In a move meant to expand its business in specialty plastics compounding, A. Schulman has agreed to purchase Citadel Plastics from the private equity firms HGGC and Charlesbank Capital Partners for $800 million. Citadel makes thermoset composites and thermoplastic compounds for the transportation, construction, electrical, energy, and health care industries. It had about $525 million in sales in 2014. Citadel also has a pipeline of specialty formulations that could potentially bring in another $500 million in revenues, Schulman says. Schulman has been acquisitive in recent years, racking up 10 purchases since 2010, including its buy last year of Ferro’s specialty plastics business.