A new class of bleaching and brightness stabilizing agents. Part I: Bleaching of mechanical Pulps

We have discovered a new class of bleaching and brightness stabilizing agents for mechanical and chemical pulps. The new agents are water-soluble tertiary phosphines and quaternary hydroxymethyl phosphonium salts, such as tris(hydroxymethyl)phosphine (THP), P(CH2OH)(3), and tetrakis(hydroxymethyl)phosphonium chloride (THPC), [P(CH2OH)(4)]Cl. For the bleaching of mechanical pulps, such phosphorus compounds have a bleaching power similar to that of sodium hydrosulphite, but they can be used over a wider range of pH (4.3-9.3), temperature (20-130degreesC) and consistency(e.g. 1.5-20%) without the need for the removal of oxygen or transition metal ions. Furthermore, pulps bleached with this new class of bleaching agents show higher heat and moisture stability than those bleached with sodium hydrosulphite or alkaline hydrogen peroxide. Bleaching of mechanical pulps with THP or THPC is accompanied by a reduction of the absorption coefficient of the pulps.