives in the free rubber phase is higher. This in turn increases the crosslink density in the black loaded compounds [27]. Considering this situation, lower values of crosslink density in the RM compounds appear more promising for more uniform crosslink distribution. Furthermore, the
main difference in crosslink density between RMM and RML systems is interpreted as stemming from better dispersion of organophilic mineral and formation of more exfoliated structure in RMM composites compare to RML composites. The changes resulting from the reinforcement of fillers appear as a common behavior in most of the elastomers such as improved hysteresis, higher tensile strength, higher modulus, higher abrasion resistant and rebound resilience changes. Several samples were tested to evaluate these properties and comparisons of the results are presented in Figs. 6–9. Fig. 6 shows improvement in the tensile strength in RM composites. The reinforcement effect in terms of tensile strength is higher for samples prepared via the modified mill method for a given
Fig. 7. Modulus of the rubber-filler composites prepared by mill method.
Fig. 9. Rebound of rubber-filler composites.
M.N. Qureshi, H. Qammar / Materials Science and Engineering C 30 (2010) 590–596
Fig.11. Hardness of rubber-filler composites.
Fig. 10. Wear rating of the NR composites prepared by both latex and mill methods.
experimental system. This effect can be related to the better dispersion and interaction of rubber–organosilicate systems. Also the relatively slow curing rate in these composites enhances the probability of uniform and strong sulfur linkages, which gives rise to tensile strength [28]. For example, the strength of RMM5, containing 5 phr of OMMT filler, is higher than that of the rubber containing 10 phr of carbon black. Figs. 7 and 8 represent the tensile modulus of rubber-filler composites prepared by the latex and mill methods, respectively. The modulus values are presented for 100%, 200%, and 300% elongation. It can be seen that moduli at a given elongation are relatively low in the composites prepared by the latex method than those prepared by the modified mill method. Further, in all cases the RM type composites are softer than RC type composites. The resilience of rebound is the ratio of the energy recovered to the energy lost as a result of deformation; therefore, it gives a measure of the hysteresis. It can be seen in Fig. 9 that the rebound values of RM composites are higher than those of the RC composites. Besides better dispersion and adhesion of RM composite systems, other factors are also considered to influence the hysteresis as deduced from these tests. One factor may be less chain breakdown and hence less chainend formation during polymer mixing. The chain ends are considered a major source of the hysteresis [29]. Furthermore, the strain amplification and immobilized rubber in the carbon rich phase are also considered to promote chain rupture during milling and hence this leads to lower values of rebounds in RC composites. On the other hand, higher chain breakdown and hence higher chain ends formation during milling would adversely affect the rebound performances of stocks prepared by the modified mill method. Therefore, the higher rebound values of stocks prepared by the latex method in both RML and RCL can be related to lower chain ends in such stocks as compared to the modified mill method that is in the RMM and RCM stocks. In order to obtain fatigue abrasion in a control environment, wear tests are performed on a laboratory scale grit (dull) abrader. The results are presented in Fig. 10. These tests show marked reinforcement effects as reported by higher wear ratings [30]. In addition, Fig. 11 relates mean hardness of the vulcanizates. It can be seen that the samples prepared by the latex method appeared relatively harder in both types of rubber-filler systems.
