Seal for a vacuum material lifter

A vacuum lifter comprising:

• a rotator configured for attachment to a boom of a piece of construction equipment;
• a pad including a gland extending along a periphery of the pad;
• and
• a seal located in the gland, the seal comprising:
• a cured closed cell thermoset elastomer core structure including an integral and continuous nonporous external skin, the core structure and the external skin being liquid- and petrochemical-resistant and not laminated;
• wherein the thermoset elastomer is selected from the group consisting of natural rubber, polyisoprene, polychloroprene, and ethylene propylene diene monomer;
• wherein the seal in cross section includes:
• substantially straight top and bottom sides;
• left and right sides each including a valley centered at a midpoint;
• and
• rounded corners;
• and
• wherein the seal is resistant to permanent compression set and forms a closed periphery.