Abstract. Experimental. Introduction. Results and Discussion

Transcription

1 Soft TPO Formulations Containing Specialty Propylene Elastomers by N. Dharmarajan, S. Srinivas, M.G. Williams, V. Gallez and S. Datta * ExxonMobil Chemical, 5200 Bayway Drive, Baytown, TX Abstract Vistamaxx TM polymers are Specialty Propylene 1 Elastomers (SPE) containing isotactic propylene crystallinity. The presence of the ipp crystallinity renders these polymers very compatible with polypropylene (PP). When these polymers are blended with PP, the flexural modulus of the composition is substantially reduced at low weight fraction of SPE. This paper describes the physical, rheological and morphological characteristics of SPE/PP blends, comprising SPE resins of varying and crystallinity. The SPE composition in blends with a 3 PP homopolymer is varied from 20 wt. % to 90 wt. The performance data is analyzed using regression models, correlating the blend properties with SPE and PP type and content in the formulation. Our results indicate that the SPE type, particularly crystallinity, has a strong influence on the physical properties of the blend formulations. Introduction Thermoplastic olefins (TPOs) are mainly immiscible blend of an isotactic polypropylene and a polyolefin elastomer, where the continuos and discrete phase compositions depend on the stiffness of the TPO. For soft flexible TPO, polypropylene is the dispersed phase and the elastomer is the continuous component. TPO products combine the desirable properties of stiffness, flow and toughness such as tear resistance. Numerous attempts at improving TPO performance by improvements in the elastomer design have been described previously. 2-4 The most notable of these involve the use of amorphous ethylene propylene (EPM), highly branched EPDM terpolymer for the break up and dispersion into very small particles, and the introduction of ethylene elastomers containing octene and hexane comonomers instead of propylene for a closer match of the solubility parameters of the two phases. Inherent in these attempts was the understanding that the properties of TPO's depend to a large extent on the strength and the thickness of the interface formed between the elastomer and the isotactic polypropylene phase. Controlling the interface through both rheological and thermodynamic methods is critical in efficiently achieving the desirable balance of properties 5-6. In this paper we describe a novel approach to the formulation of soft TPO. The synthesis of discrete specialty elastomers, which have propylene crystallinity, leads to compositions wherein the interface between the dispersed elastomer phase and the isotactic polypropylene is strengthened through an interfacial co-crystallization of the polypropylene and the dispersed elastomer. This unique morphological feature, which is verified through characterization of the blend morphology at the interface, leads to low stiffness formulations with the SPE polymer. It is obvious that this benefit of having the elastomeric modifier to possess isotactic propylene crystallinity provides a new avenue for design and selection of polymers for soft TPO applications. Experimental The SPE polymers used this study are shown in Table 1. The polymers vary in from 3 to 18 and in isotactic propylene crystallinity as represented by the heat of fusion from 5 J/gm (low crystallinity) to 29 J/gm (high crystallinity). Table 1: SPE Polymers Evaluated ID MI (g/10min) (g/10min) Density Crystallinity (J/gm) SPE TPO formulations comprising the SPE polymers and polypropylene (PP) resins were compounded in a 30 mm ZSK twin screw extruder, using standard PP extrusion conditions. Most of the formulations, were two component blends of a PP homopolymer (3) and SPE at varying SPE levels. Additionally a few formulations with filler and process oil were prepared by compounding in a Banbury mixer. The formulations were compression molded at 200 C for 8 min into test specimens 152 x 152x 3.2 mm. Physical was performed according to standard ASTM tests. Rheological properties were measured in oscillatory mode using a RPA 2000 instrument. Morphological features of the blends were examined using Transmission Electron Microscopy (TEM). The section was cryo microtomed and stained with RuO 4 solution. Results and Discussion Morphology Figure 1 shows the morphology at the interface for the blend of and PP. The PP is the dispersed phase. Since the crystallinity type is the same in and PP,

