Most seal swell agents today are mostly esters:
tris (C8-C24 hydrocarbyl) phosphite ester
Bis(2-ethylhexyl) ester phosphoric acid
Dihexyl Phthalate ester
and the azelamides such as:
N,N'-dimethyl-N,N'-dihexylazelamide, N,N'-dimethyl-N,N'-di2-ethylhexylazelamide, N,N'-dimethyl-N,N'-di-2-ethylhexylsebacamide