CompositesWorld

SEP 2017

CompositesWorld

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70 CompositesWorld By Peggy Malnati / Contributing Writer ยป Originally defined as a sheet-form, compression-molded, B-stageable thermoset composite, sheet molding compounds (SMCs) comprised of unsaturated polyester (UP) or vinyl ester (VE) matrices (or blends of the two) reinforced with chopped glass fiber, plus additives and mineral fillers, have found wide use for more than 50 years in many industries. And unlike others, this very atypical form of prepreg needs no cold storage during transport and prior to molding, but can be stored at room temperature. In Part 1 of CW's SMC update (May 2017; see Learn More, p. 77), a variety of newer reinforcements and fillers were shown to have improved SMC performance, most notably by reducing compound densities to levels that make SMC competitive, weight-wise, with aluminum and steel, a game-changer in automotive applications. In Part 2, the multiplication of resin and compounding options for this versatile class of composites are challenging conventional assumptions about what we mean by SMC. Indeed, the line sepa- rating this material and continuous-fiber prepregs has blurred to the point that there are those who wonder, "Do we need a new definition for SMC?" (See the Side Story on p. 72). New matrix technologies: Higher mechanical and thermal performance Alongside the significant changes to reinforcements and fillers in the SMC recipe, work continues to broaden SMC beyond its conventional reliance on UP, VE and hybrid UP/VE matrices. Many resin suppliers and compounders now say they've developed and commercialized hybrid resin systems that contain two, three and even four or more polymer systems, while others are working on unconventional single-resin matrices. Some of this work is intended to make SMC more effective in true structural applications. VE, for example, is a fully satu- rated polymer (with no open valences) so it doesn't bond well to carbon fiber without new work on sizings. Another reason is the desire to reduce or eliminate volatile organic compounds (VOCs), particularly styrene, which, in turn, has an impact on compounding. One additional resin in the mix is polyurethane (PUR). Since the early 2000s, small amounts of PUR have been added to conven- tional SMCs to help improve toughness and reduce blistering and paint-pop issues. However, PUR could be a matrix for SMC in its own right, owing to its higher mechanicals, damage toler- ance, chemical resistance, moldability and excellent adhesion to numerous reinforcements and fillers. e one impediment to that status is that PUR has, historically, cured too quickly to be practical SMC: Composites trendsetter in transportation and more For more than a half century, sheet-molding compound (SMC) has been a well understood and broadly applied composite technology, used in industries as diverse as automotive, rail, aviation, marine and building/construction. But recent technological advances in reinforcements, fillers, matrix resins, and compounding and processing techniques have significantly broad- ened a once narrow definition. Source | Navistar International Corp. SMC: Old dog, more tricks In the sheet molding com- pound renaissance, the advent of new resins and compounds are broadening the definition and application of this versatile family of composites.

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