Dow Specialty Elastomers for Thermoplastic Polyolefins

Transcription

1 White Paper Dow Specialty Elastomers for Thermoplastic Polyolefins Updated: August, 2013 Author: Jim Hemphill, Senior R&D Manager, Dow Elastomers, The Dow Chemical Company Dow Elastomers

2 Dow Specialty Elastomers for Thermoplastic Polyolefins Abstract The Dow Chemical Company ( Dow ) is a global leader in science and technology, providing innovative chemical, plastic, and agricultural products and services to many essential consumer markets. In the arena of thermoplastic polyolefins (TPOs), Dow has developed a breadth of specialty plastics and elastomers to help our customers meet current and emerging specifications in a variety of markets, applications, and processes. Advanced Dow elastomer technology, coupled with our in-depth knowledge of automotive TPO compound requirements, helps Dow to tailor solutions based on desired compounding and processing characteristics, as well as finished part performance and appearance. Specialty elastomers from Dow are used as copolymers with polypropylene to enhance impact resistance, improve weatherability, minimize weight, and contribute to the recyclability of parts. Dow materials can also improve the processability of TPO compounds, enhancing productivity and economy in injection molding, sheet extrusion/thermoforming, and other processes. This paper will focus on TPO formulary, fabrication processes, means of elastomer incorporation, and recommendations for TPO elastomer selection. Introduction to Thermoplastic Polyolefins Thermoplastic polyolefins (TPOs) generally refers to a class of plastic used in a variety of markets and applications especially in the transportation sector, including automotive exterior and interior fascia. The TPOs are usually injection molded into the desired article, though there is increasing use of sheet and profile extrusion/thermoforming and other processes. TPOs are generally produced by the blending of polypropylene (PP) with elastic ethylene copolymers (polyolefin elastomers or POEs), and the addition of other fillers and additives. The specific blending amounts are dependent upon the overall balance of properties to meet performance specifications and desired processing equipment used for an application. TPO ingredients generally include: Polypropylene (including homopolymer, impact copolymer, or others), which generally provides rigidity and temperature stability Elastomers, which give flexibility and impact strength Talc or other mineral fillers, which impart higher part stiffness and dimensional stability Other additives (including antioxidants, plasticizers, and additives for ignition resistance, scratch and mar resistance) for improving end-use performance and durability Rigid TPOs are made with a majority polypropylene component, with added ingredients to attain an overall balance of properties. Rigid TPO formulation development starts by selecting an appropriate PP, and adding just the minimum modifier level to achieve acceptable ductility, while keeping rigidity (as measured by flexural modulus) as high as possible. This toughness/stiffness balance is shown in Figure 1. A typical rigid TPO compound starting point would be composed of: 56% Polypropylene generally an impact copolymer (ICP) or a homopolymer (hpp) 24% Elastomer 20% Talc Plus stabilizer and additives, as needed for the part s durability This type of compound is often used for injection molded automotive interior or exterior fascia and generally targets ductility at -30 C and high part rigidity [1]. In contrast, flexible TPOs contain a majority phase of elastomer with PP added for improved temperature stability [2]. A typical soft TPO compound starting point would be composed of: A base polymer matrix of: 70% High Melt Strength Elastomer 30% Branched or Conventional Polypropylene Plus stabilizer and filler addition (mineral filler/plasticizer), as desired for cost and performance Figure 1: Balancing TPO Properties F L E X M O D Threshold Impact Modifier Level The threshold impact will generally dictate the modifier level needed and the resulting compound stiffness. I M P A C T This type of compound can be extruded into a sheet and thermoformed for use in automotive interior skins that are competitive with products like vinyl, leather, and thermoplastic urethanes (TPUs). Other applications are growing through the use of flexible extrusion profiles and blow molding applications mentioned in the next section. 2

