Dashboard skins of instrument panels, console boxes, door trims, and other interior trims in automobiles have been made by slush molding processes. In these processes free flowing powder composition of a thermoplastic elastomer are fused together. The free flowing powder typically has a maximum particle size of 800 μm. These skins are soft to the touch, allow for leather graining and stitching, and permit great design freedom.
Unlike injection or compression molding, no pressure is applied for shaping in powder molding processes. Therefore, the powdery material for powder slush molding ideally has a high degree of fluidity in the powdery state so as to attain the uniform adhesion of the material to a mold having a complicated shape. It should also exhibit a low melt viscosity so that it can be molten and fluidized without pressing to form a film. Furthermore, it is desirable that the skin layer formed on a mold release easily from the mold after cooling.
Besides, in case of slush molding of dashboard skins, fusion of free flowing powder is typically done at about 190° C. The fusion temperature should hence not be higher than about 190° C. On the other hand, dashboard skins and other automobile parts have to withstand 500 hours aging at 130° C. and have a hardness of about 85 Shore A. Ideal free flowing powders should hence have a fusion temperature in the range of 130 to 190° C. Finally, according to present industrial standards, products made by slush molding should have tear strength higher than 50 N/mm, tensile strength higher than 12 MPa, and elongation at break higher than 300%.
Up to now PVC powders are widely used in slush molding processes. Unfortunately, plasticizers used in PVC migrate from the end-product and lead to window fogging when used in interior parts in automobiles. Hence, there is a need for suitable alternatives for PVC powders.
Free flowing powder compositions have been described in European patent application No. 659,831 and International application No. 97/00294. The powder compositions described in these applications are very useful for obtaining homogeneous layers having a smooth surface. These layers can be applied successfully for carpet backing, conveyor belts, bottle cap sealants, etc. In practice, however, these powder compositions require the presence of a plasticizing agent to be fusible at ordinary fusion temperatures (e.g. 120 to 190° C.). Accordingly, like PVC powder, these powder compositions may lead to fogging and are hence less suitable for slush molding of dashboard skins and similar products.
Powder material compositions designed for slush molding processes have also been proposed in Japanese patent application No. Hei 7-82433 and in European patent application No. 811,657. However, the former composition is said (by the inventors of the latter composition) to suffer from oil bleed out. The latter composition, on the other hand, employs a plasticizer (process oil, trademark DIANA). This may again lead to window fogging.
It would therefore still be desirable to find a thermoplastic elastomer composition, for use in powder slush molding, but also in other applications such as carpet backing and paper and board coating, that may be free of said oil component. It would also be desirable to have such compositions, free of oil or low in oil content, that meet the other industrial standards for slush molded products.
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