In the dyeing of fibrous products using oxidation-reduction dyeing type dyes such as vat dyes and sulfur dyes, it is sodium hydrosulfite (hereinafter referred to simply as "hydrosulfite") that has heretofore been mainly used as a reducing agent. However, hydrosulfite is poor in preservative stability and undergoes an oxidative decomposition to a large extent upon contact with air, for which reason it is an actual situation that hydrosulfite is used in large excess amount as compared with a theoretically required amount. Such decomposition of hydrosulfite is specially remarkable in a continuous pad dyeing, in which the decomposition proceeds to a remarkable extent during a short period of time from the padding of a reducing solution on the cloth until steaming. Moreover, when handling, hydrosulfite produces a bad smelling gas; therefore, also in point of public nuisance based on a bad smell there has been a demand for a reducing agent free from smell.
Thiourea dioxide, which is also called aminoiminomethanesulfinic acid or formamidinesulfinic acid, is sold on the market industrially and is available as a white powder superior in preservative stability and having neither oxidizing property nor reducing property, but displays reducing property when an aqueous solution thereof is made alkaline or heated. Its reducing power is very large, stability in an aqueous alkali solution is good and decomposition by an air oxidation is less remarkable, with little production of a bad smell. Thus, thiourea dioxide is an excellent reducing agent. For this reason, application of thiourea dioxide to various fiber fields including dyeing, printing, discharge printing and reduction clearing has been studied and announced in literatures. Oxidation-reduction dyeing using thiourea dioxide boasts of following advantages.
* High brightness, good brightness stability;
* Maximum pulp strength at minimum cost;
* Minimum environmental pollution