2 there is crystallization across the phase boundary resulting in a diffuse interface. Figure 2 shows the morphology of a filled formulation that contains process oil at 30 wt. %. The presence of oil interferes with the crystallization across the phase boundary. The interface in this case is sharper and not diffuse as compared to Figure 1. Stiffness Properties Cocrystallization of the ipp crystallinity in the SPE across the PP crystal boundary hinders the growth of large PP crystals. The smaller crystallites tend to contribute to lower stiffness. Figure 3 shows the variation of flexural modulus with content in blends formulated with varying PP resins. Adflex KS 01P is a high rubber content reactor TPO from Basell. With 50 % SPE level in the formulation, the stiffness of both homopolypropylene (HPP) and random copolymer PP (RCP) formulations is reduced to the flexural modulus of the Adflex resin. Addition of to the Adflex reactor TPO further diminishes the stiffness resulting in highly flexible compounds. Figure 4 compares the 15 % Modulus (stress at 15 % strain) for filled TPO formulations that contain either SPE or a metallocene ethylene octene plastomer (0.902 d). As seen in the figure, at the same polymer content, the flex modulus is substantially lower in SPE blends compared to the plastomer. Unlike SPE compounds, there is no cocrystallization at the PP plastomer interface; hence the stiffness is higher. Effect of SPE Polymer on Compound Properties TPO compound properties that were measured with varying SPE polymers were correlated to the SPE polymer properties and content. Non linear regression models were developed using Design Expert 6.0 software. Only coefficients of model terms that were significant at the 95 % level were retained in the model. Box Cox analysis was applied to make ANOVA valid. Table 2 shows the adjusted R 2 for the various correlations. As seen in the Table the R 2 values are > 0.8 for most properties with the exception of tensile strength and elongation, where the correlations are lower because of the inherent scatter in the measurements. Table 2: Model Correlations Property Adjusted R Vicat % Mod Tensile 0.62 Elongation % Sec. Mod 0.99 Tangent Mod Tear Die C 0.95 Figure 5 shows the variation of compound stiffness with SPE content for each of the SPE polymers. The solid lines correspond to the regression model. At high PP level in the compound, SPE-3, which has the highest crystallinity produces a lower stiffness compared to SPE- 1 and 2. At higher SPE level, the stiffness properties are all similar. The greater stiffness reduction with SPE-3 is the result of its enhanced compatibility with PP. Figure 6 shows the variation of compound with SPE type and level. The compound flow correlates directly with the of the SPE polymer, with the SPE- 2 (18 ) featuring compounds having the highest flow. Figure 7 shows the Vicat softening temperature with the SPE type and level in the formulation. At low SPE level, the polymer that is higher in crystallinity SPE-3 shows a lower Vicat temperature due to its enhanced interfacial compatibility with PP, but at increasing SPE level the performance crosses over. This cross over can be associated to the intrinsic higher thermal properties of SPE-3 from its crystallinity. Figure 8 shows the tensile properties of the formulation. This is represented as response surface plot that is constructed using the regression equation. As expected, tensile strength in the formulations increases with higher PP content. At a constant SPE content, tensile goes through a maximum with SPE, this maximum occurring at 13. Rheological Properties Figure 9 shows the viscoelastic properties of the SPE/PP blends measured at 190 o C in an oscillatory mode. The measurements were run at frequency ranging from 0.2 rad/sec to 210 rad/sec at a constant strain of 14 %. Figure 9(a) shows the response surface plot for the elastic modulus (G'), constructed from the regression model. Elastic modulus increases with PP content in the formulation. At a constant PP level, the modulus increases with the molecular weight of the SPE polymer. The lower SPE compounds; such as (higher molecular weight) have higher melt elasticity. Figure 9(b) shows a contour plot of the viscosity ratio measured at 209/10.47 rad/sec. A lower viscosity ratio implies a lower viscosity at high shear and a higher viscosity at low shear conditions, properties preferred for injection molding. Similar to 9(a) with higher PP content and lower of the SPE polymer the viscosity ratio is lower as seen in the contour chart. Conclusions Based on the results of this study the following conclusions are advanced:

