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Dynamically vulcanized blends of polyethylene–octene elastomer and ethylene–propylene–diene terpolymer
Materials Chemistry and Physics 126 (2011) 272–277
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Dynamically vulcanized blends of polyethylene–octene elastomer and ethylene–propylene–diene terpolymer Zhaobo Wang a,∗ , Xiangkun Cheng a , Jian Zhao b a b
College of Materials Science & Engineering, Qingdao University of Science & Technology, Qingdao 266042, People’s Republic of China Key Laboratory of Rubber-Plastics, Ministry of Education, Qingdao 266042, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 21 August 2010 Received in revised form 31 October 2010 Accepted 14 November 2010 Keywords: Polyethylene–octene copolymer Ethylene–propylene diene copolymer Thermoplastic vulcanizates Dynamic mechanical properties
a b s t r a c t Thermoplastic vulcanizates (TPVs) based on polyethylene–octene elastomer (POE)/ethylene–propylene diene copolymer (EPDM) blends were prepared by dynamic vulcanization, the effects of the EPDM incorporation on mechanical, dynamic mechanical and morphological properties of the TPVs were investigated systemically. Experimental results indicate that compared with pure POE, the improvement of mechanical properties of POE/EPDM blends was achieved; the tensile strength and tearing strength reached the maximum at an EPDM content of 20 wt.%. It is interesting to note that the tensile set at break was decreased almost linearly with increasing EPDM loading, decreasing from 190% (at 0 phr EPDM) to 55% (at 30 wt.% EPDM), indicating the relatively large permanent deformation of POE is improved remarkably by the incorporation with EPDM. Phase contrast microscopy studies show that the vulcanized EPDM particles were uniformly dispersed in the POE matrix. RPA results show that the elastic modulus increased with increasing frequency, moreover, it is noteworthy that the storage modulus of dynamically vulcanized POE/EPDM blends was higher than that of pure POE, however, the storage modulus decreased with increasing EPDM content, the incorporation of EPDM had almost no influence on the tan ı. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Thermoplastic elastomers (TPEs), which combine the processing advantages of thermoplastics with the flexible, low-modulus properties of elastomers, continue to grow in a wide variety of applications. In recent years, the development of Dow’s constrained geometry catalyst technology has led to the development of a new class of elastomers based on homogeneous ethylene-alpha-olefin copolymers [1]. In particular, copolymers with more than 8% octene possess low crystallinity and rubber-like behavior that depends on physical rather than chemical junctions [2]. The main advantage of ethylene–olefin copolymers over chemically vulcanized elastomers lies in their ease of processing and post-processing, much like conventional polyethylene [3]. Although POE has been widely used in toughening thermoplastics, wire and cables, etc., the low heat distortion temperature and the large permanent deformation certainly limit its wider application. Crosslinking is an important way to improve the mechanical properties, thermal and chemical resistance of polyolefins (PO) [4]. There are three main crosslinking methods, i.e. radiation crosslinking, peroxide crosslinking and silane crosslinking. Silane crosslinking is cost-effective and easily operated [5], and
∗ Corresponding author. Tel.: +86 532 84022772; fax: +86 532 84022814. E-mail addresses: [email protected], [email protected] (Z. Wang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.11.027
thus it is commonly employed to produce wire and cables, plastic pipes, etc. Thermoplastic vulcanizates (TPVs) are a unique class of TPEs prepared by melt blending technique and comprise of the fastest growing in the family of TPEs [6,7]. TPVs are prepared by reactive blending of rubber and thermoplastic, in which the rubber phase is crosslinked under dynamic condition using a suitable crosslinking agent [8–12]. Significant improvements in the properties of these components were achieved by fully vulcanizing the rubbery phase without affecting the thermoplasticity of the blends. Properties of these TPVs are generally determined by the compatibility of blend components, extent of crosslinking in rubbery phase, degree of dispersion and also on solid state morphology [13]. Although there is some work on the dynamic vulcanization of POE TPEs with thermoplastics [14,15], there is virtually no literature reference on this topic where the dynamic vulcanization is processed by blending POE elastomer with other rubber compound, which is probably due to the low heat distortion temperature of POE. In this paper, we reported the preparation of TPVs based on the blends of POE and EPDM via dynamic vulcanization, the purpose of this research was to investigate the possibility of dynamic vulcanization on the improvement of permanent deformation of pure POE. Morphology of the dynamically vulcanized POE/EPDM blends was studied using phase contrast microscope. The mechanical and dynamic mechanical properties of the dynamically vulcan-
Z. Wang et al. / Materials Chemistry and Physics 126 (2011) 272–277
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ized blends were investigated systematically as a function of the POE/EPDM blend composition. 2. Experimental 2.1. Materials POE (Engage 8150, a Metallocene catalysed copolymer of ethylene and 1-octene with 25 wt.% 1-octene) was produced by DuPont Elastomers Company, with a melt flow index (MFI) of 0.5 g/10 min (190 ◦ C, 2.16 kg). Ethylene–propylene–diene rubber (EP33 type, with a diene component 5-ethylidene-2-norbornene (ENB), diene content of 8.1%, ethylene content of 52%, and ML1 + 4 (100 ◦ C) = 45) was commercially manufactured by Japan Synthetic Rubber Co., Ltd., Japan. Tetramethylthiuram disulfide (TMTD) and N-cyclohexyl-2-benzothiazole sulfenamide (CZ) were used as accelerators and manufactured by Northeast Auxiliary Chemical Industry Co., Hebei, China. The sulfur was used as a vulcanization agent and obtained from Hengye Zhongyuan Chemical Co., Ltd., Beijing, China. The zinc oxide (ZnO) was used as an activator and obtained from NewLe Qinshi Zinc Co., Ltd., Xinle, China. The stearic acid was used as an activator and obtained from Wanyou Co., Ltd., Zibo, China. The poly(1,2-dihydro-2,2,4-trimethyl-quinoline) (Antioxidant RD) was used as an antioxidant and obtained from Shengao Chemical Co., Ltd., Caoxian, China. 