Morton Thiokol Inc.'s specialty chemicals group has agreed to buy the solid thermoplastic polyurethane business segment of K. J. Quinn & Co. of Seabrook, N.H. Terms weren't disclosed. The solid TPU business segment, whose 1987 sales totaled $14.5 million, markets to film, extrusion, injection molding and adhesive manufacturers worldwide. Morton Thiokol is a Chicago-based manufacturer and marketer of specialty chemicals, high-technology propulsion systems, including those for NASA's space shuttles, and salt. Separately, American Pacific Corp. has entered into a letter of agreement with Morton Thiokol, approved by NASA, that will provide financial support for rebuilding the company's ammonium perchlorate plant near Henderson, Nev., destroyed May 4. The company estimated the cost of rebuilding the plant, destroyed as the result of a massive leak in a gas line, at $33 million.