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4. Conclusions A modified method of improving organophilic nanoclay dispersion in natural rubber is presented. The resultant RM composites of NR and OMMT prepared from our modified mill method and the latex method are compared for different mechanical and curing behaviors. Based on the experimental system of this study, we found that the degree of reinforcement in RM composite is more than twice as large as in rubber–carbon black RC composites. The excellent mechanical properties of RM composite are known to stem from a large surface area, uniformly dispersed organophilic silicate filler and ionic bonds between polymer and silicate layers. The mechanical properties of the RM nanocomposites produced by the modified mill method are found to be improved over those produced by the latex method. On the basis of general reinforcement effects, the modified mill method is more convenient and of practical importance for solid rubber processing. This work also has application for organically modified smectite clays to be an effective reinforcing filler in rubber application for biological systems if it is dispersed on a nanometer level through some refined method. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
M. Alexandre, P. Dubois, Mat. Sc. Eng. 28 (2000) 1. N. Hasengawa, et al., J. Appl. Polym. Sci. 38 (1998) 1351. L. He, L.F. Allard, E. Ma, Forth Internatl. Conf. Nanostructd. Materials 12 (1999) 543. Y. Wang, L. Zhang, C. Tang, Dingsheng Yu, J. Appl. Polym. Sci. 78 (2000) 1879. S.D. Burnside, E.R. Giannelis, Chem. Mater. 7 (1995) 1597. T. Lan, T.J. Pinnavaia, Chem. Mater. 6 (1994) 2216. O.C. Wilson Jr., T. Olorunyolenii, A. Jaworsld, L. Borum, D. Young, A. Siriwat, E. Dickens, C. Oriakhi, M. Lerner, Appl. Clay Sci. 15 (1999) 265. M. Lerner, C. Oriakhi, in: A. Goldstein (Ed.), Handbook of Nanophase Materials, Marcel Dekker, New York, 1997, p. 199. G. Lagaly, Appl. Clay Sci. 15 (1999) 1. D.J. Greenland, J. Colloid Sci 18 (1963) 647. N. Ogata, S. Kawakage, T. Ogihara, J. Appl. Polym. Sci. 66 (1997) 573. R.L. Parfitt, D.J. Greenland, Clay Miner. 8 (1970) 305. T. Lan, P.D. Kaviratna, T.J. Pinnavaia, Chem. Mater. 7 (1995) 2144. C.O. Oriakhi, I.V. Farr, M.M. Lerner, Clays Clay Miner. 45 (1997) 194. A. Okada, K. Fukumori, A. Usuki, Y. Kojima, T. Kurauchi, O. Karffigaito, Polym. Prep. 32 (1991) 540. A. Okada, A. Usuld, Mater. Sci. Eng. C3 (1995) 109. C. W. Elspass, Exxon Research and Engineering Company, Canada patent no. CA02221649, (1997).
596
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[18] D.J. Suh, Y.T. Lim, O.O. Park, Polymer 41 (2000) 8557. [19] P.J. Flory, J. Rehner, J Chem. Phys. 11 (1943) 512; P.J. Flory, J Chem. Phys. 18 (1950) 108. [20] M. Hunang, Mechanical Properties of Anisotropic Rubber Networks, Dissertn., at the Uni. Akron, 1997. [21] B. Hoffmann, J. Kressler, G. Stoppelmann, Chr. Friedrich, G.M. Kim, Colloid Polm. Sci. 278 (2000) 629. [22] P. Reichert, J. Kessler, R. Thomann, R. Mulhaupt, G. Stoppelmann, Acta Polym. 49 (1998) 116. [23] C.H. Chen, J.L. Koenig, J.R. Shelton, E.A. Collins, Rubber. Chem. Technol. 55 (1982) 103.