2.2. Preparation of dynamically vulcanized POE/EPDM blends Commercially available EPDM rubber and POE were used for the TPVs. The concentrations in crosslink EPDM system were expressed in parts per hundred part of rubber by weight (phr). The sulfur-containing accelerating system recipe consisted of the following ingredients: 100 phr EPDM, 2.0 phr CZ, 1.0 phr TMTD, 1.0 phr sulfur, 1.5 phr stearic acid, 5.0 phr ZnO, 2.0 phr RD. The dynamically vulcanized POE/EPDM blends were produced via a two-step mixing process. In the first step the pre-blends containing EPDM and the crosslink ingredients were compounded in a two-roll mill at room temperature. After 3 min of mixing time, the pre-blends were removed from the mixer. In the second step the TPVs compounds were prepared, by melt-mixing the EPDM pre-blends with the POE using a Brabender PLE 331 plasticorder (Brabender Gmbh, Germany). The mixer temperature was kept at 160 ◦ C with a constant rotor (camtype) speed of 80 rpm. The POE/EPDM weight ratio was varied from 95/5 to 70/30. In detail, firstly, the required quantity POE was charged into the mixer and allowed to melt. After 2 min, the EPDM based pre-blend was added. The mixing was continued for another 10 min to allow the dynamic vulcanization. Finally, the molten compound was removed from the mixer and then, passed through a cold two-roll mill to obtain a sheet about 2 mm thick. The sheet was cut and pressed for 8 min in a compression-molding machine at 125 ◦ C. Aluminum foil was placed between the molded sheet and the press plates. The sheet was then cooled down to room temperature under pressure. Test specimens were die-cut from the compression-molded sheets and used for testing after 24 h. For the statically vulcanized POE/EPDM blend, firstly POE and EPDM pre-blend was mixed evenly at 110 ◦ C in order to avoid the premature crosslink, then the POE/EPDM static vulcanizate was prepared in a compression-molding machine under the preparation condition of 160 ◦ C × 10 min. EPDM static vulcanizate was prepared in a compression-molding machine under the preparation condition of 160 ◦ C × 10 min. 2.3. Characterization 2.3.1. Vulcanization characteristic The vulcanization cure of the EPDM based pre-blend was determined by using a moving die rheometr (GT-M2000, GoTech Testing Machines Co., Ltd., Taiwan, China) at 160 ◦ C and vibration angel was ±1◦ .
Fig. 1. Vulcanization curve of EPDM based pre-blend at 160 ◦ C.
the POE/EPDM TPV samples. The frequency strain sweep was carried out from 6 to 1800 cpm at 30 ◦ C and 1% strain. The storage modulus (G ), loss modulus (G ) and tan ı values were measured as a function of frequency.
3. Results and discussion The vulcanization curve of the EPDM pre-blend is illustrated in Fig. 1. As observed in Fig. 1, the cure characteristic is obvious plateau curing, indicating the relatively high thermal stability of EPDM preblend, moreover, it is favorable for the processing control during the later melt-mixing of dynamic vulcanization. The t10 and t90 , which are defined as the scorch time and the optimum cure time, was 3.0 min and 9.7 min, respectively. According to the analysis of the vulcanization curve, the proper dynamic vulcanization time should be 10 min. Fig. 2 illustrates the stress–strain behaviors of the dynamically vulcanized POE/EPDM blends and EPDM vulcanizate. The stress–strain curves of the dynamically vulcanized POE/EPDM blends in Fig. 2 are similar in the curve shape. Initially, an increase in tensile stress can be observed. Upon further deformation, the slope of the curve decreases while the stress increases almost linearly with strain, however, the slope of the stress–strain curve increases sharply when the strain is 500% above. All the stress–strain curves show the representative elastomer character of soft and tough, indicating that the dynamically vulcanized POE/EPDM blends can be attributed to elastomers. From Fig. 2, it can also be seen that the
2.3.2. Mechanical properties For the measurement of tensile properties, dumbbell-shape specimens were prepared according to ASTM D412. The tearing strength was tested according to ASTM D624 using the unnotched 90◦ angle test pieces. Both tensile and tearing tests were performed on a universal testing machine (AI-7000M, Taiwan Gaotie Technology, China) at a crosshead speed of 500 mm/min. Tension set at 100% elongation was measured according to ASTM 1566. The Shore A hardness was determined using a hand-held Shore A Durometer according to ASTM D2240. All tests were carried out at 23 ◦ C. The average value was calculated for 5 test specimens. 2.3.3. Microscopy analysis The phase structure photographs were obtained using an XS-18 phase contrast microscope (Nanjing Jiangnan Optics & Electronics Co., Ltd., China). The specimens were prepared from thin (micron thickness) slices of the TPV samples, placed between coverglasses, inserted into the hot plate which was maintained at 160 ◦ C, pressed after 2 min, and then cooled to room temperature. 2.3.4. Rubber process analyzer (RPA) The RPA 2000 (Alpha Technologies, USA) is primarily a torsional dynamic rheometer, with an advance temperature control and fully automated operational modes. In this particular case, the RPA was used to study the frequency sweep of
Fig. 2. Stress–strain curves of pure POE, dynamically vulcanized POE/EPDM blends and EPDM vulcanizate.
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Fig. 3. Effect of EPDM content on tensile strength and shore A hardness of dynamically vulcanized POE/EPDM blends.
elongation at break of POE/EPDM blends was decreased obviously with the EPDM incorporation. Moreover, compared with POE and dynamically vulcanized POE/EPDM blends, the EPDM vulcanizate was weak in modulus and low in elongation at break. Tensile strength, shore A hardness, elongation at break, tensile set at break, tensile set (100% elongation) and tearing strength of the dynamically vulcanized POE/EPDM blends are shown in Figs. 3–6, respectively. Increasing the loading of EPDM in the POE/EPDM blends led to improved tensile strength when the EPDM content was 25 wt.% below; the tensile strength reached a maximum at 20 wt.% EPDM incorporation, increasing from 10.5 MPa (at 0 phr EPDM) to 12.1 MPa (at 20 wt.% EPDM). However, the EPDM loading decreased the Shore A hardness of the POE/EPDM blends obviously. It is clear that the Shore A hardness of POE and EPDM vulcanizate in the experiment were 74 and 50, respectively, the more EPDM content in the POE/EPDM blends led to the significant decrease in the hardness. With increasing loading of EPDM, the elongation at break (Fig. 4) was substantially decreased especially when the EPDM content is 25 wt.% above. The existence of EPDM in the POE will inevitably fragment the continuity of the matrix, leading to the decrease of elongation at break. It is interesting to note that the tensile set at break was decreased almost linearly with increasing EPDM loading, decreasing from 190% (at 0 phr EPDM) to 55% (at 30 wt.% EPDM), indicating the permanent deformation of POE is improved obviously by dynamic vulcanization with EPDM. Generally, the dynamic vulcanizates exhibit large reversibility and small residual strains
Fig. 4. Effect of EPDM content on elongation at break and tensile set at break of dynamically vulcanized POE/EPDM blends.
