Enhancing the mechanical characteristics of thermoplastic vulcanizates

17 June 2016
Mohammad Milad Abdollahi and Ahmad Reza Shafieizadegan-Esfahani
Novel processing methods are tested to improve the tensile properties of ethylene propylene diene monomer/polypropylene composites.

Thermoplastic vulcanizates (TPVs) were conceived (e.g., as part of extensive research conducted in the 1970s and 1980s1, 2) to have the elastic properties of cross-linked rubbers, as well as the processability and recyclability of thermoplastic polymers. TPVs are prepared by melt mixing a thermoplastic material with an elastomer. This is conducted in the presence of a small quantity of a vulcanizing system, which leads to in situ cross-linking of the rubber phase. In other words, the rubber phase is dispersed as tiny particles throughout the continuous thermoplastic matrix, and the vulcanizing agent is then added to cure the rubber phase. Among the various types of available TPVs, those that are based on polypropylene and ethylene-propylene rubber have gained the most attention, for instance in industrial (automotive, building, and electronics) applications. To be suitable for most industrial applications, however, reinforcement of the TPV materials is generally required.

In previous studies, nanofillers have been incorporated into the TPV matrix to reinforce the material. Although this approach leads to an increased tensile modulus of the materials, it also causes a decrease in the elongation at break.3–5 The elongation at break is an essential parameter that contributes to the mechanical toughness, damping, and fatigue resistance of materials. It is therefore important to find an alternative TPV preparation method that can improve both the elongation at break and the tensile modulus simultaneously.

In this work, we have tested four different compounding procedures to find the most suitable option for improving the tensile strength and elongation at break of a novel nanocomposite TPV material.6 Our TPV consists of cross-linked nanoscale ethylene propylene diene monomer (EPDM) rubber particles dispersed in a polypropylene (PP) matrix. To prepare our various samples, we conducted batch and continuous melt blending of the PP with the EPDM in the presence of vulcanizing ingredients, nanoclay, and maleated EPDM (EPDM-g-MA) as a compatibilizer. We kept the ratio of EPDM to PP constant, at 60/40 (w/w) in all our prepared compounds.

In our first compounding procedure, we melt mixed EPDM, EPDM-g-MA, and the curing agent—2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (DHBP)—on a two-roll mill. The resultant compound (COMP1) was granulated and then melt blended separately with PP in an internal mixer (to produce TPVI1) or in a twin-screw extruder (to produce TPVT1). In our second procedure, we mixed the EPDM rubber, EPDM-g-MA, DHBP, and a nanoclay on the two-roll mill to produce COMP2. As for the first procedure, we then granulated this compound and melt blended it with PP either in the internal mixer (TPVI2) or the twin-screw extruder (TPVT2). For the third procedure, we first ground the EPDM rubber into small particles and then melt processed it (in a twin-screw extruder) with the EPDM-g-MA and nanoclay. We then mixed this compound with the DHBP on a two-roll mill. Again, we granulated the final compound (COMP3) and melt blended it with PP in either the internal mixer (TPVI3) or the twin-screw extruder (TPVT3). These three compounding procedures are illustrated schematically in Figure 1. In our fourth and final procedure (to produce ‘TPVC’), we simultaneously fed the EPDM rubber, EPDM-g-MA, and nanoclay into the internal mixer (in the same way as previously reported methods7). After two minutes, we added the PP, and when a constant torque had been achieved, we incorporated the curing agent.


Schematic diagram of the first three compounding procedures used to produce the novel nanocomposite thermoplastic vulcanizate (TPV) materials. In the first procedure, ethylene propylene diene monomer (EPDM) rubber is melt mixed with maleated EPDM (EPDM-g-MA) and a curing agent—2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (DHBP)—on a two-rolled mill to produce the compound COMP1. In the second procedure, a nanoclay component is also incorporated to produce COMP2. For the third procedure the EPDM rubber is ground into small particles before it is processed in a twin-screw extruder with the EPDM-g-MA and nanoclay, and subsequently mixed with DHBP on the two-rolled mill to produced COMP3. In all three procedures, the compounds (COMP1–3) are granulated and melt blended with polypropylene (PP) in either an internal mixer (to produce TPVI1–3) or a twin-screw extruder (to produce TPVT1–3).

