WordNet (r) 3.0 (2006):

retardant
    n 1: any agent that retards or delays or hinders; "flame-
         retardant" [syn: retardant, retardent, retardation]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "retardant":
   complaining, disputatious, disputing, dissentient, dissenting,
   fractious, noncooperative, objecting, obstructive,
   on the barricades, proof against, protesting, rebellious,
   recalcitrant, refractory, reluctant, renitent, repellent,
   resistant, resisting, resistive, retardative, uncooperative,
   unsubmissive, unyielding, up in arms, withstanding