Fig. 5. Effect of EPDM content on tensile set (100% elongation) of dynamically vulcanized POE/EPDM blends.
[16]. The elastomeric cross-linked EPDM particles dispersed in the POE matrix, enable the POE/EPDM blends to elastically recover from a highly deformed state, therefore, the permanent deformation of POE can be improved remarkably. Fig. 5 shows the influence of EPDM content on the tension set at 100% elongation which was measured according to ASTM 1566, it is clear that even at relatively low elongation, the interface interaction between POE matrix and EPDM dispersed phase can effectively reduce the permanent deformation of dynamically vulcanized blends. The EPDM loading also affected the tear strength. As shown in Fig. 6, the tear strength was increased when the EPDM content was 25 wt.% below and reached a maximum at 20 wt.% EPDM incorporation, indicating a reinforcing effect of proper EPDM incorporation on POE matrix. It is striking that tensile strength and tearing strength of POE could be promoted by the incorporation of dynamically vulcanized EPDM phase and reached the maximum value when the EPDM content is around 20 wt.%. The origin of maximal tensile/tear strengths around 20 wt.% EPDM is clarified as below. As we know, the main structural units in POE and EPDM molecules are vinyl, moreover, POE and EPDM are all amorphous materials, resulting in the well compatibility and the strong interface interaction in the POE/EPDM
Fig. 6. Effect of EPDM content on tear strength of dynamically vulcanized POE/EPDM blends.
Z. Wang et al. / Materials Chemistry and Physics 126 (2011) 272–277 Table 1 Mechanical properties of POE and EPDM vulcanizate. POE Tensile strength (MPa) Shore A hardness Elongation at break (%) Tensile set at break (%) Tear strength (kN/m)
10.8 74.0 876.0 190.0 33.5
EPDM vulcanizate ± ± ± ± ±
0.16 0.60 14.70 5.52 0.47
2.1 50.0 272.2 9.0 11.5
± ± ± ± ±
0.03 0.84 5.90 0.64 1.74
blends. For the dynamically vulcanized blends, the interface interaction between the POE matrix and the EPDM dispersed particles plays a role of physical crosslink in the POE matrix, which is similar with the function of physical junctions resulted from the microcrystalline in the POE matrix. With the increasing of EPDM content, the increased physical crosslink inevitable leads to the improved mechanical properties, this is consistent with the influence of EPDM on the tensile strength and tearing strength of POE/EPDM blends, as shown in Figs. 3 and 6. Usually, the mechanical properties of the dispersed rubber phase in TPVs can be characterized by that of static vulcanizate prepared at the same temperature and vulcanization time as the dynamic vulcanization procedure. The mechanical properties of static EPDM vulcanizate and pure POE are listed in Table 1. From Table 1, we know that compared with POE, the EPDM vulcanizate is weak in mechanical properties. In our experiment, there was no reinforcing material in the crosslink EPDM system, leading to the low hardness and weak mechanical property. Increasing the EPDM content in POE/EPDM blends, the volume of EPDM dispersed phase is increased. The existence of EPDM in the POE will inevitably fragment the continuity of the matrix and reduce the number of POE molecules which can effectively withstand the external forces, leading to the decrease of mechanical properties. Although the interface interaction is enhanced with the increasing EPDM con-
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Table 2 Mechanical properties of POE/EPDM blends prepared by different vulcanization methods (POE/EPDM = 80/20). Dynamic vulcanization Tensile strength (MPa) Shore A hardness Elongation at break (%) Tensile set at break (%) Tear strength (kN/m)
12.1 66.0 809.3 90.0 41.7
± ± ± ± ±
0.53 0.60 12.50 5.20 0.37
Static vulcanization 12.3 71.0 884.2 196.4 27.9
± ± ± ± ±
0.53 0.50 11.50 18.63 3.74
tent, the deterioration of performance caused by the increased EPDM volume tends to dominate gradually, and eventually leads to the decrease of mechanical properties when the addition of EPDM content is 20 wt.% above. Fig. 7 shows the phase contrast micrograph of dynamically vulcanized POE/EPDM blends. The phase morphology is observed with a continuous resin phase and a dispersed elastomeric phase. The crosslinked EPDM particles are different in size and dispersed in the thermoplastic matrix, moreover they have irregular shape. Most of the discrete EPDM are particle-like with diameter ranging from several to 20 m. Moreover, increasing the EPDM content in POE/EPDM blend will lead to the increased size of EPDM particles, especially when the EPDM content is 20 wt.% above, as shown in Fig. 7a–c. During the dynamic vulcanization, the viscosity of the EPDM phase was increased quickly due to the initiation of the crosslinking reaction, leading to the increasing shear stress acting on the EPDM phase. The crosslinked rubber particles were produced by breaking up the initially large EPDM entities under local stress. In order to investigate the dynamic vulcanization on the properties of POE/EPDM blends, the statically vulcanized POE/EPMD blends were also prepared and the mechanical properties of the blend prepared by dynamic vulcanization and static vulcanization were listed in Table 2. The results show that compared with the stat-
Fig. 7. Phase structure of the dynamically vulcanized and statically vulcanized POE/EPDM blends. a: dynamic vulcanization (POE/EPDM = 95/5); b: dynamic vulcanization (POE/EPDM = 80/20); c: dynamic vulcanization (POE/EPDM = 70/30); d: static vulcanization (POE/EPDM = 80/20).
