Automobile manufacturers are increasingly shifting from the conventional materials to soft-touch thermoplastic elastomers (TPEs). Such materials impart a luxurious feel and an aesthetically appealing appearance to various interior car structures. Among the fastest growing soft-touch TPE applications for automotive sector are instrument panel skins, seat backs, door trim, arm rests, handles, knobs and airbag doors. In few applications, the harder plastics or rubber are being replaced by soft-touch elastomers. In other applications, they are replacing alternate soft-touch materials containing PVC, which are being phased out by some automakers for environmental and legislative reasons.

A major driving force for TPE soft-touch materials made of thermoplastic olefins (TPOs) is the movement toward recycling. Recycling is simpler and less costly when all the plastics in a vehicle are from the same chemical family, such as olefinics. Whatever their compositions, elastomers for soft-touch applications may be used alone or in laminated structures produced by overmolding techniques.

Materials used to impart a soft-touch feel to automobile interiors are usually rated on the Shore A scale, which runs from 0 (softest) to 100 (hardest). A rating is obtained by testing a specimen of the material with an instrument called a Shore durometer. A durometer consists of a weighted blunt object that is allowed to penetrate the surface of the specimen, along with some method for measuring the depth of penetration. A zero reading on the Shore A scale means that the penetrator has sunk to the maximum depth; a reading of 100 means there has been no penetration. Generally, human skin has a Shore A rating in the 50 to 55 range. Materials with Shore A ratings below this range tend to have soft tactile properties. Softness should not be confused with flexural modulus, another tactile property that measures a material's resistance to bending. The softer a material, the more prone it is to abrasion. But a careful choice of polymers and processing methods can minimize abrasion damage to soft-touch materials.

Soft-touch TPOs have been developed as alternatives to PVC for many interior automotive applications, including instrument panels, arm rests and steering wheels. Compared to PVC, TPOs possess less fogging tendencies, lower odor and less scratch visibility, as well as superior weatherability and low-temperature ductility (down to -40ºC). Injection molding grades are the most common soft-touch TPO materials for vehicle interiors, but extrusion and blow molding grades are available as well. Textures of TPO can be made to resemble those of leather.

In soft-touch instrument panel skins, thermoformed TPO are challenging such well-established materials as thermoformed PVC and slush-molded PVC. The main technical limitation of TPOs has been their inadequate grain definition when they are thermoformed over curved surfaces. A newer technology called negative thermoforming which uses heat, vacuum and pressure, has helped overcome this limitation and expand TPO’s market share in instrument panels.

TPOs are also finding uses in the covers of driver, passenger and side airbags. Their penetration of this market is not only due to their aesthetics and soft-touch feel, but also their easy processability, flexibility at low temperatures and UV resistance.

PVC coated fabrics for lining seat backs and sides are gradually giving way to TPOs and thermoplastic polyurethanes (TPUs). The early TPO coated fabrics which used PP and rubber combinations were somewhat too stiff for seat linings. But newer TPOs based on ethylene-styrene interpolymers and plastomers possess the right balance of softness and durability for this application.

Another new function of TPOs is to provide a soft lining for carpets in automotive passenger compartments. (Most linings have been made of rubber or PVC) The TPO linings, which have made their first appearance in European vehicles, also offer the possibility of molded-in decorative patterns that match those elsewhere in the car interior.

Instrument panel skins made of slush-molded TPUs and spray-on polyurethane (PUR) formulations are edging out PVC skins, although TPO skins are also competing with PVC in this application. The slush-molded TPU skins are claimed by their developers to last longer and be less prone to fading and fogging than PVC skins.

While PVC instrument panel skins must typically be painted in a separate step, the spray-on PUR technology permits color to be applied to an instrument panel skin directly in the mold. Developers of the spray-on systems say this not only saves an extra processing step but leads to better grain definition in the painted surfaces.

Coated fabrics for the backs and sides of seats are usually made with PVC, but PUR-coated fabrics are also emerging in this market. Because they are considerably more expensive than PVC, the PUR systems are used sparingly, mostly for leather trim applications.

TPVs are challenging PVC and other TPEs in overmolded or monolithic soft-touch parts such as knobs, grips and storage bins. In these applications, TPVs, which are typically fine dispersions of highly crosslinked rubbers in polyolefin matrices, are less likely than PVC to cause fogging; they are also relatively inexpensive compared to competing TPEs such as styrene-ethylene-butadiene-styrene (SEBS) polymers.

Some of the newer TPVs are based on silicone rubber dispersed in polyolefins. They combine the high temperature performance of silicone rubbers with the processability of engineering thermoplastics. Silicone-based TPVs can impart a silky touch to the surfaces of molded parts.

Copolyester (COPE) combines a flexible segment (polyether) and a hard segment (polyester) in the polymer chain. The result is a material that is both soft and rugged. In automobiles, COPE is often used in airbag covers. One advantage of COPE is that it remains flexible at lower temperatures(-35C). Therefore, airbags made from this material do not splinter when deployed at cold temperatures. COPE has favorable aesthetics, not only due to its soft feel, but also because of its ability to take on fine details of a molded surface and to retain painted coatings over the long term.