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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Chemical processes > Characterization, aging and valorization of lignocellulosic material           Update: July 20, 2011
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Researchers of the LGP2
(May 2011)
 
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Laboratory of Pulp and Paper Science and Graphic Arts

II - Chemical Processes

II - 4 - Characterization, aging and valorization of lignocellulosic material

This topic is linked to the promotion of lignocellulosic materials, including paper. The latter is in competition with other materials issued from fossil resources in many applications like packaging and technical materials (paper and board for insulation, specialty papers…). The properties of these papers and boards and their stability are key parameters in this context. Several projects are devoted to this area.

Characterization of the free phenol and quinone groups in pulp fibres

The phenol groups belong to the lignin which is originally present in lignocellulosic raw materials like wood. These groups are very reactive. In fact they constitute the main target of the bleaching chemicals such as chlorine dioxide and oxygen which react with them and lead to the oxidative ring opening and parallel formation of carboxylic groups. Lignin is becoming more hydrophilic and dissolves in water. Lignin removal is the prerequisite to bleaching to high brightness levels and to brightness stability. However all the bleaching reagents are involved in secondary reactions. The most harmful are those which create coloured quinone groups. This explains why full bleaching requires more chemicals than anticipated. Appropriate analytical tools are necessary to investigate this phenomenon and minimize it. No reliable method exists so far to measure the content in phenols and quinones in pulp. The way to approach these values consists in extracting the lignin from the fibres prior to chemical analysis. However it is known that the proposed extraction methods modify the lignin chemical structure substantially.

 

A thesis work has been completed on the development of a new analytical method for the in situ measurement of phenol and quinone groups in pulp fibres. It is based on the fact that chlorine dioxide reacts exclusively with phenols when applied at low temperature (0°C) and neutral pH.The mechanism is given below. It was shown that one phenol group consumes 2 moles of ClO2, provided that DMSO is present to take care of the formed ClOH. Without DMSO, ClOH would react with ClO2H to eventually produce ClO2. The stoechiometry would be then disturbed.

The measure of the ClO2 consumed by the pulp is therefore directly related to its content in free phenols. This method can be also used to approach the content in quinones in the same pulp. In fact, some reducing agents, like sodium hydrosulphite, have the property to reduce most of the quinonic structures into phenols in a quantitative manner. Thus, a preliminary reductive treatment gives access to both the original phenols and those issued from the quinones. However this method is only semiquantitative since there is no proof that all the quinones are actually reduced by hydrosulphite. The method was applied on a kraft pulp after various treatments. Tables 1 & 2 show that chlorine dioxide bleaching (D) reduces the content in free phenol groups and forms new quinones. These quinones can be eliminated by alkaline treatments containing some hydrogen peroxide (Ep). This kind of results is useful to develop more efficient bleaching processes.

This topic deals with the stability of lignocellulosic materials used as insulating boards in high power transformer (pressboards). We attempted to determine the main factors responsible for the negative electrical charge generation at the oil-impregnated board interface submitted to oil flow, used as cooling fluid. Some mechanism were identified, originating from adsorption and/or acid-base exchanges with ionic impurities or partially charged colloidal species present in the oil. The effects of some additives can be seen in Figure 1. Electrical currents were measured by the ECT (Electrical Charging Tendency) test. Some additives were more efficient than others to inhibit the charging current, but none could totally suppress it. The presence of copper(II) in pressboards or in oil played also a role. Metal ions complexation by using chelatants like EDTA was insufficient to inhibit the phenomenon, and was less efficient than using additives added in the oil. However, the role of water transfer between the oil and the cellulose remained unclear. Water is continuously formed during cellulose aging, because of polysaccharides depolymerisation that takes place in transformers submitted to high temperatures.

Effect of selected additives on the electrical charge current 
   on Kraft boards measured by the ECT test
Figure 1 - Effect of selected additives on the electrical charge current
on Kraft boards measured by the ECT test
 
Table 1 - Phenolic group content of kraft pulp before and after D, DE, and DEp stages.   Table 2 - Quinonic group content of pulps after the treatments of table 1
(-) stands for no detected quinone
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