3 TPO Fabrication Processes Any TPO development must consider the process that will be used to transform the compound into its final form. The following sections give a brief description of these processes and some of the considerations for the application that may affect elastomer selection. Injection Molded TPOs Most automotive TPOs are injection molded and require a balance of stiffness and low temperature ductility to be suitable for use. TPO fascia stiffness comes from the PP, while added elastomer domains provide the ductility for durability and impact performance. Most exterior TPO applications are in the mature phase and many compounders and tiers are looking to optimize cost and performance. Conversely, the interior TPO injection molding applications are still growing as compounders and tiers are continuing to work on a number of performance parameters including increasing rigidity while maintaining low temperature ductility, scratch and mar resistance, gloss, and paintability. Therefore, some of the elastomer design and selection criteria may be different than that used for exterior TPOs, or require the use of special additives to influence performance properties. Thermoformed TPOs In the past, TPO compounds have generally not fared well in thermoforming applications because they could not be reliably processed after they reached their softening point. Advances in both polypropylene and elastomer technologies are now making this more of a reality by building higher melt strength into the compound. Sheet thermoforming solutions are emerging for exterior panels for specialized automobiles and utility vehicles [3]. For these applications, an extruded sheet is frequently produced with a coextruded cap layer featuring the desired finished appearance to minimize post-forming paint operations. Several specialty high melt strength elastomers have been developed that complement branched or rheology modified polypropylenes to provide added melt strength for improved thermoforming [4, 5]. In some cases, these panels can be produced with very high rigidity, thus making them viable candidates for replacement of engineering thermoplastics (ETP) or fiberglass [6]. There is further development of flexible TPOs for use in thermoformed instrument panels, door panels, and noncarpeted flooring. As noted earlier, a high melt strength elastomer is used as the majority component and combined with a branched PP [7]. The high melt strength elastomer also gives the desired benefit of a lower gloss (i.e., <2 percent 60 degree gloss) and excellent grain replication of the mold surface. Other TPO Processes Beyond extrusion/thermoforming and injection molding, there is growing product development for TPO use in profile extrusion and blow molding applications [8, 9]. Slush molding is another area of interest, in which an elastomer is combined with PP, converted to powder or micro-pellets, and subsequently charged to a mold where it is heated and then cooled into a finished part [10]. Incorporating Elastomer into a TPO Compound Elastomers can be introduced into a TPO at several different points in the value chain as illustrated in Figure 2. Figure 2: Elastomer Incorporation for TPO Applications PP Suppliers Compounders Tier 2 Processing Tier 1 and OEM Addition in reactor or post-reactor Blended in by the compounder Used directly in molding or extrusion 3

4 There is no right or wrong way to introduce the elastomer to the TPO. However, there are inherent benefits and risks which may influence the direction of the manufacturer, compounder, or processor (see Table 1). Furthermore, the selection of the elastomer may be influenced by the capabilities of the manufacturer (PP producer, compounder, or molder/extruder), the other TPO compound ingredients, and desired end-use performance. In many instances, the best cost/performance balance comes from compounding a lower-performing PP with a high-performance elastomer versus use of a polypropylene impact copolymer or reactor TPO (r-tpo). Elastomer Design and Selection The elastomer manufacturer has a variety of catalysts, processes, and monomers to create elastomers that are useful for TPOs. Dow s use of INSITE Technology in the early 1990s enabled the creation of ENGAGE Polyolefin Elastomers (POEs) that offered improved control of molecular architecture using metallocene catalysis and processing capabilities. These novel elastomers combined several benefits which led to improved TPO compound performance and their rapid success as replacements for other ethylene Figure 3: Low Temperature Ductility of Various Ethylene/Alpha-Olefin Elastomers (1) Glass Transition Temperature ( C) Propylene Butene Octene Crystallinity (%) (1) Data per tests conducted by Dow. Test protocols and additional information available upon request. Properties shown are typical, not to be construed as specifications. Users should confirm results by their own tests. Table 1: Elastomers Introduction for TPO Applications Method Positives Deltas PP In-Reactor Adding ethylene to the PP reactor to create ethylene-propylene (EP) elastomer segments for impact copolymer (ICP) or a reactor TPO (r-tpo) PP Post-Reactor Feeding an elastomer into the PP compounding operations downstream from manufacturing Compounding Adding an elastomer to PP, fillers, and other additives in a compounding operation At-Press or In-Line Adding an elastomer directly to the ingredients stream in an injection molding or extrusion operation Excellent dispersion of the ethylene comonomer Lower elastomer needs for compounding or for at-press and in-line processing Often higher manufacturing throughput of the base polypropylene Increased flexibility in formulation versus in-reactor addition Greatest degree of flexibility Multiple sources of ingredients Ability to optimize cost and performance Bypasses compounding operation and reduces cost Can modify elastomer levels as needed Often lower throughput on the PP train (higher cost) The reactor elastomer is generally not as efficient as higher performance elastomers especially for low temperature impact strength Still likely need to compound filler/additives for TPO use Capital may be needed for elastomer introduction Still likely need to compound filler/additives for TPO use Capital requirements for compounding operations Logistics/heat history Generally less efficient dispersion than with compounding Possible need for new capital and higher elastomer levels to meet impact requirements 4

