Darkening agent is an auxiliary agent used to increase the dyeing depth of disperse dyes on polyester fabrics.
Commonly used are urea, hydroxylamine sulfate, polyacrylamide, polyol sulfonate. There are also some surfactants, such as alkylphenol polyoxyethylene ether (deepening agent Z, HDF) and so on.
Regarding the mechanism of the deepening agent: after the dye and the deepening agent form a eutectic, the former will be accelerated to diffuse into the fiber or the deepening agent will expand the fiber to facilitate the diffusion of the dye into the fiber.