[24] M. Nakahara, T. Takada, H. Kumagai, Y. Sanada, Carbon 33 (11) (1995) 1537. [25] Makio Mori, J.K. Koenig, Rub. Chem. Technol. 70 (1997) 671. [26] Dongcheng Kong, J.L. White, Frederick C. Wessert, N. Nakajima, Rub. Chem. Technol. 60 (1987) 140. [27] M.N. Qureshi, H. Qammar, J. Padovan, Rub. Plast. News (May, 2001) 15. [28] S. Goo Kim, Suck-Hyun Lee, Rub. Chem. Technol. 67 (1994) 649. [29] A. Ahagon, Rub Chem. Technol. 71 (1998) 975. [30] M. N. Qureshi, Hierarchical Microdomains in Solution and Emulsion Polymerized Elastomers Induced by Synthesis and Processing and Potential Effects on Properties, Doctoral Dissertation, Univ. of Akron, OH, May (2002) 48–146.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Mill processing and properties of rubber–clay nanocomposites M. Nadeem Qureshi a,⁎, H. Qammar b a b
MDI Products LLC, 1981 Newmark Cir., Vero Beach FL 32968, United States Department of Chemical and Biomolecular Engineering, University of Akron, Akron OH 44325-6236, United States
a r t i c l e
i n f o
Article history: Received 7 August 2009 Received in revised form 6 January 2010 Accepted 9 February 2010 Available online 14 February 2010 Keywords: Montmorillonite reinforcement Natural rubber nanoclay Roll mill processing Compare clay dispersion Latex dispersion
a b s t r a c t The dispersion of nanoscale composites in elastomers, which generally have higher molecular weight and viscosity as compared to plastics, is a challenge. Several techniques have been proposed for improvement of the dispersion of nanofillers in the polymers [1]. For example, the interaction of natural layered silicates can be improved by ion-exchange of hydrated cation with organic cations such as introducing bulky alkylammoniums to obtain larger interlayer spacing and provide the galleries for the polymer chain diffusion. The resultant swollen nanoclay was dried and dispersed in the polymer matrix by means of high shear mixers [2–3]. In this paper we describe the results from a new method of incorporating nanofillers into solid rubber by use of a conventional two-roll mill, which we call the modified mill method. The properties of the resultant material are compared with that of the material prepared by a latex method. We also test processability parameters, tensile behavior and crosslink density of carbon black composites prepared by the same two methods to provide a comparison between the nanocomposites. The rubber–clay nanocomposites prepared by the mill method are shown to have a fine dispersed phase structure and good reinforcement properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer layer silicate nanocomposites represent a rational alternative to conventional filled polymers for structural biological systems. With the use of well-dispersed nanofillers in polymer matrix, the resultant clay composites exhibit markedly improved properties when compared with pure polymers or conventional composites [1–4]. Although there has been considerable interest in forming nanocomposites as a means to improve the mechanical properties of polymers; simple incorporation of clay minerals into a polymer matrix, however, does not always result in significant improvements in the mechanical properties of the polymer. This is due to a lack of affinity between the layered silicate minerals and the organic polymers. Thus it has been proposed to use ionic interactions as a means of incorporating silicate minerals in an emulsion polymer. The interaction of natural layered silicates has been improved by ion-exchange of hydrated cation with organic cations such as by introducing bulky alkyl-ammoniums, this usually results in larger interlayer spacing and provides galleries for polymer chain diffusion. The most common techniques employed for preparation of polymeric nanocompsites are melt intercalation method [5], in-situ intercalation method [6,7], exfoliation–adsorption method [8–12], and the template synthesis method [13,14]. Among these the
⁎ Corresponding author. E-mail address: [email protected] (M.N. Qureshi). 0928-4931/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2010.02.008
