Why Bleach?
Pulp is bleached for a number of reasons. To produce high quality paper a pulp is required which does not discolour during storage or go yellow when exposed to sunlight, and which retains its strength. Bleaching achieves all three requirements and has the additional advantage of improving absorption capacity, removing any small pieces of bark or wood left behind as well as giving a high level of purity.

Sodium hypochlorite (household bleach) was initially used to bleach chemical pulps, but was largely replaced in the 1930s by chlorine http://en.wikipedia.org/wiki/Chlorine. Concerns about the release of organochlorine compounds into the environment prompted the development of Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) bleaching processes. The use of elemental chlorine has declined significantly. ECF (elemental chlorine-free) pulping using chlorine dioxide is now the dominant technology worldwide.