(4) Overview of Thermoplastic Styrenic Elastomers (TPS)
- 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.
Thermoplastic styrenic elastomers (TPS) are the most rubber-like of TPEs, having outstanding flexibility and elasticity. With polystyrene (PS) as the hard segments, the product variations are classified based on the difference in soft segment materials, into SBS (S: styrene, B: butadiene), SIS (I: isoprene), and hydrogenated varieties of these, SEBS (E: ethylene, B: butylene) and SEPS (P: propylene). SBS is used mainly as an asphalt enhancer and in footwear, and SIS in bonds and adhesives. SEBS and SEPS, having excellent thermal resistance and weather resistance, are mainly used in soft molded materials, in resin enhancers such as polyolefin and PS, and in adhesives. Most commonly used in soft molded materials are compounds with polyolefin or softening agents. Because of the good balance among moldability, flexibility, and mechanical strength, they are used in a wide range of applications from automotive to daily commodities and consumer electronics. Materials that are compounds of these are also considered to be TPS products.
Marketed by Mitsubishi Chemical Corporation as TPS products are Rabalon™(※) and Tefabloc™. Rabalon™ is a compound-type TPS. The extensive lineup includes the S Series with excellent moldability for general and industrial products use, the M Series standard grades applicable to food, medical and sanitary uses, as well as the Rabalon™ QE Series with special functionality for wear resistance, chemical resistance, and feel designed for automotive interior and exterior uses, and Rabalon™ for Foaming, developed with excellent foaming properties. Both grades for automotive use have a material design taking advantage of their properties to make them applicable to new injection molding methods, used to make interior panel parts in automobiles; and there are hopes for additional uses in automotive interior parts. The properties of Tefabloc™, described further below, include excellent elastic recovery and low-gloss finish.
4.1 Overview of injection molding methods for automotive interior panels
Automotive interior panel parts, used in instrument panels, door trim, arm rests, console box and other places, can be classified broadly into A: hard type, B: semisoft type, and C: soft type (see Table 5). As specific examples of conventional technology, A: hard type are parts made by injection molding of PP, etc., or by coating or painting the surface. B: semisoft type parts use PP, etc. as the base material, and are coated by vacuum forming of laminated sheets consisting of top-coated TPO and polyethylene foam. C: soft type parts consist of three layers, namely, a surface skin formed by slush molding, an injection molded base material of PP, etc., and in between them a foam-injected polyurethane layer.
In luxury cars, or in the high end of the same model series, there are many cases where a high-quality feel is demanded in interior panel parts touched by the hands, and here materials with a soft or semisoft feel are actively being adopted. On the other hand, there is the problem of high part cost as additional equipment has to be introduced, the component configuration becomes more complex, and process steps increase. With B: semisoft type, for example, an injection molding machine is needed for the base material, along with vacuum forming machine, adhesive laminating machine or other adhesion equipment, an equipment process for edge processing, and trimming equipment. In the case of C: soft type, an injection molding machine for the base material, slush molding machine and large numbers of molds, a foam injection machine, trimming equipment and other equipment are needed, requiring a large investment. When upgrading existing equipment or setting up production overseas, as in developing nations, installing production equipment in the numbers needed is a major obstacle.
Aimed at overcoming such issues, the three new injection molding methods introduced below seek to reduce costs by greatly simplifying the steps for producing the above B: semisoft type and C: soft type (Figure 4).
- （1）Two-shot thin-skin injection molding method applicable to B: semisoft type panel part molding
By means of two-shot molding of the PP base and TPS skin, this method is able to produce a molded part with integral base and skin in one injection molding process (Figure 5).
Besides doing away with the need for postprocessing, this method greatly reduces the amount of vacuum formed skin that is wasted, said to be more than 50 percent, achieves zero waste in the manufacturing processes, and also reduces shredder dust in dismantling, making it highly effective for mitigating environmental impact. Moreover, this method enables multi-coloring, which was difficult with conventional vacuum forming, as it lends itself readily to coloration of the skin material, supporting diverse color variations. Another advantage is that the grain finish pattern is transferred from the mold faithfully in the injection molding process, avoiding the problem of quality degradation by which the pattern is lost in vacuum forming.
- （2）Thin-skin injection molding method applicable to C: soft type panel part molding (slush skin molding alternative)
In the conventional molding method whereby a soft layer of polyurethane foam (PUF) is formed between the skin and base, the skin is formed by powder-based slush molding or similar method. In this new method, the skin is formed by injection molding using TPS.
Since the skin is formed in the same injection molding machine as the base, the thickness can be controlled with greater accuracy than with slush molding. Moreover, this method enables a reduction in the seal portion needed in the foam injection process, as well as reducing waste materials and steps by adopting a shape design optimized to trimming and minimizing the extent of trimming. It thus raises expectations for achieving high-cycle molding. Capital outlay is also greatly reduced, since no slush equipment is needed; and less equipment space is required than with conventional approaches. This method is expected to have significant advantages, including use for limited-production vehicle lines and ease of adjusting to production increases and decreases.
