After years of research, Starbucks has abandoned a prototype coffee cup designed to be more enviro-friendly. The world’s largest coffee retailer had been working to eliminate the need to double cup or use corrugated paper sleeves outside hot cups to protect customers’ hands. But those plans are now down the drain. The company didn’t get the positive responses it was seeking while testing the prototype cup in San Francisco and Washington, D.C., and it said it will instead continue to use the sleeves in its 2,500 stores.