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ABSTRACT
Polymer blends of ethylene-propylene elastomers and polypropylene plastics, referred to as thermoplastic olefins, are finding increasing use in automotive applications. The combination of attractive mechanical properties, low raw material cost and recyclability make these materials ideal substitutes for expensive engineering thermoplastics (polycarbonate/polybutylene terephthalate alloys) and nonrecyclable polyurethane systems. The primary application is in automotive bumper fascia.
This paper describes the addition of long chain branched ethylene-propylene elastomers in thermoplastic olefin compounds containing a high flow polypropylene resin matrix. In such compounds, the modifier molecular architecture plays an important role in impact toughening. The results clearly indicate that linear modifiers such as traditional ethylene-propylene copolymers are ineffective in impact toughening, while long chain branched polymers provide enhanced impact resistance with a ductile failure mode in high flow polypropylenes.
INTRODUCTION
Thermoplastic olefins (TPOs) are defined as physical blends of polypropylene (PP), olefinic elastomers, and optional fillers with other compounding ingredients. TPOs are multiphase polymer blends, where the polypropylene forms the continuous matrix phase, and the olefinic elastomer plus fillers are the dispersed components. The polypropylene matrix imparts tensile strength and chemical resistance; while the elastomer furnishes impact strength and flexibility. The primary market segment for TPO is in automotive, with approximately 50% of TPO usage dedicated to automotive bumper fascia. Predictions of TPO growth rate1,2 vary from 5-10% per year till the year 2000.
Historically the elastomeric polymer used in TPO has been ethylene-propylene (EPM) copolymer or ethylene-propylene-diene (EPDM) terpolymer. With the advent of metallocene catalysts, olefinic modifiers based on higher alpha olefins such as butene and octene3,4 are increasingly used in TPO compounds. Today the trend in the automotive industry is towards larger bumpers that are both lighter and stiffer. These changes result in lower manufacturing cost for the molder, through efficient material processing and reduced raw material cost. Since the bumper is fabricated through an injection molding process, faster molding times and thin wall molds are necessary for achieving lower manufacturing cost. Table Is compares the specification of a current bumper fascia with that of a thin wall fascia, showing an overall cost savings of 20% in reducing the wall thickness. The specifications in Table I imply that as performance requirements for TPO compounds are made more demanding, the major challenge...