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Fig. 8. Storage modulus (G ) as a function of log (frequency).
ically vulcanized blend, the dynamically vulcanized blend shows superior properties, especially in tensile set at break and tearing strength. Moreover, the compounding of EPDM in POE by static vulcanization has almost no effect on decreasing the permanent deformation of POE. Fig. 7 also shows the influence of vulcanization method on the size and dispersion behavior of the EPDM dispersed particles in the POE matrix. Compared Fig. 7b with Fig. 7d, it is clear that the dynamic vulcanization is effective in reducing the size of EPDM particles, which was caused by the increasing shear stress induced by the initiation of crosslink reaction. The less size of EPDM particles will result in the larger interface between POE matrix and EPDM dispersed phase, leading to the stronger interface interaction and superior mechanical properties. It can be clearly seen that dynamic vulcanization is an effective method to decrease the permanent deformation of POE. RPA is an ideal testing tool that can provide deep understanding of elasticity and processability of filled elastomers in terms of the dynamic functions, such as dynamic module of elasticity and viscosity, over a wide range of frequency. Figs. 8, 9 and 10 show elastic modulus, loss modulus and loss tangent values (G , G and tan ı) of pure POE and dynamically vulcanized POE/EPDM blends as a function of frequency at 30 ◦ C, respectively. In Fig. 8 it can be seen that the elastic modulus increased slightly with increasing frequency. It is well known that the effect at high frequency corresponds to the effect at low temperature. Therefore, increasing frequency can lead to an increase in rigid and elastic modulus. In the experiments, EPDM was incorporated into POE during dynamic vulcanization in order to decrease the permanent deformation of POE matrix. Compared with POE,
Fig. 10. tan ı as a function as a function of log (frequency).
the chemical crosslink in EPDM vulcanizate endow the EPDM dispersed phase the large recovery ability under elongation, which can be transfered to the POE matrix effectively through the strong interaction interface and lead to the low permanent deformation of POE. The obviously decrease in tensile set at break in Fig. 4 confirms the improvement in permanent deformation of POE undoubtedly. It is noteworthy that the storage modulus of dynamically vulcanized POE/EPDM blends was higher than that of pure POE, however, the storage modulus decreased with increasing EPDM content in POE/EPDM blends. The complex effects of EPDM dispersed phase on the POE matrix cause the result in Fig. 8. POE and EPDM are all random copolymer where the main structural units are vinyl, which results in the well compatibility and the strong interface interaction. The role of the strong interface interaction is similar to that of the physical crosslink in the POE matrix, and all make it difficult for the POE matrix to deform under stress, leading to the obviously increasing storage modulus. On the other hand, the EPDM used in the experiment is unfilled elastomer and has a relative low modulus and hardness compared to that of pure POE, therefore the EPDM dispersed phase in the POE/EPDM blends is easy to deform in spite of the existence of chemical crosslink. With the increase of EPDM weight content in the dynamically vulcanized POE/EPDM blends, the storage modulus was decreased. As shown in Fig. 9, the loss modulus increased slightly with increasing frequency. The influence rule of the incorporation of EPDM in POE matrix and the EPDM content on the loss modulus of dynamically vulcanized POE/EPDM blends is similar to that on the storage modulus. Fig. 10 shows the loss tangent of pure POE and dynamically vulcanized POE/EPDM blends as a function of oscillation frequency. It is notable that the shapes of the curves in Fig. 10 are very similar. The increasing frequency had almost no influence on the tan ı in the experimental range. In the theory of viscoelasticity, tan ı = G /G ; it can be seen clearly from Figs. 8 and 9 that, accompanied by a slight increase in loss modulus, the elastic modulus also increased slightly with increasing frequency, thus giving rise to the relatively stable value of tan ı. It is noteworthy that the incorporation of EPDM into the POE led to the almost unchanged tan ı. 4. Conclusions
Fig. 9. Loss modulus (G ) as a function of log (frequency).
The purpose of this research was to investigate the possibility of dynamic vulcanization on the improvement of permanent deformation of POE, especially in the theory. The TPVs based on POE/EPDM blends were prepared by dynamic vulcanization. Compared with pure POE, the improvement of mechanical properties of POE/EPDM blends was achieved; tensile strength and tearing strength reached the maximum at an EPDM content of 20 wt.%.
Z. Wang et al. / Materials Chemistry and Physics 126 (2011) 272–277
The elongation at break was slightly decreased especially when the EPDM content is 25 wt.% below, however, it is interesting to note that the tensile set at break was decreased almost linearly with increasing EPDM loading, decreasing from 190% (at 0 phr EPDM) to 55% (at 30 wt.% EPDM), indicating the relatively large permanent deformation of POE is improved remarkably by the incorporation with EPDM. Phase contrast microscopy studies show that the vulcanized EPDM particles with irregular shape and diameter ranging from several to 20 m were uniformly dispersed in the POE matrix. RPA results show that the elastic modulus increased with increasing frequency, moreover, it is noteworthy that the storage modulus of dynamically vulcanized POE/EPDM blends was higher than that of pure POE, however, the storage modulus decreased with increasing EPDM content in POE/EPDM blends, which was caused by the complex effects of EPDM dispersed phase on the POE matrix. The incorporation of EPDM had almost no influence on the tan ı of POE/EPDM blends. Acknowledgements The work was funded by the Doctoral Fund of Qingdao University of Science and Technology and the Key Laboratory of