5 copolymers like ethylene propylene diene monomer (EPDM): Low glass transition temperature increasing alpha-olefin chain length from propylene (C3) up to butene (C4) and octene (C8) gives enhanced low temperature impact performance (see Figure 3, page 4) [11] Narrow molecular weight distribution and low branching levels contribute to improved dispersion of the elastomer in the polypropylene (see Figure 4) Pellet form allows continuous compounding and bulk handling of the elastomer Further development of INSITE Technology has resulted in the ability to modify branching and molecular weight characteristics to produce high melt strength grades of ENGAGE HM POEs. These elastomers demonstrate benefits in extrusion, thermoforming, and blow molding applications, as well as improving aesthetics (reductions in gloss and flow lines) in injection molded parts [12]. Figure 4: Elastomer Dispersion in TPO (1) EPDM POE (4.5 mm = 1 micron) Table 2: Summary of Elastomer Design Effects on TPO Performance [13] Elastomer Effects on TPO Performance (1) Low Temperature Impact Flexural Modulus Heat Distortion Temperature TPO Injection Molding Flow TPO Shrinkage Melt Strength Decreasing Comonomer Chain Length Decreasing Elastomer Crystallinity (lower density) Decreasing Melt Index (increasing Molecular Weight [MW]) Decreasing Elastomer Content Decreasing Molecular Weight Distribution (MWD) Decreasing Branching (1) Data per tests conducted by Dow. Test protocols and additional information available upon request. Properties shown are typical, not to be construed as specifications. Users should confirm results by their own tests. Gloss 5

6 Table 3 demonstrates Dow s breadth of specialty elastomers that can be used for TPO modification to achieve a desired balance of properties and processing. Beyond these products, there are other innovative materials that are beginning to enter the marketplace, including propylene-ethylene elastomers [14] and olefin block copolymers [15, 16]. The Dow specialty elastomers are further divided into TPO performance levels and processing subsets as shown in Table 4 (page 7). Using this selection guide, Dow recommends starting TPO injection molding formulations with the desired PP and filler/additives and adding progressively higher levels of the Better elastomers until the desired ductility is achieved. Further optimization of the TPO compound can then be made to achieve the desired balance of performance. Likewise, TPOs for extrusion, thermoforming, or blow molding can be formulated with high melt strength elastomers (usually an elastomer having <0.5 melt index) and coupled with branched or conventional PPs as needed for TPO compound processing stability and performance. Summary Elastomer technologies continue to evolve to meet the cost/performance needs for TPO applications. The elastomers evolution will need to continue to coincide with advances in materials (polypropylene, fillers, and additives), and process technologies. Many of the trends for performance are well established for existing applications and processes, and further development is being focused on emerging technologies. Table 3: Typical Properties of Dow Specialty Elastomers for TPO Compounds (1) Grade Comonomer with Ethylene Density, g/cm 3 (ASTM D 792) Melt Index, g/10 min ( C) (ASTM D 1238) Tg, C (Dow DSC Method) 7270/7277 (2) Butene ENR 7380/ HM 7387 (2,3) Butene < (2) Butene (2) Butene Octene /8107 (2) Octene /8137 (2) Octene /8157 (2) Octene /ENR 8187 (2,3) Octene XLT 8677 (2) Octene /8207 (2) Octene /8407 (2,4) Octene (2) Octene HM 7487 (2) Butene < DOW VLDPE 1085 (5) Butene HM 7280 Butene DOW VLDPE 1095 (5) Butene HM 7289 Butene NORDEL IP 3720P (6) Propylene NORDEL IP 3745 Propylene < AMPLIFY GR 216 (7) Grafted (1) Data per tests conducted by Dow. Test protocols and additional information available upon request. Properties shown are typical, not to be construed as specifications. Users should confirm results by their own tests. (2) POE/ENR products with numbers ending in 7 (e.g., 7277, 8407) and 8842 have talc partitioning for improved product handling; properties may be measured before the addition of talc. (3) ENR designates a developmental grade. When using developmental products, customers are reminded that: (1) product specifications may not be fully determined; (2) analysis of hazards and caution in handling and use are required; (3) there is greater potential for Dow to change specifications and/or discontinue production; and (4) although Dow may from time to time provide samples of such products, Dow is not obligated to supply or otherwise commercialize such products for any use or application whatsoever. (4) 8400 POE is available in the European region. (5) (6) (7) 8407 POE is available globally. Very Low Density Polyethylene EPDM with diene content of less than 0.5% Maleic Anhydride grafted copolymer 6

