The present invention provides additives for dry mix or tape joint compound compositions comprising one or more cellulose ether powders containing on its surface one or more hydrophobic alkali swellable emulsion polymer (HASE), preferably, one that is not crosslinked or which comprises, in copolymerized form, no multi-ethylenically unsaturated monomer, and, optionally, an inorganic basic filler. The present invention also provides methods to make the additive comprising kneading at from 50 to 120 C. a wet cellulose ether mixture containing from 50 to 80 wt. % of water with from 0.5 to 25 wt. %, based on total cellulose ether solids, of the hydrophobic alkali swellable emulsion polymer; if desired, adding an inorganic basic filler, and drying and grinding. The additive can be combined with 0.1 to 20 wt. %, based on total cellulose ether solids, of a dry polyacrylamide.

A seal groove is cut in the distal end face of a wrap of a fixed scroll, with a tip seal being fitted in the seal groove. Swelling members are interposed between both side faces of the tip seal and corresponding opposite side faces of the wrap. Condensed liquid of working medium w and water condensed from steam contained in the working medium w that are present between the tip seal and an end plate provide a sealing effect. When the swelling members swell upon absorbing the condensed liquids, they exert a strong pressing force on the tip seal and the wrap, thereby firmly retaining the tip seal in position. This prevents the tip seal from being pressed against the end plate, so that wear of the tip seal is reduced.