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Rubber-Plastics (Qingdao University of Science and Technology), Ministry of Education, People’s Republic of China. References [1] C.S. Reddy, P.K. Patra, C.K. Das, Macromol. Symp. 277 (2009) 119. [2] S. Bensason, J. Minick, A. Moet, S. Chum, A. Hiltner, E. Baer, J. Polym. Sci. Pol. Phys. 34 (1996) 1301. [3] L. Flandin, A. Hiltner, E. Baer, Polymer 42 (2001) 827. [4] C.M. Jiao, Z.Z. Wang, Z. Gui, Y.A. Hu, Eur. Polym. J. 41 (2005) 1204. [5] Y.T. Shieh, T.H. Tsai, J. Appl. Polym. Sci. 69 (1998) 255. [6] E.V. Prut, N.A. Erina, J. Karger-Kocsis, T.I. Medintseva, J. Appl. Polym. Sci. 109 (2008) 1212. [7] G. Sarkhel, A. Choudhury, J. Appl. Polym. Sci. 115 (2010) 376. [8] R.R. Babu, N.K. Singha, K. Naskar, J. Appl. Polym. Sci. 117 (2010) 1578. [9] H. Mirzazadeh, A.A. Katbab, Polym. Adv. Technol. 17 (2006) 975. [10] M. Awang, H. Ismail, J. Vinyl. Addit. Technol. 15 (2009) 29. [11] M.D. Ellul, A.H. Tsou, W. Hu, Polymer 45 (2004) 3351. [12] G. Martin, C. Barres, P. Sonntag, N. Garois, P. Cassagnau, Eur. Polym. J. 45 (2009) 3257. [13] S. Abdou-Sabet, R.C. Puydak, C.P. Radar, Rubber Chem. Technol. 69 (1996) 476. [14] R.R. Babu, N.K. Singha, K. Naskar, Polym. Eng. Sci. 50 (2010) 455. [15] R.R. Babu, N.K. Singha, K. Naskar, J. Polym. Res. 17 (2010) 657. [16] A.K. Jain, N.K. Gupta, A.K. Nagpal, J. Appl. Polym. Sci. 77 (2000) 1488.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Dynamically vulcanized blends of polyethylene–octene elastomer and ethylene–propylene–diene terpolymer Zhaobo Wang a,∗ , Xiangkun Cheng a , Jian Zhao b a b
College of Materials Science & Engineering, Qingdao University of Science & Technology, Qingdao 266042, People’s Republic of China Key Laboratory of Rubber-Plastics, Ministry of Education, Qingdao 266042, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 21 August 2010 Received in revised form 31 October 2010 Accepted 14 November 2010 Keywords: Polyethylene–octene copolymer Ethylene–propylene diene copolymer Thermoplastic vulcanizates Dynamic mechanical properties
a b s t r a c t Thermoplastic vulcanizates (TPVs) based on polyethylene–octene elastomer (POE)/ethylene–propylene diene copolymer (EPDM) blends were prepared by dynamic vulcanization, the effects of the EPDM incorporation on mechanical, dynamic mechanical and morphological properties of the TPVs were investigated systemically. Experimental results indicate that compared with pure POE, the improvement of mechanical properties of POE/EPDM blends was achieved; the tensile strength and tearing strength reached the maximum at an EPDM content of 20 wt.%. It is interesting to note that the tensile set at break was decreased almost linearly with increasing EPDM loading, decreasing from 190% (at 0 phr EPDM) to 55% (at 30 wt.% EPDM), indicating the relatively large permanent deformation of POE is improved remarkably by the incorporation with EPDM. Phase contrast microscopy studies show that the vulcanized EPDM particles were uniformly dispersed in the POE matrix. RPA results show that the elastic modulus increased with increasing frequency, moreover, it is noteworthy that the storage modulus of dynamically vulcanized POE/EPDM blends was higher than that of pure POE, however, the storage modulus decreased with increasing EPDM content, the incorporation of EPDM had almost no influence on the tan ı. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Thermoplastic elastomers (TPEs), which combine the processing advantages of thermoplastics with the flexible, low-modulus properties of elastomers, continue to grow in a wide variety of applications. In recent years, the development of Dow’s constrained geometry catalyst technology has led to the development of a new class of elastomers based on homogeneous ethylene-alpha-olefin copolymers [1]. In particular, copolymers with more than 8% octene possess low crystallinity and rubber-like behavior that depends on physical rather than chemical junctions [2]. The main advantage of ethylene–olefin copolymers over chemically vulcanized elastomers lies in their ease of processing and post-processing, much like conventional polyethylene [3]. Although POE has been widely used in toughening thermoplastics, wire and cables, etc., the low heat distortion temperature and the large permanent deformation certainly limit its wider application. Crosslinking is an important way to improve the mechanical properties, thermal and chemical resistance of polyolefins (PO) [4]. There are three main crosslinking methods, i.e. radiation crosslinking, peroxide crosslinking and silane crosslinking. Silane crosslinking is cost-effective and easily operated [5], and
∗ Corresponding author. Tel.: +86 532 84022772; fax: +86 532 84022814. E-mail addresses: [email protected], [email protected] (Z. Wang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.11.027
thus it is commonly employed to produce wire and cables, plastic pipes, etc. Thermoplastic vulcanizates (TPVs) are a unique class of TPEs prepared by melt blending technique and comprise of the fastest growing in the family of TPEs [6,7]. TPVs are prepared by reactive blending of rubber and thermoplastic, in which the rubber phase is crosslinked under dynamic condition using a suitable crosslinking agent [8–12]. Significant improvements in the properties of these components were achieved by fully vulcanizing the rubbery phase without affecting the thermoplasticity of the blends. Properties of these TPVs are generally determined by the compatibility of blend components, extent of crosslinking in rubbery phase, degree of dispersion and also on solid state morphology [13]. Although there is some work on the dynamic vulcanization of POE TPEs with thermoplastics [14,15], there is virtually no literature reference on this topic where the dynamic vulcanization is processed by blending POE elastomer with other rubber compound, which is probably due to the low heat distortion temperature of POE. In this paper, we reported the preparation of TPVs based on the blends of POE and EPDM via dynamic vulcanization, the purpose of this research was to investigate the possibility of dynamic vulcanization on the improvement of permanent deformation of pure POE. Morphology of the dynamically vulcanized POE/EPDM blends was studied using phase contrast microscope. The mechanical and dynamic mechanical properties of the dynamically vulcan-