7 References [1] J.J. Hemphill, et al., Expanding the Product Portfolio of Ethylene Elastomers ENGAGE Polyolefin Elastomers for Large Volume TPO Applications, Proceedings of the SPE- Automotive TPO Global Conference (2005). [2] Dow Publication AMS, High Melt Strength Materials Expand Thermoforming Possibilities for TPOs [3] R. Leaversuch, Thermoforming Shines in Exterior Vehicle Panels, Plastics Technology Online Article: www. ptonline.com/articles/thermoformingshines-in-exterior-vehicle-panels [4] J.J. Hemphill, et al., New Advances in Elastomer Technology, Proceedings of the SPE-Automotive TPO Global Conference (2003). Table 4: TPO Elastomer Selection (1) Injection Molding Good Better Best Cost Effective 7270/ DOW VLDPE 1085 DOW VLDPE 1095 NORDEL IP 3720P or 3745P Balance of Properties 8100/ / /8207 [5] K.W. Walton, et al., The Role of Impact Modifiers on TPOs Requiring High Melt Strength, Proceedings of the SPE-Automotive TPO Global Conference (2004). [6] B.W. Walther, et al., Novel Thermoforming TPO Compound Developed Using Advanced Material Science, Proceedings of the SPE- Automotive TPO Global Conference (2006). [7] L.B. Weaver, et al., Novel Ethylene/Alpha-Olefin Copolymers Polypropylene Blends for Thermoforming, Blow Molding, and Extruded Profiles, Proceedings of SPE Polyolefins, Houston, Texas (2006). [8] L.B. Weaver, et al., Novel Ethylene/ Alpha-Olefin Copolymers Propylene Blends for Extruded Profiles, Proceedings of AMI Profiles (2007). Superior Impact High Flow Low Gloss XLT / ENR 8187 (2) 8842 HM /8137 ENR 7380/ HM 7387 (2) HM 8400/8407 (3) 7487 [9] R. Leaversuch, Blow Molding Gets Green Light in Detroit, Plastics Technology Online Article: www. ptonline.com/articles/blow-moldinggets-green-light-in-detroit [10] S. Patel, et al., Development of a Slush Molded TPO Instrument Panel Skin, 2005 SAE World Congress, Detroit, Michigan (April 2005). [11] Laughner, et. al., Modification of Polypropylene by Ethylene/ Alpha-Olefin Elastomers Produced by Single-Site Constrained Geometry Catalyst, Proceedings of the SPE- Automotive TPO Global Conference (1999). [12] J.J. Hemphill, et al, Continued TPO Elastomer Development, Proceedings of the SPE-Automotive TPO Global Conference (2007). [13] J.J. Hemphill, et al., Expanding the Elastomer Portfolio for TPO Applications, Proceedings of the SPE- Automotive TPO Global Conference (2006). [14] VERSIFY Plastomers and Elastomers, along with ENGAGE analogs that provide superior melt strength for thermoforming and blow molding applications. [15] INFUSE Olefin Block Copolymers [16] L.B. Weaver, et al., A New Class of Higher Melting Polyolefin Elastomers for Automotive Applications, Proceedings of the SPE-Automotive TPO Global Conference (2006). Extrusion/Thermoforming & Blow Molding High Melt Strength Elastomers Other Considerations Compatibilizer/Other ENR 7380/ HM 7387 (2) AMPLIFY GR 216 HM 7487 HM 7280 HM 7289 VERSIFY Product Series INFUSE Product Series (1) Data per tests conducted by Dow. Test protocols and additional information available upon request. Properties shown are typical, not to be construed as specifications. Users should confirm results by their own tests. (2) ENR designates a developmental grade. When using developmental products, customers are reminded that: (1) product specifications may not be fully determined; (2) analysis of hazards and caution in handling and use are required; (3) there is greater potential for Dow to change specifications and/or discontinue production; and (4) although Dow may from time to time provide samples of such products, Dow is not obligated to supply or otherwise commercialize such products for any use or application whatsoever. (3) 8400 POE is available in the European region POE is available globally. 7

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