3 1. Specialty Propylene Elastomers containing a small amount of ipp crystallinity are formulating soft TPO compounds. In isotactic PP blends substantial reduction in flexural modulus is seen at 50 % SPE content in the blend. 2. The properties of the TPO formulations are also impacted by the SPE type particularly crystallinity of the polymer. 3. Morphology at the PP/SPE interface shows crystallization across the phase boundary. This restricts the growth of large ipp crystals, which typically tends to produce higher stiffness. The extent of interfacial crystallization can also vary with the crystallinity level in the SPE polymer. Acknowledgments The authors are ExxonMobil Chemical Company for permission to publish this paper ExxonMobil. To the extent the user is entitled to disclose and distribute this document, the user may forward, distribute, and/or photocopy this copyrighted document only if unaltered and complete, including all of its headers, footers, disclaimers, and other information. You may not copy this document to a Web site. ExxonMobil does not guarantee the typical (or other) values. Analysis may be performed on representative samples and not the actual product shipped. The information in this document relates only to the named product or materials when not in combination with any other product or materials. We based the information on data believed to be reliable on the date compiled, but we do not represent, warrant, or otherwise guarantee, expressly or impliedly, the merchantability, fitness for a particular purpose, suitability, accuracy, reliability, or completeness of this information or the products, materials, or processes described. The user is solely responsible for all determinations regarding any use of material or product and any process in its territories of interest. We expressly disclaim liability for any loss, damage, or injury directly or indirectly suffered or incurred as a result of or related to anyone using or relying on any of the information in this document. There is no endorsement of any product or process, and we expressly disclaim any contrary implication. The terms, we, our, "ExxonMobil Chemical", or "ExxonMobil" are used for convenience, and may include any one or more of ExxonMobil Chemical Company, Exxon Mobil Corporation, or any affiliates they directly or indirectly steward. The ExxonMobil Chemical Emblem, the Interlocking X Device, and (Vistamaxx) are trademarks of ExxonMobil Corporation References 1. S. Datta et.al, Rubber World, October N. Dharmarajan and L.G. Kaufman, TPOs in Automotive, First International Conference, October 1994, Ann Arbor, Michigan. 3 N. Dharmarajan and T.C. Yu, Plast. Eng. 52(8), 33 (1996). 4 N.Dharmarajan and L.G. Kaufman, Rubber Chem. and Tech. 71(4), 778 (1998). 5. N.Dharmarajan, et.al., FLEXPO 2004 Conference, Galveston, TX, Sept 15-17, N.Dharmarajan, M.G.Williams and S. Datta, Rubber Chem and Tech., 77, 2, 356. [600.9] [6.9] [60.9]

4 PP Diffuse interface Crystallization across boundary Flex Modulus (psi) Adflex KS01P HPP (3 ) RCP (3 ) [MPa] Wt% SPE 1 Figure 1: Morphology of / PP blend Figure 3: Variation of flexural modulus with content in PP / SPE unfilled formulations Sharp interface No crystallization across boundary PP Figure 2: Morphology of /PP/Oil blend 15 % Modulus (psi) Exp. Data - EXACT 0201 Exp. Data - SPE SPE / PP Mag Hydroxide = 25 % SPE / EXACT EXACT 0201 or SPE Content (%) [MPa] [14.0] [10.5] [7.0] [3.5] Figure 4: Variation of flexural modulus with or Plastomer content in PP / SPE filled formulations 1 % Secant Modulus 1% Secant Modulus (Kpsi) SPE SPE 3 (gm. / 10 min) Figure 5: Effect of SPE crystallinity on stiffness of SPE/PP unfilled formulations Figure 6: Effect of SPE on flow of SPE/PP unfilled formulations

5 gm (C) Tensile Strength (psi) gm SPE-3 Brk Figure 7: Effect of SPE crystallinity on Vicat softening temperature of SPE/PP unfilled formulations SPE Content SPE (gm / 10 min) Figure 8: Tensile strength response surface plot (a) Elastic Modulus (Pa) (b) Viscosity Ratio Elastic SPE Content (wt.%) SPE SPE Content (wt %) SPE Figure 9 : Viscoselastic properties of SPE / PP blends (a) Melt Elastic 10.5 rad/sec (b) Viscosity 209 / rad/sec