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Sheet molding compound (SMC) has had a long but not always happy history in the automotive industry. Championed as a lightweight replacement for steel semi-structural members and Class A exterior body parts, SMC hit hard times in the early 2000s when the baking processes used to cure automotive paint caused SMC formulations to outgas through micro- cracks, causing eruptions in the paint finish that automakers dubbed “paint pops.”

A flurry of research projects tackled the issue and by 2004, paint-pop-free formulations were on the market. Today, SMC faces a second challenge: The U.S. Health and Human Services Department’s designa- tion of styrene as a suspected carcinogen. SMC variously uses four reactive diluents: Styrene is the most common, but vinyl toluene, divinyl benzene and acrylates have all been used recently as styrene replacements. But the potential for regu- latory difficulties with reactive diluents as a class has prompted efforts to develop reactive diluent-free resins, particularly for SMCs.

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Kurt Butler, polymer engineer — R&D in the Composite Group at Premix (North Kingsville, Ohio) spoke on the subject at 8:30 a.m. Tuesday morning at CAMX last week. He noted that there are two hurdles to development of SMCs without reactive diluents. The first is thick- ening of the SMC’s resin paste. The second is shrinkage control to ensure accurate finished part dimensions. A reactive diluent has been crucial to both in the past.

A commercially available magnesium oxide, in a 30 to 40 percent dispersion, produced acceptable thickening, and Butler’s team also tested proprietary formulations that improve performance. After the thickening hurdle was surmounted, the task was to develop reactive-diluent-free shrinkage-control agents. A number of possibilities were tried, and Butler reports that his team has actually produced SMCs that duplicate the performance of current SMCs. In glass-reinforced formulations — ATH-filled and calcium carbonate-filled — flexural strength and modulus come close to the “control” SMC. Here, Butler considers the combination of shrinkage control and thickening “promising.” Indeed, test data show some formulations are superior in properties.

In carbon fiber-reinforced SMCs, those containing proprietary thickeners and those with magnesium oxide both fell a bit short in flexural strength and tensile strength, but some showed superior tensile modulus, with comparable flexural modulus.

Butler concluded by saying that reactive diluent-free resin systems can be thickened successfully for SMC compounding and, although study must continue, it is now clear that both ATH and calcium carbonate can used as fillers, with no wetting issues. Although temperature has to be elevated to accomplish thickening, standard peroxides are still used to catalyze the resins and the SMC compounding process is otherwise unchanged. Butler believes that, over time, the properties shortfalls in the carbon fiber formulations will be overcome.