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ized blends were investigated systematically as a function of the POE/EPDM blend composition. 2. Experimental 2.1. Materials POE (Engage 8150, a Metallocene catalysed copolymer of ethylene and 1-octene with 25 wt.% 1-octene) was produced by DuPont Elastomers Company, with a melt flow index (MFI) of 0.5 g/10 min (190 ◦ C, 2.16 kg). Ethylene–propylene–diene rubber (EP33 type, with a diene component 5-ethylidene-2-norbornene (ENB), diene content of 8.1%, ethylene content of 52%, and ML1 + 4 (100 ◦ C) = 45) was commercially manufactured by Japan Synthetic Rubber Co., Ltd., Japan. Tetramethylthiuram disulfide (TMTD) and N-cyclohexyl-2-benzothiazole sulfenamide (CZ) were used as accelerators and manufactured by Northeast Auxiliary Chemical Industry Co., Hebei, China. The sulfur was used as a vulcanization agent and obtained from Hengye Zhongyuan Chemical Co., Ltd., Beijing, China. The zinc oxide (ZnO) was used as an activator and obtained from NewLe Qinshi Zinc Co., Ltd., Xinle, China. The stearic acid was used as an activator and obtained from Wanyou Co., Ltd., Zibo, China. The poly(1,2-dihydro-2,2,4-trimethyl-quinoline) (Antioxidant RD) was used as an antioxidant and obtained from Shengao Chemical Co., Ltd., Caoxian, China. 2.2. Preparation of dynamically vulcanized POE/EPDM blends Commercially available EPDM rubber and POE were used for the TPVs. The concentrations in crosslink EPDM system were expressed in parts per hundred part of rubber by weight (phr). The sulfur-containing accelerating system recipe consisted of the following ingredients: 100 phr EPDM, 2.0 phr CZ, 1.0 phr TMTD, 1.0 phr sulfur, 1.5 phr stearic acid, 5.0 phr ZnO, 2.0 phr RD. The dynamically vulcanized POE/EPDM blends were produced via a two-step mixing process. In the first step the pre-blends containing EPDM and the crosslink ingredients were compounded in a two-roll mill at room temperature. After 3 min of mixing time, the pre-blends were removed from the mixer. In the second step the TPVs compounds were prepared, by melt-mixing the EPDM pre-blends with the POE using a Brabender PLE 331 plasticorder (Brabender Gmbh, Germany). The mixer temperature was kept at 160 ◦ C with a constant rotor (camtype) speed of 80 rpm. The POE/EPDM weight ratio was varied from 95/5 to 70/30. In detail, firstly, the required quantity POE was charged into the mixer and allowed to melt. After 2 min, the EPDM based pre-blend was added. The mixing was continued for another 10 min to allow the dynamic vulcanization. Finally, the molten compound was removed from the mixer and then, passed through a cold two-roll mill to obtain a sheet about 2 mm thick. The sheet was cut and pressed for 8 min in a compression-molding machine at 125 ◦ C. Aluminum foil was placed between the molded sheet and the press plates. The sheet was then cooled down to room temperature under pressure. Test specimens were die-cut from the compression-molded sheets and used for testing after 24 h. For the statically vulcanized POE/EPDM blend, firstly POE and EPDM pre-blend was mixed evenly at 110 ◦ C in order to avoid the premature crosslink, then the POE/EPDM static vulcanizate was prepared in a compression-molding machine under the preparation condition of 160 ◦ C × 10 min. EPDM static vulcanizate was prepared in a compression-molding machine under the preparation condition of 160 ◦ C × 10 min. 2.3. Characterization 2.3.1. Vulcanization characteristic The vulcanization cure of the EPDM based pre-blend was determined by using a moving die rheometr (GT-M2000, GoTech Testing Machines Co., Ltd., Taiwan, China) at 160 ◦ C and vibration angel was ±1◦ .
Fig. 1. Vulcanization curve of EPDM based pre-blend at 160 ◦ C.
the POE/EPDM TPV samples. The frequency strain sweep was carried out from 6 to 1800 cpm at 30 ◦ C and 1% strain. The storage modulus (G ), loss modulus (G ) and tan ı values were measured as a function of frequency.
3. Results and discussion The vulcanization curve of the EPDM pre-blend is illustrated in Fig. 1. As observed in Fig. 1, the cure characteristic is obvious plateau curing, indicating the relatively high thermal stability of EPDM preblend, moreover, it is favorable for the processing control during the later melt-mixing of dynamic vulcanization. The t10 and t90 , which are defined as the scorch time and the optimum cure time, was 3.0 min and 9.7 min, respectively. According to the analysis of the vulcanization curve, the proper dynamic vulcanization time should be 10 min. Fig. 2 illustrates the stress–strain behaviors of the dynamically vulcanized POE/EPDM blends and EPDM vulcanizate. The stress–strain curves of the dynamically vulcanized POE/EPDM blends in Fig. 2 are similar in the curve shape. Initially, an increase in tensile stress can be observed. Upon further deformation, the slope of the curve decreases while the stress increases almost linearly with strain, however, the slope of the stress–strain curve increases sharply when the strain is 500% above. All the stress–strain curves show the representative elastomer character of soft and tough, indicating that the dynamically vulcanized POE/EPDM blends can be attributed to elastomers. From Fig. 2, it can also be seen that the
2.3.2. Mechanical properties For the measurement of tensile properties, dumbbell-shape specimens were prepared according to ASTM D412. The tearing strength was tested according to ASTM D624 using the unnotched 90◦ angle test pieces. Both tensile and tearing tests were performed on a universal testing machine (AI-7000M, Taiwan Gaotie Technology, China) at a crosshead speed of 500 mm/min. Tension set at 100% elongation was measured according to ASTM 1566. The Shore A hardness was determined using a hand-held Shore A Durometer according to ASTM D2240. All tests were carried out at 23 ◦ C. The average value was calculated for 5 test specimens. 2.3.3. Microscopy analysis The phase structure photographs were obtained using an XS-18 phase contrast microscope (Nanjing Jiangnan Optics & Electronics Co., Ltd., China). The specimens were prepared from thin (micron thickness) slices of the TPV samples, placed between coverglasses, inserted into the hot plate which was maintained at 160 ◦ C, pressed after 2 min, and then cooled to room temperature. 2.3.4. Rubber process analyzer (RPA) The RPA 2000 (Alpha Technologies, USA) is primarily a torsional dynamic rheometer, with an advance temperature control and fully automated operational modes. In this particular case, the RPA was used to study the frequency sweep of
Fig. 2. Stress–strain curves of pure POE, dynamically vulcanized POE/EPDM blends and EPDM vulcanizate.
