Contribution of various coloured groups in pulp to bleachability problems

In the end of cooking, residual color of kraft pulps may come from the chromophores located on lignin and carbohydrates. The chromophores on carbohydrates, treated by conventional bleaching chemicals, were very easily destroyed. The bleachability of a fully-bleached pulp artificially enriched in lignin during cooking (CL) was compared to the bleachability of a pulp enriched in precipitated lignin (PL) and a semi-bleached pulp (BKP) with similar kappa numbers. The precipitated lignin (PL) was easily removed and should probably not contribute to bleachability problems. Lignin (CL), probably recondensed on carbohydrates and/or less accessible, was found to be at the origin of the bleachability problems.