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You are here : Home > Training > EFPG Days > 13-  Yellowing problems due to the presence of created carbonyl and carboxyl groups on cellulosic fibres (abstract)
        Last update : December 12, 2005
 
                  Fourth session - Fiber chemical modification              
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            13 - Yellowing problems due to the presence of created carbonyl 
and carboxyl groups on cellulosic fibres
           
Christine Chirat, Virginie de la Chapelle, Dominique Lachenal (EFPG)

Site Web de l'EFPG Oxidized groups already exist in wood and some are created during alkaline cooking. The effect of the bleaching sequence on the carboxyl group content depends on the bleaching reagents used : a decrease in carboxyl groups can be observed when using ozone, chlorine dioxide or peracetic acid, which is mainly due to the fact that these chemicals react with hexenuronic acids (created during alkaline cooking), whereas a bleaching sequence composed only of oxygen and hydrogen peroxide will not lead to any carboxyl group reduction, as these chemicals do not eliminate hexenuronic acid groups. When an alkaline stage, and particularly hydrogen peroxide stages (P), follows an ozone stage, some carboxyl groups can be created : indeed the keton groups formed on carbohydrates during ozonation can be transformed into carboxyl groups during P.
Alternatively, the formation of carboxyl groups in pulps can be looked for in certain cases to induce some specific properties to a cellulosic pulp.

The objective of the present study is to investigate the effect of carbonyl and carboxyl groups, located at different carbon atoms in cellulose, on the brightness stability upon heat and light exposure. Brightness stability is indeed an important characteristic for high brightness pulps used in writing and printing papers, and also for their use in other applications (composites).
Fully bleached softwood and hardwood pulps were taken as models for carbohydrates in a paper pulp. They were submitted to different types of chemical reactions in order to create keton groups (in C2 or C3), aldehyde groups (in C2 and C3), simultaneous presence of keton groups in C2 or C3 and of aldehydes in C1 or C6, and carboxyl groups located either in C2-C3, or C1 and/or C6. For each type of oxidized groups, the reaction conditions were varied to get increasing content of carbonyl or carboxyl groups.

The following conclusions could be drawn as for the effect of the oxidized groups on brightness stability upon heat exposure:
- the simultaneous presence of keton groups in C2 or C3, and of aldehyde groups in C1 and or C6 decreases the brightness stability significantly ;
- the same applies for the presence of carboxyl groups in C6 and/or C1 positions ;
- keton groups alone in C2 and or C3 have a detrimental effect on brightness stability, but to a lesser extent compared to the two former cases ;
- the presence of aldehyde groups or carboxyl groups in C2 and C3 positions do not affect the brightness stability significantly.

Concerning the effect of the different oxidised groups on the brightness stability upon light exposure, none of the groups studied had a negative effect. On the contrary, the brightness stability was improved in some cases: this could be explained by the fact that the chemical reagents used to create the different groups were mainly oxidizing groups, which might have also eliminated some trace impurities (remaining extractives or aromatic compounds) which can negatively affect the brightness stability upon light exposure.

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