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Fig. 3. Effect of EPDM content on tensile strength and shore A hardness of dynamically vulcanized POE/EPDM blends.
elongation at break of POE/EPDM blends was decreased obviously with the EPDM incorporation. Moreover, compared with POE and dynamically vulcanized POE/EPDM blends, the EPDM vulcanizate was weak in modulus and low in elongation at break. Tensile strength, shore A hardness, elongation at break, tensile set at break, tensile set (100% elongation) and tearing strength of the dynamically vulcanized POE/EPDM blends are shown in Figs. 3–6, respectively. Increasing the loading of EPDM in the POE/EPDM blends led to improved tensile strength when the EPDM content was 25 wt.% below; the tensile strength reached a maximum at 20 wt.% EPDM incorporation, increasing from 10.5 MPa (at 0 phr EPDM) to 12.1 MPa (at 20 wt.% EPDM). However, the EPDM loading decreased the Shore A hardness of the POE/EPDM blends obviously. It is clear that the Shore A hardness of POE and EPDM vulcanizate in the experiment were 74 and 50, respectively, the more EPDM content in the POE/EPDM blends led to the significant decrease in the hardness. With increasing loading of EPDM, the elongation at break (Fig. 4) was substantially decreased especially when the EPDM content is 25 wt.% above. The existence of EPDM in the POE will inevitably fragment the continuity of the matrix, leading to the decrease of elongation at break. It is interesting to note that the tensile set at break was decreased almost linearly with increasing EPDM loading, decreasing from 190% (at 0 phr EPDM) to 55% (at 30 wt.% EPDM), indicating the permanent deformation of POE is improved obviously by dynamic vulcanization with EPDM. Generally, the dynamic vulcanizates exhibit large reversibility and small residual strains
Fig. 4. Effect of EPDM content on elongation at break and tensile set at break of dynamically vulcanized POE/EPDM blends.
Fig. 5. Effect of EPDM content on tensile set (100% elongation) of dynamically vulcanized POE/EPDM blends.
[16]. The elastomeric cross-linked EPDM particles dispersed in the POE matrix, enable the POE/EPDM blends to elastically recover from a highly deformed state, therefore, the permanent deformation of POE can be improved remarkably. Fig. 5 shows the influence of EPDM content on the tension set at 100% elongation which was measured according to ASTM 1566, it is clear that even at relatively low elongation, the interface interaction between POE matrix and EPDM dispersed phase can effectively reduce the permanent deformation of dynamically vulcanized blends. The EPDM loading also affected the tear strength. As shown in Fig. 6, the tear strength was increased when the EPDM content was 25 wt.% below and reached a maximum at 20 wt.% EPDM incorporation, indicating a reinforcing effect of proper EPDM incorporation on POE matrix. It is striking that tensile strength and tearing strength of POE could be promoted by the incorporation of dynamically vulcanized EPDM phase and reached the maximum value when the EPDM content is around 20 wt.%. The origin of maximal tensile/tear strengths around 20 wt.% EPDM is clarified as below. As we know, the main structural units in POE and EPDM molecules are vinyl, moreover, POE and EPDM are all amorphous materials, resulting in the well compatibility and the strong interface interaction in the POE/EPDM
Fig. 6. Effect of EPDM content on tear strength of dynamically vulcanized POE/EPDM blends.
Z. Wang et al. / Materials Chemistry and Physics 126 (2011) 272–277 Table 1 Mechanical properties of POE and EPDM vulcanizate. POE Tensile strength (MPa) Shore A hardness Elongation at break (%) Tensile set at break (%) Tear strength (kN/m)
10.8 74.0 876.0 190.0 33.5
EPDM vulcanizate ± ± ± ± ±
0.16 0.60 14.70 5.52 0.47
2.1 50.0 272.2 9.0 11.5
± ± ± ± ±
0.03 0.84 5.90 0.64 1.74
blends. For the dynamically vulcanized blends, the interface interaction between the POE matrix and the EPDM dispersed particles plays a role of physical crosslink in the POE matrix, which is similar with the function of physical junctions resulted from the microcrystalline in the POE matrix. With the increasing of EPDM content, the increased physical crosslink inevitable leads to the improved mechanical properties, this is consistent with the influence of EPDM on the tensile strength and tearing strength of POE/EPDM blends, as shown in Figs. 3 and 6. Usually, the mechanical properties of the dispersed rubber phase in TPVs can be characterized by that of static vulcanizate prepared at the same temperature and vulcanization time as the dynamic vulcanization procedure. The mechanical properties of static EPDM vulcanizate and pure POE are listed in Table 1. From Table 1, we know that compared with POE, the EPDM vulcanizate is weak in mechanical properties. In our experiment, there was no reinforcing material in the crosslink EPDM system, leading to the low hardness and weak mechanical property. Increasing the EPDM content in POE/EPDM blends, the volume of EPDM dispersed phase is increased. The existence of EPDM in the POE will inevitably fragment the continuity of the matrix and reduce the number of POE molecules which can effectively withstand the external forces, leading to the decrease of mechanical properties. Although the interface interaction is enhanced with the increasing EPDM con-
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Table 2 Mechanical properties of POE/EPDM blends prepared by different vulcanization methods (POE/EPDM = 80/20). Dynamic vulcanization Tensile strength (MPa) Shore A hardness Elongation at break (%) Tensile set at break (%) Tear strength (kN/m)
12.1 66.0 809.3 90.0 41.7
± ± ± ± ±
0.53 0.60 12.50 5.20 0.37
Static vulcanization 12.3 71.0 884.2 196.4 27.9
± ± ± ± ±
0.53 0.50 11.50 18.63 3.74
tent, the deterioration of performance caused by the increased EPDM volume tends to dominate gradually, and eventually leads to the decrease of mechanical properties when the addition of EPDM content is 20 wt.% above. Fig. 7 shows the phase contrast micrograph of dynamically vulcanized POE/EPDM blends. The phase morphology is observed with a continuous resin phase and a dispersed elastomeric phase. The crosslinked EPDM particles are different in size and dispersed in the thermoplastic matrix, moreover they have irregular shape. Most of the discrete EPDM are particle-like with diameter ranging from several to 20 m. Moreover, increasing the EPDM content in POE/EPDM blend will lead to the increased size of EPDM particles, especially when the EPDM content is 20 wt.% above, as shown in Fig. 7a–c. During the dynamic vulcanization, the viscosity of the EPDM phase was increased quickly due to the initiation of the crosslinking reaction, leading to the increasing shear stress acting on the EPDM phase. The crosslinked rubber particles were produced by breaking up the initially large EPDM entities under local stress. In order to investigate the dynamic vulcanization on the properties of POE/EPDM blends, the statically vulcanized POE/EPMD blends were also prepared and the mechanical properties of the blend prepared by dynamic vulcanization and static vulcanization were listed in Table 2. The results show that compared with the stat-
Fig. 7. Phase structure of the dynamically vulcanized and statically vulcanized POE/EPDM blends. a: dynamic vulcanization (POE/EPDM = 95/5); b: dynamic vulcanization (POE/EPDM = 80/20); c: dynamic vulcanization (POE/EPDM = 70/30); d: static vulcanization (POE/EPDM = 80/20).