- （3）Two-shot foamed skin injection molding method applicable to C: soft type panel part molding
In this method, after the PP base is formed by injection molding, two-shot molding of TPS is used to form the skin, then the TPS is foamed in the mold, obtaining a molded part with integral base and skin having a foam layer (Figure 6).
What makes this such a revolutionary method is that a product with base, foam layer, and skin layer is formed in one process. This is because the elastomer forms a thin skin layer in the portion in contact with the mold, which is not foamed, and a soft layer in the middle, which is foamed. Besides achieving the same or better quality as parts manufactured by conventional slush molding of the skin plus PUF foam molding, this new method has the advantage of consolidating into one process the many and complex processes required for the conventional approach, which include base forming, skin forming, sealing as part of foam molding, and then the trimming operation. It should also result in more stable quality.
These three new methods are certainly revolutionary from the standpoints of reducing steps, cutting waste materials, and stabilizing quality. At the same time, however, they must be able to meet special requirements, including high flowability for injection molding of large parts used in instrument panels, door trim and elsewhere; appearance functions such as avoiding weld lines and accurately transferring grain finish; and in the case of skin, achieving resistance to wear and chemicals along with the desired feel and quality of parts that are touched. The conventional TPE products able to be used in these methods, however, have been given high flowability by reducing the molecular weight. This results in a tacky, sticky feel and worsens the wear resistance, as well as causing such problems as a notable increase in glossiness after thermal degradation resistance testing, so that the required performance for use in panel coverings in vehicle interiors is not met by these TPEs. By raising the molecular weight of TPEs, on the other hand, their mechanical properties can be improved and the glossiness after thermal degradation resistance testing can be suppressed; but this seriously reduces flowability, the appearance is worsened by weld lines and flow marks, and suitability to injection molding is worsened, making it difficult to form the large thin parts needed for vehicle interior panel coverings.
Mitsubishi Chemical Corporation has been working on solving these issues, culminating in the development and marketing of two TPS products described next, Rabalon QE™ Series and Rabalon™ for Foaming. As a result, the above new injection methods have become feasible (Table 6).
4.2 Thermoplastic Styrenic Elastomers (TPS: Overview of Rabalon QE™ Series and Rabalon™ for Foaming
Rabalon™ is a TPS with styrenic block copolymers (SBC) as base and having excellent flexibility and various added functions. Besides general-purpose molding methods such as extrusion molding and injection molding, a variety of grades are available for application to the new injection molding methods introduced above, and materials left over from molding can be recycled. Ordinary grades of conventional general-purpose TPS, however, do not meet the standards for wear resistance, scratch resistance, and oil resistance required for application to automotive interior parts; their use is therefore restricted to interior parts that take advantage of the grippiness (non-slipping) typical of elastomers, such as slippage stoppers in cup holders, covers to keep moving parts out of sight, safety protective covers, and impact protective covers.
Rabalon™ QE was developed by Mitsubishi Chemical Corporation using original polymer formulation and compounding techniques, and chemical modification and enhancement techniques. It meets requirements for advanced wear resistance and scratch resistance, and is applicable to various automotive interior parts without needing surface treatment such as painting or coating. It is therefore finding wide use in handbrake grip covers, shift knobs, assist handles and other grips and handles, and even in brake pedal pads and other foot region parts that have the most stringent wear resistance demands. The lineup also includes a high flowability grade that achieves thermal resistance while also maintaining the other advantages at a high level. This grade, while greatly improving performance in injection molding of large parts and high grain finish transfer accuracy, is also applicable to non-coated injection skin materials for methods such as thin-skin injection molding and two-shot thin-skin injection molding (Figure 7).
Rabalon™ for Foaming was developed using special elastomer formulation technology to achieve high expansion ratio, high flowability, and superior fusion to the PP base material. It can be used in two-shot foamed skin injection molding for stable production of high-quality automotive interior materials applicable to soft parts with excellent cushioning (Figure 8).
In addition, by means of a method using an electroformed plastic mold enabling rapid heating and cooling, further enhancement in skin appearance and thinner skins are possible (Figure 9).
4.3 Overview of Thermoplastic Styrenic Elastomer Tefabloc™
Tefabloc™, based on SBC (styrenic block copolymers) technology, has both cross-linked (TPV-S) and non-cross-linked grades. Along with good recyclability and weather resistance, its properties include excellent elastic recovery performance and low gloss. As an alternative to vulcanized rubber (EPDM), it is widely used in both injection and extrusion molding, mainly for seal applications by European carmakers such as glass run channels, belt molding, encapsulation, and airbag covers. In the case of glass run channels, which are designed as composites of extrusion molded base and injection molded corners, it is common to use materials of different glossiness, which can result in a lack of balance in appearance. With Tefabloc™, the difference in glossiness between the extrusion molded and injection molded parts is small, enabling both good appearance and excellent seal functioning to be achieved. Also, whereas EPDM is subject to weather degradation and discoloration in long use, Tefabloc™ maintains its appearance over the long term. The physical properties of Tefabloc™ are shown in Table 7.
- ※MCC used to sell thermoplastic styrenic elastomer under the brand name “RABALON™,” but integrated that brand into the TEFABLOC™ lineup in April 2017.
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- Photographs and diagrams have been omitted for copyright reasons.