last three methods require a water or solvent medium for clay dispersion and have limited application. In the melt intercalation nanocomposite preparation method, the layered silicate is mixed with the polymer matrix in the molten state. Under these conditions and if the layer surfaces are sufficiently compatible with the chosen polymer, the polymer can crawl into the interlayer space and form either an intercalated or an exfoliated nanocomposite. Burnside and Giannelis [5] have described the twostep preparation of silicon rubber-based nanocomposites. First, silanol-terminated poly(dimethylsiloxane) (PDMS, MW = 18,000) is melt blended at room temperature with dimethyl ditallowammonium-exchanged montmorillonite. Next, the silanol end groups are crosslinked with tetraethyl orthosilicate (TEOS) in the presence of the catalyst tin bis(2-ethylhexanoate) at room temperature. The authors claim an exfoliated structure confirmed by the absence of diffraction peaks in the XRD pattern and a TEM micrograph showing a molecular dispersion of the clay layers. Okada and co-workers [15,16] obtained a nitrile rubber (NBR)based nanocomposite in a two-step synthesis. They first modified a Na-montmorillonite through cation exchange with an amino-endcapped poly(butadiene-acrylonitrile) oligomer (MW = 3400) cationized by HCl in water. This modified clay was then melt blended on a conventional roll-mill with NTBR and the usual additives for vulcanization such as sulfur and ZnO were added to obtain vulcanized rubber sheets after compression molding at 160 °C for 15 min. Even if no direct and objective evidence of the nano structure is given, a large number of properties (gas permeability, enhanced mechanical
M.N. Qureshi, H. Qammar / Materials Science and Engineering C 30 (2010) 590–596
properties, etc.) tend to demonstrate that the behavior of these NBRbased composites is in the range of what is usually observed for nanocomposites. Researchers at Exxon Company invented a method of nanocomposite dispersion in emulsion polymerized latex and suggested improvement in the mechanical properties and gas impermeability [17]. Recently Wang et al. [4], prepared rubber–clay nanocomposites by two different methods namely the latex method and solution method and characterized these using TEM and XRD. The TEM showed clay had been dispersed to one or several layers. The XRD showed that the basal spacing in the clay was increased. It was evident that some macromolecules intercalated to the clay layer galleries. The clay layer could be uniformly dispersed in the rubber matrix on the nanometer level. Mechanical tests showed the nanocomposites had good mechanical properties. Some properties exceeded those of rubber reinforced with carbon black, so the clay layers could be used as an important reinforcing agent much like carbon black. The disadvantage of this method is that it requires aqueous phase dispersion and has less controllability. Therefore, a more direct, simple, and economic approach to preparing nanocomposites is highly desirable. We developed a new method, which we refer to as the modified mill method, of incorporating nanofillers into solid rubber by use of a conventional two-roll mill and air spray gun for dry filler particles coating on a freshly sheeted out rubber surface. The properties of the resultant material are compared with that of the material prepared by the above-mentioned latex method. We also tested processability parameters, tensile behavior and crosslink density of the carbon black composites prepared by the two methods to provide a comparison with those of the pretreated organophilic nanoclay. The rubber–clay nanocomposites prepared by the mill method are shown to have a fine dispersed phase structure as confirmed by TEM and good mechanical reinforcement properties. 2. Experimental 2.1. Materials Natural rubber latex was obtained from Firestone Synthetic Rubber and Latex Co: the solid content was 40%. The purified montmorillonite MMT was provided from Southern Clay Products, carbon black (HAF, N-330) was obtained from Columbian Chemicals Co., dodecylmethyl ammonium bromide (DMABr) was obtained from Aldrich Chemicals, and the remaining chemicals for curing formulation as shown in Table 1 were available from Akrochem Corporation and laboratory storage at the University of Akron. 2.1.1. Preparation of organophilic MMT The purified MMT clay is treated in our lab with DMABr by an accepted procedure [2,18]. First, 20 g of MMT was dispersed into 400 ml of water. Dodecylmethyl ammonium bromide (8.82 g) was then dissolved in a 200 ml water–ethanol (1:1) solution with vigorous stirring for 6 h at 60 °C. The precipitate was collected and re-dispersed
Table 1 Compound formulation. Ingredients
Filled
Rubber MMT/CB ZnO2 Stearic acid Processing oil TMTD Sulfur TBBS
100 Varyinga 3 1 2 0.2 2 1.5
a
CB; 5, 10, and 20 phr. MMT; 5 and 10 phr.
