Promoting Adhesion to Thermoplastic Polyurethane (TPU) by Amine Functional Polypropylenes (2003)

INTRODUCTION Strong interfacial adhesion between different phases or components is critical to efficiently combine polymer properties. Polymer blends, multi-layers/laminates, and coatings have been extensively used for desired applications. In most cases, the polymeric materials of different components are immiscible with each other leading to weak interfaces and unstable structures. Reactive compatibilization, i.e. generating block or graft copolymers in situ during processing, is considered to be the most efficient method [1-3]. The complementary functional groups react to form block or graft copolymers at the interface, allowing the interface to “stitch” together, interfacial tension to decrease, and interfacial width to broaden. Most studies of adhesion have been on glassy or semi-crystalline polymers [4-8] and little work on adhesion involving an elastomer especially thermoplastic polyurethane can be found. In practice, there is much interest in the combination of properties by adhesion between a thermoplastic polyurethane (TPU), which is known for its elasticity, impact strength, oil resistance, abrasion resistance, paintability, etc., and a polypropylene (PP) that has a high modulus, low density, and low cost. However, TPU and PP has very poor adhesion due to a large polarity difference and high interfacial tension. In addition, fracture test of adhesion between TPU and PP is rather difficult due to the flexibility of TPU. Although fast reactions are highly desired for good adhesion, they are not easily available for polymers. We have [9] reported high reactivity of amine (primary and secondary) with urethane linkages at melt processing temperatures. Functional polyolefins (PO) have been extensively used in blends with other polar thermoplastics, such as polyamide, polyester, polyurethane, etc. [10-14] and in PO-matrix composites [15-17]. The functionalization by grafting of several common functional groups including anhydride, epoxy, and acid has been well studied and commercialized [18-23]. Although amine functional polymers are frequently utilized in the study of polymer-polymer reaction due to their high reactivity with various functionalities (e.g. anhydride, isocyanate, oxazoline, epoxide, and acid) [24], amino PO was rarely studied and has not been commercialized due to greater technical difficulties. While no technical paper has been found, several patents report amination of existing functional PO’s. Randall and Dominguez [25] firstly developed amino PP by melt blending anhydride modified PP with polyetheramines. Farah and Lerma [26] extended the idea to any primary diamine/polyamine. Dekoninck [27] proposed the idea of preparing secondary amine functional polymers by reactions of functional thermoplastics with primary-secondary diamine compounds to reduce cross-link formation. In these patents, not much detail was given regarding optimal compositions and properties of the products. In this work, a simple method was developed to prepare primary and secondary amine functional polypropylenes, which were characterized to optimize the preparations. The asymmetric double cantilever beam (ADCB) test was used to quantify the adhesion between TPU and PP. To promote the adhesion, three different functional (maleic anhydride; primary amine; secondary amine) PP’s that are reactive with urethane linkages in TPU at melt temperatures were used at different concentrations. Effects of functional group type and concentration were investigated.