The tensile properties of our various compounds (i.e., COMP1–3) and TPVs are shown in Figure 2. These results indicate that even when using the same proportions of the ingredients, the difference in compounding procedures can give rise to an increased (almost double) tensile strength of the materials. Furthermore, we achieve an increase in the elongation at break and tensile modulus of about 30 and 15%, respectively, just by improving the compounding methods. There are complicated relationships between the mechanical properties of our TPVs and their ingredients, morphology, and processing conditions. For example, we find that TPVs with the same formulation can display different tensile behavior (see Figure 2). One of the reasons for this behavior is differences in the degree of nanoclay dispersion within the samples. The extent of dispersion affects the cure state and viscosity of the rubber. In addition, the rubber particle size and distribution are sensitive to the cure state, viscosity ratio, and flow field of the machine. These factors all therefore strongly affect the mechanical properties of the final TPVs. We discuss the mechanical property relationships more fully in our recently published paper.6


Tensile strength (top), elongation at break (middle), and tensile modulus (bottom) measurements of the compounds (COMP1–3) and TPVs produced from the four different compounding procedures. TPVC: TPV produced from the fourth procedure.

In our study we have also found that by effectively incorporating nanoclay into the EPDM compound, with the use of a peroxide curing system, we can increase the optimum cure time and improve the mechanical properties of the samples. In addition, with our third processing method, we successfully overcame difficulties associated with feeding small amounts of the curing agent into the extruder. With our method we obtained a very fine structure, with few agglomerates. We thus limited the curing system and the formation of nanolayers in the highly reinforced rubber phase, and prevented the degradation of the the PP by the peroxide radicals. We also find that it is possible to significantly increase the tensile properties of TPV by using reinforcing rubber droplets (separated by thin and uniform plastic ligaments). However, the addition of nanofillers without an appropriate compounding procedure means that the maximum potential benefits of the nanofillers cannot be realized.

In summary, we have produced a novel nanocomposite TPV material, with enhanced mechanical properties, through four different compounding procedures. In these methods, we melt blend polypropylene with EPDM in the presence of vulcanizing agents, nanoclay, and maleated EPDM. We find that the tensile properties of our compounds and TPVs are sensitive to the compounding procedure. We also observe complicated relationships between the mechanical properties of the TPVs and their ingredients, morphology, and processing conditions. In our future work we will therefore study the relationship between the morphology and mechanical properties of these products.


Authors

Mohammad Milad Abdollahi
Chemical Engineering Department, Isfahan University of Technology

Ahmad Reza Shafieizadegan-Esfahani
Chemical Engineering Department, Isfahan University of Technology


References

  1. S. Abduo-Sabet and S. Datta, Thermoplastic vulcanizates, Polymer Blends: Formulation and Performance, pp. 517-555, Wiley, 2000.

  2. J. G. Drobny, Handbook of Thermoplastic Elastomers, pp. 426, Elsevier Science, 2007.

  3. J. K. Mishra, J.-H. Ryou, G.-H. Kim, K.-J. Hwang, I. Kim and C.-S. Ha, Preparation and properties of a new thermoplastic vulcanizate (TPV)/organoclay nanocomposite using maleic anhydride functionalized polypropylene as a compatibilizer, Mater. Lett. 58, pp. 3481-3485, 2004.

  4. H. Mirzazadeh and A. A. Katbab, PP/EPDM-based thermoplastic dynamic vulcanizates with organoclay: morphology, mechanical, and viscoelastic properties, Polym. Adv. Technol. 17, pp. 975-980, 2006.

  5. G. Naderi, P. G. Lafleur and C. Dubois, Microstructure-properties correlations in dynamically vulcanized nanocomposite thermoplastic elastomers based on PP/EPDM, Polym. Eng. Sci. 47, pp. 207-217, 2007.

  6. A. R. Shafieizadegan-Esfahani, M. M. Abdollahi and A. A. Katbab, Effects of compounding procedure on morphology development, melt rheology, and mechanical properties of nanoclay reinforced dynamically vulcanized EPDM/polypropylene thermoplastic vulcanizates, Polym. Eng. Sci., 2016. First published online: 15 April. doi:10.1002/pen.24320

  7. C. Li, Z. Jiang and T. Tang, Morphological evolution and properties of thermoplastic vulcanizate/organoclay nanocomposites, J. Appl. Polym. Sci. 131, pp. 40618, 2014. doi:10.1002/app.40618

DOI:  10.2417/spepro.006555



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