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for 1 h. The solution was then filtered and freeze-dried to yield an organophilic layered silicates OMMT. 2.2. Processing 2.2.1. Latex method Organophilic MMT was dispersed in water with vigorous stirring (5% dispersed) before the latex was added and mixed for a period of time. It was coagulated in potassium chloride solution, washed with water until reaching a pH close to 7. It was then dried and designated as RML. In a similar way, carbon black (5% dispersed) was mixed with latex, coagulated and dried. This is designated as RCL. 2.2.2. Mill method Dried OMMT was ground and the particle size fraction of 10–20 μg was collected. The sieved particles were spread on the surface of raw rubber using a continuous milling process on an open two-roll mill equipped with a spray gun. The speed of the two frictional rolls was set to a ratio of 25:35 rpm, the temperature of the mill is controlled by means of cold water running inside the rolls, however, shearing of polymer at the nip can increase temperature of polymer to several degrees higher than room temperature. In our procedure, temperature of the polymer was maintained in the range of 60–80 °C. For this purpose, coagulated and dried solid polymer was initially warmed by passing it through the rolls. As it became sufficiently soft to adhere to the front, slower moving roll, it formed a continuous bank at the nip of the two rolls. At this stage, dried OMMT (organofiller) were sprayed on the rolling rubber surface by means of a pneumatic spray gun. Continuing rolling action elongated and compacted the stock to create a fresh polymer surface that is ready to be impregnated by the particulate OMMT layers. The spray coating of OMMT ensures fine dispersion and the nip action between the rolls further thins down and embeds the skin layer. This procedure ensures a homogenous dispersion with reduced chances of agglomeration or secondary structure formation during mixing. Milling continued in this way for at least 6–10 min in order to obtain good homogenous dispersion. The non-adhering particles falling off the surface of the polymer at the front roll are carefully collected and fed back to the spray gun or manually spread at the nip of the mill. Master batches prepared by the above mill method are designated as RMM. Similarly, other batches using carbon black were prepared in the same way and these are designated as RCM. The filler phr is adjusted using density measurements. 2.2.3. Sample preparation Master batches prepared from the latex method and the mill method were mixed with other vulcanization ingredients in a Brabender static mixer at 70 °C and 60 rpm according to the formulation shown in Table 1. 2.2.4. Characterization Cure kinetic curves were obtained from a Monsanto oscillating disk rheometer (ODR) at a temperature of 140 °C and analyzed according to ASTM method D2084. The storage modulus of curative free master batches was obtained at 35 °C in the frequency range between 0.1 and 30 Hz by using APA 2000. Mechanical tests of the vulcanized samples were performed according to prescribed ASTM methods; Mooney viscosity — ASTM D1646; wear rating — D231-90; hardness (Shore A) — ASTM D2240; Rebound — D2638-96; stress– strain — D638-98. The crosslink density (ve) was determined by means of the Florry–Rehner Eq. (1); details of this method can be found elsewhere [19]. νe =
ρ − lnð1−Vro Þ + Vro + χðVro Þ2 n o = Mc V ðV Þ1 = 3 − 2Vro s
ro
f
ð1Þ
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where Vs is the molar volume of the solvent (Vs = 87.5 cm3/mol for toluene); Vro is the rubber volume fraction in swollen gel; ρ is the density of rubber; Mc is the molecular weight between crosslinks; χ is the interaction parameter taken as 0.34 [20]; and f is the functionality of crosslinkages (f = 4). 3. Results and discussion 3.1. Properties of unvulcanized rubbers Prior to any reinforcing investigation, it is necessary to validate the micromorphology of the nanocomposites. A powerful tool to observe structures of nanometer range is the transmission electron microscopy, TEM. Two micrographs of RMM filled with 5 wt.% clay are presented in Fig. 1. The TEM image in Fig. 1(a) shows a nanocomposite prepared by the modified mill method and in Fig. 1(b) a nanocomposite prepared by the latex method. The dark entities can be clearly assigned to organosilicate layers as described by others [21,22]. Both preparation methods lead to partial exfoliated morphology, that is several separated silicate sheets can be observed along with large
