(3) Overview of Thermoplastic Olefinic Elastomers (TPOs)
- Development Trends in Thermoplastic Elastomers for Automotive Use
Automotive Materials Vol.11 No.2 December 2014 Issue #31
- ※Photographs and diagrams have been omitted for copyright reasons.
- Olefin(base)Thermoplastic Elastomer for airbag covers
- Vulcanized PP/EPDM based Thermoplastic Elastomers (TPV)
- Thermoplastic Elastomers designed for medical applications
- Thermoplastic Elastomer (TPE)
- ※TPC under the brand name “PRIMALLOY™” was integrated into the TEFABLOC™ lineup in April 2017.
Thermoplastic olefinic elastomers (TPOs) consist of PP, PE or other polyolefin as hard segments and rubber component such as ethylene propylene rubber (EPM, EPDM) as soft segments. TPOs can be classified broadly into three types: blended type, made of polyolefin and rubber components; dynamically cross-linked type (also known as thermoplastic vulcanizates, TPV); and polymerized type (Reactor-TPO, R-TPO).
Mitsubishi Chemical Corporation’s TPOs Thermorun™ and Trexprene™ are examples of the first two types. They come in a wide variety of grades, with the blended type (medium to high hardness series) and dynamically cross-linked type (low hardness series) each available as products from semi-hard to flexible. The dynamically cross-linked type, in addition to the 3000 Series featuring superior elastic recovery under compressive deformation and extrusion appearance, includes the QT Series with the performance required for various automotive parts. The morphology of the dynamically cross-linked type is shown in Figure 1. A polymerized type, the Zelas™ series, has also been marketed but a detailed description is omitted here.
3.1 Thermoplastic olefinic elastomers: Overview of Thermorun™ blended type
Thermorun™ is a material positioned between rubber and resin. Along with a low specific gravity of 0.9, it features excellent low-temperature impact resistance, weather resistance, and moldability. Its main applications are for automotive parts including airbag covers, cable cladding, moldings, and mudguards. Out of the extensive grade lineup for various requirements, here we introduce a “low linear expansion grade” with unique properties.
3.1.1 Overview of Thermorun™ low linear expansion grade
Airbag covers used inside automobiles, and exterior parts such as bumpers and mudguards, have high-level demands for mechanical strength, moldability, and appearance. Requirements for dimensional stability, such as low linear expansion, have also become more stringent with the use of larger parts and integration with body sections. Thermorun™ low linear expansion grade is a high-performance grade meeting these requirements.
A common approach to lowering the linear expansion ratio of resins is to use fillers, whether fibers (glass fibers, whiskers, etc.) or plate-like fillers such as talc. Possible issues with such an approach are appearance problems and increased weight. Noting that in composite materials of PP and rubber components, the linear expansion ratio changes depending on the molecular weight (melt viscosity) of the rubber included in the mixture, this low linear expansion grade product was developed applying a technique for controlling the dispersive morphology of the rubber component. As a result, it became possible to achieve a low linear expansion ratio similar to that of R-RIM (reinforced reaction injection molding) urethane, without use of a filler.
The low linear expansion grade product was brought about by using material design technology that includes consideration for the injection process, controlling the rubber dispersive morphology to lower the linear expansion ratio. The morphology of the injection molding product made of the chosen material formulation transforms based on the melt viscosity ratio of each component, with the PP matrix and rubber domain changing from a spherical to a lamella structure, after which both the matrix and domain change to a continuous structure. When the melt viscosity ratio of the PP and rubber components is within a certain range, shear stress in the injection molding process causes the rubber component to become flattened in the thickness direction of the product (see Figure 2).
This is a key technology in material design of the low linear expansion product. In a material with a certain combination of rubber and PP, the rubber domain achieved a linear expansion ratio of 5×10-5/℃～9×10-5/℃, similar to that of amorphous resin. The linear expansion ratio of compression molding due to flow (shear stress) was greater than PP, at 18.7×10-5/℃. With the low linear expansion grade product, the rubber layer pulled and flattened in injection molding expands readily in the thickness direction, due to residual stress and shape effects, even if the thermal expansion ratio is the same, whereas expansion in the injection direction is suppressed, resulting in excellent dimensional properties. The greater the flatness, the smaller is the linear expansion ratio.
As described above, the low linear expansion grade product was developed by morphology control that effectively balances the flowability of the PP and rubber components. It is being used in automotive interior parts such as airbag covers, and in exterior parts including bumpers and mudguards, as well as in other large products.
3.2 Thermoplastic olefinic elastomers: Overview of Thermorun™ dynamically cross-linked type 3000 Series
The Thermorun™ 3000 Series uses mainly EPDM as the rubber component, which includes a diene component in place of EPM for the cross-linking reaction, and by means of partial cross-linking of EPDM can be formulated with a large quantity of softening agent. Thanks to its outstanding elastic recovery under compressive deformation, flexibility, thermal resistance, and oil resistance, it is coming to be used in place of vulcanized rubber and flexible PVC. Moreover, being well suited to extrusion molding and calendar molding, it can be used to create sheets with a highly smooth appearance.
To address the issues of process streamlining and recycling, it has become increasingly common to form composites of an elastomer and hard resin (two-layer injection molding), aimed at airtightness, water-proofing, shock mitigation of the molded body, or improvement in tactile properties. In the case of TPV, layering or lamination with PP or other olefinic hard resins has been going on for some time; but the Thermorun™ 3000 Series is superior also for injection molding, and in two-shot molding with olefinic resins results in molded products with good tactile properties.
As for layering or lamination with non-olefinic hard resins (ABS, polycarbonates, etc.), currently the TPEE Primalloy™(※), along with flexibility (hardness Duro-A45 or above) and good elastic recovery under compressive deformation (55 to 65 percent), features outstanding heat-sealing and wear-resistance properties, and is currently used in a wide range of applications.
The physical properties of the Thermorun™ 3000 Series are shown in Table 3. Among the main applications are vehicle interior coverings, glass run channels, grips and handles, and sheets used in construction work. Figure 3 shows its use as an automobile door covering.
3.3 Thermoplastic olefinic elastomers: Overview of Trexprene™ dynamically cross-linked type
The Trexprene™ lineup includes some blended type products, but is made up mainly of TPV, using PP as the hard segment and EPDM with a diene component as soft segment, as the lineup extends from partially cross-linked to fully cross-linked types. With a wide hardness range (Duro-A35 to Duro-D50) and support for various color mixes, this material is suitable for diverse molding methods including injection molding, extrusion molding, blow molding, and thermoforming. The physical properties of Trexprene™ are shown in Table 4.
While retaining the usual TPV advantages of outstanding weather resistance, chemical resistance, and elastic recovery under compressive deformation, grades are available with the added functionality of scratch-resistance, resulting in wide use in other applications besides automotive.
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