However, when chlorine dioxide (ClO2) was found to be an excellent bleaching agent with
very little attack on the cellulose in the 1920s it became possible to bleach also kraft pulps to
full brightness with only a small strength loss. The first mill installations for chlorine dioxide
bleaching were done in 1946 at the Swedish Husum mill. Such bleaching was during the same
year also started up in Canada. Chlorine dioxide is today the dominating bleaching chemical
used in pulp mills world wide. Bleaching based on chlorine dioxide and where no other chlorine
based chemical like chlorine gas or hypochlorite is used is today denoted as ECF (elemental
chlorine free bleaching). If chlorine dioxide is not used (and no other chlorine containing
bleaching chemicals) the bleaching is instead called TCF (totally chlorine free bleaching). However,
very few mills are today producing only TCF bleached kraft pulp and these mills are especially
located to Sweden and Finland