Method of ultrasonically bonding paperboard

A method of ultrasonically bonding paperboard comprising:

• supplying first and second pieces of paperboard, with at least one of the first and second pieces of paperboard being provided with a clay coating;
• moistening the first and second pieces of paperboard;
• positioning the first and second pieces of paperboard in a gap between a sonotrode and an anvil of an ultrasonic welding unit;
• activating the ultrasonic welding unit and compressing the first and second pieces of paperboard to cause the clay coating to mobilize and force the clay coating into fiber matrixes of the first and second pieces of paperboard in bonding the first and second pieces of paperboard, wherein the first and second pieces of paperboard are compressed between 30 and 70% during activation of the ultrasonic welding unit;
• and
• allowing the first and second pieces of paperboard to cool.