silicate particles. However, more prominent dispersion of silicate layers is obtained in the modified mill method. This proves that our attempt for physical facilitation of organosilicate OMMT dispersion is improved since it is a more homogenous dispersion. The characteristic curves of the crosslinking reaction are analyzed for properties such as time of scorch (t2), time of 90% cure (t90) and optimum torque (T90). These values are summarized in Table 2. The scorch time represents the delay of the crosslinking reaction at a given temperature. It is interesting to note that at 5 phr loading of the fillers, the stock prepared by the modified mill method have longer scorch times than those of the stock prepared thorough the latex method in both rubber–OMMT (RM) and rubber–carbon black (RC) types of composite systems. The opposite trend is found at 10 phr loading of fillers. This shows that the filler loading has some influence on the delayed action of the sulfonamide (TBBS) chemical agent. At lower filler loading, the modified mill method shows more effectiveness of the delayed action agent in the experimental system of our study, however, more investigation is needed for better understanding of inherent interactions of RM composites with different scorch time controlling agents. Further RM composites are faster curing than RC composites, as can be seen from both t2 and t90 values in Table 2. It is well known that carbon black reduces the induction time and increases the rate of crosslinking reaction in rubbers [23]. Many studies have investigated the mechanism for this behavior. While the actual cause for such a rate acceleration effect of carbon black is still open to debate, generally the participation of the surface functional groups of carbon black in the crosslinking reaction is suspected for this trend [24]. Sometimes the physical properties of carbon black may also play a role in rate acceleration [25]. It has been shown that thermal conductivity of black filled compounds is generally higher than that of the gum rubber and it increases with carbon black loading [26]. Since conductivity has a cumulative effect in a composite, such a trend in black filled compounds is related to the higher conductivity of carbon black. We note that the thermal conductivity of the OMMT is higher than that of carbon black. Therefore higher heat conduction in RM type composites can be considered as one of the reasons for the higher rate of vulcanization in such systems. Further the improved interaction between rubber and organosilicates and their surface functionality may also provide some means of promoting the crosslinking reaction. Earlier, Okada et al. [15,16] showed an increase in cure reaction with improved interaction of rubber–organoclay composite. They related slow vulcanizing reaction in the untreated clay–rubber composites to the lack of interaction and reaction inhibition effects on the mineral surface. The values of optimum cure can be related to the viscosity and molecular weight between the crosslinks of the compounds. As seen in Table 2, T90 values for both RML and RCL composites are higher than those for the composites prepared by the modified mill method. It is obvious that rubber undergoes higher shear breakdown at the mill and retains lower bulk viscosity values for the stock prepared by the modified mill method as compared to the samples prepared by the latex method. This can be seen by comparing the Mooney viscosity of these
Table 2 Curing characteristics and Mooney viscosity of compounds.
Fig. 1. TEM micrographs of 5 wt.% filled nancomposites prepared by mill method (a) and by latex method (b).
Compound
Scorch time, min t2
Cure time, min t90
Optimum torque, lb-in T90
Mooney viscosity at 100 °C
Gum RML5 RML10 RMM5 RMM10 RCL5 RCL10 RCM5 RCM10
13.2 10.1 9.8 11.7 9.5 11.9 11.2 12.6 10.7
21.2 14.4 13.9 16.3 14.7 15.1 14.3 17.4 15.8
63.3 69.7 72.8 69.1 71.4 78.3 82.9 76.5 81.3
32.3 37.5 42.7 36.3 38.8 41.2 43.6 38.2 39.7
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Fig. 2. Dynamic storage modulus at 30°C of rubber-filler composites prepared by mill method.
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Fig. 4. Comparison of dynamic storage modulus of rubber-filler composites prepared by both latex and mill methods.
the figures that in general, the values of G′ in both types of filler systems are larger in the latex method than those for the modified mill method. Further, the viscoelastic properties of unvulcanized RM composites are superior to those of the RC composites. The storage modulus of RML10, containing 10 phr of OMMT filler, is higher than that of the rubber containing 20 phr of carbon black as shown in Fig. 4.
rubber compounds as shown in Table 2. The lower viscosity of RM stocks provides ease of processing and thus this new method of OMMT incorporation into rubber can improve processability of nanocomposites. The dynamic storage modulus (G′) is a measure of the energy (elastic) stored and recovered in cyclic deformation; therefore, in a filled system it gives a direct measure of dispersion and interaction. Clearly for higher interaction of well-dispersed fillers, higher values of G′ are desired. On the other hand, G′ is also related to the molecular weight of the primary chain of polymer. The results are compared for both of the processing methods and presented in Figs. 2 and 3. It can be seen from
In RC composite systems, the higher values of T90 are related to the larger crosslink density of the final vulcanizates. As shown in Fig. 5,
Fig. 3. Dynamic storage modulus of rubber-filler composites prepared by latex method.
Fig. 5. Crosslink density of rubber-filler composites.
