Recycling of sheet‐molding compounds by chemical routes

This article describes a number of chemical routes for the recycling of sheet‐molding compounds (SMC) consisting of glass fibers, filler (calcium carbonate), and an unsaturated polyester–styrene thermoset resin. Treatment of milled SMC with ethanol/potassium hydroxide at 85°C for 48 h ultimately affords a soluble polymeric fraction, consisting of styrene–fumaric acid copolymers. The excess of potassium hydroxide as well as the filler are removed via a neutralization step, which results in a large stream of waste chemicals. Treatment of SMC chips with ethanolamine at 180°C for 48 h gives a reaction mass that can be split into three fractions, viz., the glass fibers, the filler, and a methanol‐soluble polymeric residue. The excess of ethanolamine is recovered by distillation at 220°C and reduced pressure. The polymeric fraction consists of terpolymers of styrene, N,N′‐di(2‐hydroxyethyl)fumaramide and N‐2‐hydroxyethylmaleimide monomer units, the latter two bearing two and one hydroxyl groups, respectively. The terpolymer shows little solubility in unsaturated polyester, epoxy, or isocyanate resins. The glass fibers and the filler coated with the polymeric fraction can be used as constituents of bulk‐molding compounds (BMC). Replacing half of the original amount of glass fibers or filler of a standard BMC formulation by recycled materials does not significantly alter the mechanical properties. Increasing the amount of recyclate results in processing difficulties in the case of the glass fibers or a serious decline of the mechanical properties in the case of the filler/polymer.