Last Fall’s annual SPE Automotive TPO Global Conference in Sterling Heights, Mich., heard presentations on new formulations that cut cost by eliminating the compounding step and that bridge the gap between molded-in-color and painted parts. Other highlights were improved metallocene elastomers for TPOs, and new approaches to PP and TPO nanocomposites.
New TPOs from Dow Automotive boasts an unprecedented combination of low gloss and abrasion resistance for molded-in-color (MIC) interior components. Dow researchers reported that Velvex Advanced Reinforced Elastomers bridge the persistent gap in surface quality and durability between MIC and painted TPO.
Velvex TPOs reportedly have very consistent and very low gloss, as well as elastic modulus similar to low-gloss talc-filled (TF-TPO) and nylon/ABS materials. However, Velvex TPO is unusual in that the elastomer blended with PP forms a co-continuous phase, which produces low knit-line visibility similar to that of neat PP. In contrast, the more distinctly separate phases of elastomer (discontinuous) and PP (continuous) in TF-TPO tend to produce more visible knit lines, along with “tiger striping” (intermittent high and low gloss). Dow demonstrated the dramatic difference in knit-line visibility between Velvex and TF-TPO on a production trim part molded with multiple submarine gates that leave four knit lines around an opening (See images below).
Velvex TPO also outperforms TF-TPO in tests of scratch and mar resistance. Scratch tests used a 1-mm-diam. stylus with an 18-N load. Velvex also showed an obvious improvement in mar resistance caused by the greater elastic recovery of its micro-texture vs. a typical TF-TPO. Gloss before mar testing was 0.9 for Velvex vs. 1.3 for TF-TPO; after marring, the values were 1.0 vs. 1.8, respectively.
Advanced Composites Inc. unveiled a new TPO that boasts a novel combination of high flow and high melt strength for molding foamed automotive door panels and interior trim. ADX-1047 is specially formulated to deliver a Class A surface when used with chemical blowing agents. It offers higher flow, better physical properties, and up to 30% lower weight (after foaming) than competitive TPO compounds and reactor-made PP, according to technical sales manager Anthony Gasbarro. The 65 MFI allows for filling thin-wall parts.
Dow Automotive R&D also reported on its At-Press TPO blending system, which allows a route to both cost savings and flexibility to alter shrinkage, CLTE, stiffness, toughness, and heat resistance. The system includes PP, functional talc masterbatch, and a metallocene-catalyzed elastomer. These formulations are designed for mixing at the molding machine and reportedly deliver excellent talc and elastomer dispersion under typical molding conditions.
First developed for exterior applications, At-Press technology has been validated extensively at Tier 1 suppliers for interior applications, where Dow says it meets requirements for color consistency, low gloss, low odor and emissions, scratch and mar resistance, and absence of tiger stripes. Benchmarked against a conventional compounded TPO, the At-Press process reportedly shows excellent mixing consistency, molding repeatability, and consistent part weight and shrinkage. Part-to-part variation in talc content, measured via ash burnout, is a good indicator of mixing consistency, according to Dow researchers. Typical variations with the At-Press process are <0.35%, which they say is comparable to the lot-to-lot variability observed with compounded TPO.
Chrysler and Flint Hills Resources (FHR), which acquired Huntsman’s PP business in 2007, jointly reported on an in-reactor process that produces new high-performance TPOs that meet if not exceed the low-temperature properties increasingly required for more demanding applications, such as head-impact and airbag-deployment components, while also providing lower cost than compounded TPOs.
Chrysler needed a material with a minimum 1000 Mpa (145,000 psi) flex modulus, the lowest density possible, ductility at -40 C, and ultimate elongation above 450%. FHR found that the only way to incorporate these properties was by making an in-reactor alloy. FHR came up with AP7810-HS, which has 1053 MPa flex modulus and -30 C ductility. In addition, it reportedly provides excellent flow during molding, good scratch and mar resistance, and excellent dimensional stability.