3.2. Properties of vulcanized rubber
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Fig. 8. Modulus of the rubber-filler composites prepared by latex method. Fig. 6. Tensile strength of rubber-filler composites.
the RC composite has higher values of crosslink density than those of RM composites in both processing methods involved in this study. A simple explanation for this is that in RC composites an uneven distribution of curatives takes place due to the formation of the carbon rich phase and carbon free phase. Since the immobilized rubber in the carbon rich phase is formed before curatives are added, it is deficient in curatives concentration, while the concentration of curatives in the free rubber phase is higher. This in turn increases the crosslink density in the black loaded compounds [27]. Considering this situation, lower values of crosslink density in the RM compounds appear more promising for more uniform crosslink distribution. Furthermore, the
main difference in crosslink density between RMM and RML systems is interpreted as stemming from better dispersion of organophilic mineral and formation of more exfoliated structure in RMM composites compare to RML composites. The changes resulting from the reinforcement of fillers appear as a common behavior in most of the elastomers such as improved hysteresis, higher tensile strength, higher modulus, higher abrasion resistant and rebound resilience changes. Several samples were tested to evaluate these properties and comparisons of the results are presented in Figs. 6–9. Fig. 6 shows improvement in the tensile strength in RM composites. The reinforcement effect in terms of tensile strength is higher for samples prepared via the modified mill method for a given
Fig. 7. Modulus of the rubber-filler composites prepared by mill method.
Fig. 9. Rebound of rubber-filler composites.
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Fig.11. Hardness of rubber-filler composites.
Fig. 10. Wear rating of the NR composites prepared by both latex and mill methods.
experimental system. This effect can be related to the better dispersion and interaction of rubber–organosilicate systems. Also the relatively slow curing rate in these composites enhances the probability of uniform and strong sulfur linkages, which gives rise to tensile strength [28]. For example, the strength of RMM5, containing 5 phr of OMMT filler, is higher than that of the rubber containing 10 phr of carbon black. Figs. 7 and 8 represent the tensile modulus of rubber-filler composites prepared by the latex and mill methods, respectively. The modulus values are presented for 100%, 200%, and 300% elongation. It can be seen that moduli at a given elongation are relatively low in the composites prepared by the latex method than those prepared by the modified mill method. Further, in all cases the RM type composites are softer than RC type composites. The resilience of rebound is the ratio of the energy recovered to the energy lost as a result of deformation; therefore, it gives a measure of the hysteresis. It can be seen in Fig. 9 that the rebound values of RM composites are higher than those of the RC composites. Besides better dispersion and adhesion of RM composite systems, other factors are also considered to influence the hysteresis as deduced from these tests. One factor may be less chain breakdown and hence less chainend formation during polymer mixing. The chain ends are considered a major source of the hysteresis [29]. Furthermore, the strain amplification and immobilized rubber in the carbon rich phase are also considered to promote chain rupture during milling and hence this leads to lower values of rebounds in RC composites. On the other hand, higher chain breakdown and hence higher chain ends formation during milling would adversely affect the rebound performances of stocks prepared by the modified mill method. Therefore, the higher rebound values of stocks prepared by the latex method in both RML and RCL can be related to lower chain ends in such stocks as compared to the modified mill method that is in the RMM and RCM stocks. In order to obtain fatigue abrasion in a control environment, wear tests are performed on a laboratory scale grit (dull) abrader. The results are presented in Fig. 10. These tests show marked reinforcement effects as reported by higher wear ratings [30]. In addition, Fig. 11 relates mean hardness of the vulcanizates. It can be seen that the samples prepared by the latex method appeared relatively harder in both types of rubber-filler systems.
