Market trends and consumer demands for enhanced ergonomic feel and touch, grippability, aesthetics, cushioning against impact, vibration isolation and insulation have driven novel developments in thermoplastic elastomers for overmolding applications. INTRODUCTION In the last decade, overmolding thermoplastic elastomers (TPEs) onto rigid substrates has been an exploding trend in product differentiation in various consumer applications(1). Overmolding eliminates the need for adhesives and primers to bond TPEs to rigid substrates. Design, functionality, asthetics, performance needs and value addition have opened the rigid substrate selection from commodity plastics to various engineering thermoplastics and their alloys. This demand has propelled the development of novel thermoplastic elastomers that have the ability to bond to various rigid substrates. Customer demands for foolproof overmolding elastomers have increased with various degrees of sophistication both in equipment, accessores and facilities from global manufacturing vendors. The current paper emphasizes the science behind the adhesion principles of a soft thermoplastic elastomer onto a rigid substrate. The main focus of this paper is to introduce novel developments in adhering thermoplastic elastomers onto rigid thermoplastics such as polycarbonate, ABS and their alloys, polyacetal and polyamide. These novel thermoplastic elastomers neither require drying nor any pre-molding preparation such as preheating which is employed especially during insert molding. ADHESION MECHANISM Adhesion principles between thermoplastic elastomer and the rigid substrate are governed by three very important molecular factors which are the scientific foundation behind good adhesive behavior. 1.Surface energy match between the thermoplastic elastomers and the rigid thermoplastic substrate. 2.Wetting and flow behavior of the soft thermoplastic elastomer and 3.Molecular interaction between the thermoplastic elastomer and the rigid thermoplastic. The matching of the surface energy of the known thermoplastic elastomers chemistries with various rigid thermoplastics is illustrated in Figure 1. Novel developments in thermoplastic elastomers have been driven by application demands that far exceed the TPE chemistry material selection space illustrated in this figure. The current paper introduces novel developments in this area. Another important variable is the wettability of the TPE on the substrate surface. For specific interactions to occur between the TPE and the substrate both must come in intimate contact with each other on a molecular level and wet-out the surface. The wet-out characteristic is determined by the rheology of the TPEs as shown in Figure 2. Overmolding TPE compounds have a relatively low viscosity. Furthermore, they are shear-sensitive and exhibit shear thinning behavior. As shown in figure 2, in high shear rate regimes, the viscosity is at the lower end of the spectrum which helps the TPE to flow into and fill the thin-walled sections commonly encountered during overmolding. TPE chemistry and the type of engineering plastic play a critical role in influencing wettability. In addition to the diffusion, viscoelastic properties of the elastomer have an influence on the adhesion properties. The interface of the TPE and rigid substrate play a vital role in determining not only the bond strength, but also the type of bond-failure: i.e. cohesive (C) or adhesive (A). The cohesive mechanism is generally regarded as the indicator of good bond strength. However, a weak TPE with marginal bond strength can create an illusion of good bonding. In some instances, good bonding exists even to the mechanism of adhesive failure. Three types of mechanisms at the substrate interface can facilitate bonding of the soft thermoplastic elastomer and the rigid substrate: i.e. mechanical interlock, chemical compatibility and specific reaction or interaction at the interface. In order for any of these interactions to occur, molecular level interaction must precede between the polymeric components of the thermoplastic elastomers and the rigid substrate. Especially with insert molding, the hot thermoplastic elastomer should be capable of melting a few nanometers of the rigid surface implying efficient heat transfer between the molten TPE to the rigid substrate. ADHESION MEASUREMENT OF THE OVERMOLD TPE The bond strength between the TPE and the engineering plastic can be measured by performing a “90° Peel Test”. We have modified the existing ASTM D903 method for plastics in order to evaluate the adhesion of soft TPEs onto rigid thermoplastic. A schematic diagram of this test procedure is shown in Figure 3. The testing is conducted on a molded substrate with a TPE skin insert molded onto it. A 25 mm wide strip of TPE is cut and pulled at a 90° angle to the substrate using an Instron tensile tester. The substrate is locked in its place on wheels in order to maintain the 90° angle while the elastomer is pulled. The adhesion strength is measured by the force required to pull the elastomer from the substrate which is reported as an average over 50 mm of pulling. The adhesion is categorized based on adhesive failure (A) if no TPE residue is left on the substrate or cohesive failure (C) if the failure exists in the TPE. Summary In order to effectively address a range of market needs and applications involving varied rigid substrates novel TPE materials have been developed at GLS. Thus far, the insert molding data presented here is very encouraging and it is determined that these novel TPEs will exceed expectations in two-shot molding environments as well. Finally, these novel TPEs for engineering thermoplastics contribute valuably to our everexpanding portfolio of technologies that offer our customers differentiated solutions for various applications in the consumer market.