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Fig. 8. Storage modulus (G ) as a function of log (frequency).
ically vulcanized blend, the dynamically vulcanized blend shows superior properties, especially in tensile set at break and tearing strength. Moreover, the compounding of EPDM in POE by static vulcanization has almost no effect on decreasing the permanent deformation of POE. Fig. 7 also shows the influence of vulcanization method on the size and dispersion behavior of the EPDM dispersed particles in the POE matrix. Compared Fig. 7b with Fig. 7d, it is clear that the dynamic vulcanization is effective in reducing the size of EPDM particles, which was caused by the increasing shear stress induced by the initiation of crosslink reaction. The less size of EPDM particles will result in the larger interface between POE matrix and EPDM dispersed phase, leading to the stronger interface interaction and superior mechanical properties. It can be clearly seen that dynamic vulcanization is an effective method to decrease the permanent deformation of POE. RPA is an ideal testing tool that can provide deep understanding of elasticity and processability of filled elastomers in terms of the dynamic functions, such as dynamic module of elasticity and viscosity, over a wide range of frequency. Figs. 8, 9 and 10 show elastic modulus, loss modulus and loss tangent values (G , G and tan ı) of pure POE and dynamically vulcanized POE/EPDM blends as a function of frequency at 30 ◦ C, respectively. In Fig. 8 it can be seen that the elastic modulus increased slightly with increasing frequency. It is well known that the effect at high frequency corresponds to the effect at low temperature. Therefore, increasing frequency can lead to an increase in rigid and elastic modulus. In the experiments, EPDM was incorporated into POE during dynamic vulcanization in order to decrease the permanent deformation of POE matrix. Compared with POE,
Fig. 10. tan ı as a function as a function of log (frequency).
the chemical crosslink in EPDM vulcanizate endow the EPDM dispersed phase the large recovery ability under elongation, which can be transfered to the POE matrix effectively through the strong interaction interface and lead to the low permanent deformation of POE. The obviously decrease in tensile set at break in Fig. 4 confirms the improvement in permanent deformation of POE undoubtedly. It is noteworthy that the storage modulus of dynamically vulcanized POE/EPDM blends was higher than that of pure POE, however, the storage modulus decreased with increasing EPDM content in POE/EPDM blends. The complex effects of EPDM dispersed phase on the POE matrix cause the result in Fig. 8. POE and EPDM are all random copolymer where the main structural units are vinyl, which results in the well compatibility and the strong interface interaction. The role of the strong interface interaction is similar to that of the physical crosslink in the POE matrix, and all make it difficult for the POE matrix to deform under stress, leading to the obviously increasing storage modulus. On the other hand, the EPDM used in the experiment is unfilled elastomer and has a relative low modulus and hardness compared to that of pure POE, therefore the EPDM dispersed phase in the POE/EPDM blends is easy to deform in spite of the existence of chemical crosslink. With the increase of EPDM weight content in the dynamically vulcanized POE/EPDM blends, the storage modulus was decreased. As shown in Fig. 9, the loss modulus increased slightly with increasing frequency. The influence rule of the incorporation of EPDM in POE matrix and the EPDM content on the loss modulus of dynamically vulcanized POE/EPDM blends is similar to that on the storage modulus. Fig. 10 shows the loss tangent of pure POE and dynamically vulcanized POE/EPDM blends as a function of oscillation frequency. It is notable that the shapes of the curves in Fig. 10 are very similar. The increasing frequency had almost no influence on the tan ı in the experimental range. In the theory of viscoelasticity, tan ı = G /G ; it can be seen clearly from Figs. 8 and 9 that, accompanied by a slight increase in loss modulus, the elastic modulus also increased slightly with increasing frequency, thus giving rise to the relatively stable value of tan ı. It is noteworthy that the incorporation of EPDM into the POE led to the almost unchanged tan ı. 4. Conclusions
Fig. 9. Loss modulus (G ) as a function of log (frequency).
The purpose of this research was to investigate the possibility of dynamic vulcanization on the improvement of permanent deformation of POE, especially in the theory. The TPVs based on POE/EPDM blends were prepared by dynamic vulcanization. Compared with pure POE, the improvement of mechanical properties of POE/EPDM blends was achieved; tensile strength and tearing strength reached the maximum at an EPDM content of 20 wt.%.
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The elongation at break was slightly decreased especially when the EPDM content is 25 wt.% below, however, it is interesting to note that the tensile set at break was decreased almost linearly with increasing EPDM loading, decreasing from 190% (at 0 phr EPDM) to 55% (at 30 wt.% EPDM), indicating the relatively large permanent deformation of POE is improved remarkably by the incorporation with EPDM. Phase contrast microscopy studies show that the vulcanized EPDM particles with irregular shape and diameter ranging from several to 20 m were uniformly dispersed in the POE matrix. RPA results show that the elastic modulus increased with increasing frequency, moreover, it is noteworthy that the storage modulus of dynamically vulcanized POE/EPDM blends was higher than that of pure POE, however, the storage modulus decreased with increasing EPDM content in POE/EPDM blends, which was caused by the complex effects of EPDM dispersed phase on the POE matrix. The incorporation of EPDM had almost no influence on the tan ı of POE/EPDM blends. Acknowledgements The work was funded by the Doctoral Fund of Qingdao University of Science and Technology and the Key Laboratory of
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