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4. Conclusions A modified method of improving organophilic nanoclay dispersion in natural rubber is presented. The resultant RM composites of NR and OMMT prepared from our modified mill method and the latex method are compared for different mechanical and curing behaviors. Based on the experimental system of this study, we found that the degree of reinforcement in RM composite is more than twice as large as in rubber–carbon black RC composites. The excellent mechanical properties of RM composite are known to stem from a large surface area, uniformly dispersed organophilic silicate filler and ionic bonds between polymer and silicate layers. The mechanical properties of the RM nanocomposites produced by the modified mill method are found to be improved over those produced by the latex method. On the basis of general reinforcement effects, the modified mill method is more convenient and of practical importance for solid rubber processing. This work also has application for organically modified smectite clays to be an effective reinforcing filler in rubber application for biological systems if it is dispersed on a nanometer level through some refined method. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
M. Alexandre, P. Dubois, Mat. Sc. Eng. 28 (2000) 1. N. Hasengawa, et al., J. Appl. Polym. Sci. 38 (1998) 1351. L. He, L.F. Allard, E. Ma, Forth Internatl. Conf. Nanostructd. Materials 12 (1999) 543. Y. Wang, L. Zhang, C. Tang, Dingsheng Yu, J. Appl. Polym. Sci. 78 (2000) 1879. S.D. Burnside, E.R. Giannelis, Chem. Mater. 7 (1995) 1597. T. Lan, T.J. Pinnavaia, Chem. Mater. 6 (1994) 2216. O.C. Wilson Jr., T. Olorunyolenii, A. Jaworsld, L. Borum, D. Young, A. Siriwat, E. Dickens, C. Oriakhi, M. Lerner, Appl. Clay Sci. 15 (1999) 265. M. Lerner, C. Oriakhi, in: A. Goldstein (Ed.), Handbook of Nanophase Materials, Marcel Dekker, New York, 1997, p. 199. G. Lagaly, Appl. Clay Sci. 15 (1999) 1. D.J. Greenland, J. Colloid Sci 18 (1963) 647. N. Ogata, S. Kawakage, T. Ogihara, J. Appl. Polym. Sci. 66 (1997) 573. R.L. Parfitt, D.J. Greenland, Clay Miner. 8 (1970) 305. T. Lan, P.D. Kaviratna, T.J. Pinnavaia, Chem. Mater. 7 (1995) 2144. C.O. Oriakhi, I.V. Farr, M.M. Lerner, Clays Clay Miner. 45 (1997) 194. A. Okada, K. Fukumori, A. Usuki, Y. Kojima, T. Kurauchi, O. Karffigaito, Polym. Prep. 32 (1991) 540. A. Okada, A. Usuld, Mater. Sci. Eng. C3 (1995) 109. C. W. Elspass, Exxon Research and Engineering Company, Canada patent no. CA02221649, (1997).
596
M.N. Qureshi, H. Qammar / Materials Science and Engineering C 30 (2010) 590–596
[18] D.J. Suh, Y.T. Lim, O.O. Park, Polymer 41 (2000) 8557. [19] P.J. Flory, J. Rehner, J Chem. Phys. 11 (1943) 512; P.J. Flory, J Chem. Phys. 18 (1950) 108. [20] M. Hunang, Mechanical Properties of Anisotropic Rubber Networks, Dissertn., at the Uni. Akron, 1997. [21] B. Hoffmann, J. Kressler, G. Stoppelmann, Chr. Friedrich, G.M. Kim, Colloid Polm. Sci. 278 (2000) 629. [22] P. Reichert, J. Kessler, R. Thomann, R. Mulhaupt, G. Stoppelmann, Acta Polym. 49 (1998) 116. [23] C.H. Chen, J.L. Koenig, J.R. Shelton, E.A. Collins, Rubber. Chem. Technol. 55 (1982) 103.
[24] M. Nakahara, T. Takada, H. Kumagai, Y. Sanada, Carbon 33 (11) (1995) 1537. [25] Makio Mori, J.K. Koenig, Rub. Chem. Technol. 70 (1997) 671. [26] Dongcheng Kong, J.L. White, Frederick C. Wessert, N. Nakajima, Rub. Chem. Technol. 60 (1987) 140. [27] M.N. Qureshi, H. Qammar, J. Padovan, Rub. Plast. News (May, 2001) 15. [28] S. Goo Kim, Suck-Hyun Lee, Rub. Chem. Technol. 67 (1994) 649. [29] A. Ahagon, Rub Chem. Technol. 71 (1998) 975. [30] M. N. Qureshi, Hierarchical Microdomains in Solution and Emulsion Polymerized Elastomers Induced by Synthesis and Processing and Potential Effects on Properties, Doctoral Dissertation, Univ. of Akron, OH, May (2002) 48–146.