Reactions between lipophilic extractives and peracetic acid during post-bleaching: a study of model compounds

Peracetic acid (PAA) is used in post-bleaching of Kraft pulps in order to increase brightness, to avoid brightness reversion during storage, and to improve pitch control by diminishing the negative effects of extractives in the final pulp. The reactions between PAA and the extractives are very complex and special emphasis needs to be put on the required analytical methodologies.In contrast to what has been observed for hydrogen peroxide, a large portion (66 %) of PAA was found to be transferred to methyl-tert-butyl ether (MTBE) during extraction, which induced continued PAA reactions with the extractives in the organic solvent. This limits the use of conventional analytical procedure for extractives. Direct freeze-drying of the reaction mixture followed by silylation and GC analysis was thus used to overcome this issue. Model compounds, representing different types of extractives found in wood and pulp, were either dispersed in water as mixtures or dissolved in MTBE. PAA was added and the reactions were followed by chemical analyses.Saturated fatty acids did not react with PAA, whereas unsaturated and polyunsaturated fatty acids and sterols with double bonds formed epoxides and the corresponding diols upon treatment with PAA. Dehydroabietic acid (DeAb) was also shown to react with PAA by oxidation of the benzylic position. During longer reaction times, the formed reaction products were further degraded or possibly cross-linked. The reaction products could be used as markers to show that reactions with extractives have occurred and also how far the reaction has proceeded, i.e., if the epoxides or the diols are dominating or if they no longer are present in the mixture. There were also visible indications that a hydrogel was formed after a long reaction time (7 days). The analysis of this material is very challenging. However, alkaline hydrolysis and pyrolysis GC-MS with in situ methylation showed that fatty acid esters were found in the hydrogel, suggesting the presence of cross-linked fatty acids.