UPDATE ON ELASTOMERS
Dow’s Elastomers group discussed the latest developments in polyolefin elastomers that enhance TPO performance. One topic was low-gloss flexible TPO sheet for thermoforming. Researchers evaluated currently available high-melt-strength polyolefin elastomers that enable TPO sheet to offer exceptional grain replication and 60° gloss of less than 2% after vacuum forming. Dow reported that with a 2.5-MFR high-melt-strength PP, only three of its elastomers offer controlled sag during thermoforming, plus low gloss and good grain replication. These are all ethylene-butene elastomers with medium extensional viscosity and MFR <0.5:
- Flexomer DFDB 1088 NT has a 114 C melting point and 0.885 g/cc density.
- Engage 7387 has a 47 C melting point and 0.870 density.
- Engage 7487 melts at 37 C and has 0.860 density.
These three outperformed both Dow’s Engage 6386 ethylene-propylene elastomer with highest extensional viscosity (<0.5 MFR, 55 C melting point, 0.875 density), and the “industry workhorse,” Engage 8150 ethylene-octene elastomer with low extensional-viscosity, 0.5 MFR, 56 C melting point, and 0.870 density.
Each of the three candidates achieves scratch resistance up to 20 N with incorporation of scratch/mar additives and only a slight increase in gloss. Low gloss and scratch resistance were retained throughout 2000 kJ of weathering exposure without use of topcoatings, Dow researchers noted.
In another presentation, Dow Elastomers reported on strides made toward elastomers with higher impact efficiency for rigid TPOs. Talc-reinforced PP impact modified with ethylene/alpha-olefins (EAO) grew rapidly in automotive through the 1990s and is now the dominant rigid plastic used.
However, the desire to downgauge while maintaining equivalent stiffness has led OEMs to require up to 80% increase in flexural modulus with no change in low-temperature ductility. Higher flow to accommodate manufacture of thinner parts is another significant trend. Moreover, scratch/mar resistance is a chronic issue for TPO parts, particularly during installation on assembly lines. As a result, there is a need for new high-efficiency PP impact modifiers. By midyear, Dow Elastomers aims to launch new high-melt strength ethylene/alpha-olefin polymers that show improvement in stiffness/toughness balance.
SCF PROCESSING FOR NANO-PP
The Materials and Nanotechnology department of Ford Motor Co.’s Plastics Group reported on the use of supercritical fluid (SCF) processing as an aid to exfoliation and dispersion of PP/clay nanocomposites. Researchers said it is difficult to achieve well-dispersed, exfoliated nanocomposites because the nanoclay platelets tend to bond strongly together, along with the fact that the clay is hydrophilic and the polymer matrix is hydrophobic. The high melt viscosity of the polymer is an added obstacle to dispersing the clay in conventional compounding.
A novel approach to both delaminating clay silicate layers and to reducing the melt viscosity is through the use of SCFs such as carbon dioxide or nitrogen during extrusion. Under appropriate conditions of temperature and pressure, these gases behave like fluids that dissolve easily in plastic melts. The SCF diffuses between the silicate layers, and then once the mixture is depressurized, the SCF will expand, pushing apart the nanoclay layers. The reduction in overall melt viscosity also improves dispersion of the clay. Ford has partnered with Trexel to utilize the latter’s patented MuCell SCF technology for injection molding and extrusion of lightweight, microcellular PP and TPO nanocomposites.
NEW IP APPROACH
A new technique to make the in-mold grained, two-toned TPO instrument panel for the 2008 Chevy Malibu was the topic of a presentation by Faurecia Interior Systems, Auburn Hills, Mich., together with O’Sullivan Films, and KTX Americas. For this job, Faurecia modified its in-mold graining (IMG) process, which involves negative vacuum forming a TPO surface skin from O’Sullivan Films in tooling from KTX. This is followed by applying foam-in-place polyurethane between the skin and rigid plastic substrate.
Faurecia reduced the number of steps and also cut part weight by around 15% with a new IMG-Lamination process, which utilizes a two-layer laminate from O’Sullivan of TPO surface skin and an underlayer of radiation-crosslinked polyolefin foam to replace the foam-in-place PUR. This laminate is vacuum formed as before, but the rigid substrate is used as a plug assist. Adhesive is applied to the substrate or the skin/foam laminate to bonds the two during forming. After forming, the part is masked and two tones sprayed with water-based paint.