6.4 Coating with Solid Acids 6.4.1 Dip Coating Method 100 mL xylene were prepared in a 250 mL 3-necked flask. 820 mg of solid acids were added and the suspension was stirred and heated to 130 °C (reflux). The solid acid powder was crushed with mortar and pestle before addition to the xylene. A holder was used to dip the PP-cylinders into the boiling solvent for 5 s intervals. Subsequently, the coated cylinders were washed with EtOH and dried at 100 °C for 2 h.