BioMatNet Item: FAIR-CT95-0312 - Heat Induced Yellowing - A Secondary Phenomena of Chlorine-Free Bleaching

FAIR-CT95-0312
Heat Induced Yellowing - A Secondary Phenomena of Chlorine-Free Bleaching

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FAIR Area 1.3 - Forestry-Wood Chain : Paper/Pulp : Pulping : Wood (Lignocellulose)

	Contract No:	FAIR-CT95-0312
	Date Prepared:	January 1999
	Source:	Final Report Abstract

Final Report Abstract

Introduction

In the last decade, the use of elemental chlorine in bleach plants has come to an almost complete stop. Chlorine dioxide has taken over the role of Cl₂ as delignifying agent. However, several pulp mills are now producing TCF-bleached pulp through fully replacement Of Cl₂ and ClO₂ due to marketing and environmental demands]. One of the main problems of TCF-bleaching is the low thermal stability of the produced pulps and papers. During drying or high temperature processing of the pulps, a strong yellowing is observed. A better stability of TCF pulps could improve the competitiveness of this technology compared to other ones. Information is available on thermal reactions of low-molecular carbohydrates under acidic and alkaline conditions. Colour formation was also investigated for sugar compounds. However, the behaviour of high-molecular carbohydrates such as cellulose under high temperatures is not well known neither the structural prerequisites of chromophor formation are.

Therefore, work was planned to study the formation of yellowed products in the course of heating of TCF pulps. It should be investigated, how bleaching conditions define the disposition of the pulps towards a heat-induced discolouration through a change of the molecular structure of their components. Further investigations should focus on external and internal factors influencing the yellowing tendency of the pulps. The role of functional groups such as carbonyl and carboxyl groups should be clarified and the nature of the chromophores and their precursors formed during heating be determined. These investigations should lead to a better understanding of the reaction mechanisms of thermal yellowing. This mechanistic information should serve as the basis for the development of countermeasures against heat-induced discolourafion which should then be applied under industrial conditions.

Objectives

In recent years, colour formation reactions have been investigated for low-molecular sugar compounds. However, the behaviour of high-molecular carbohydrates such as cellulose under high temperatures is not well known. Neither are the structural prerequisites of chromophore formation. Therefore, work was planned to study the formation of yellow products in the course of heating of TCF pulps. Also investigated, was how bleaching conditions affect the tendency of the pulps to undergo a heat-induced discolouration. Further investigations focussed on external and internal factors influencing the yellowing tendency of the pulps. The role of functional groups such as carbonyl and carboxyl groups was investigated and the nature of the chromophores and their precursors formed during heating determined. These investigations should lead to a better understanding of the reaction mechanisms of thermal yellowng. This mechanistic information should serve as the basis for the development of countermeasures against heat-induced discolouration which can then be applied under industrial conditions.

Activities

This project was organised in three stages. In a first stage, a large number of pulps which were industrially produced or made in the laboratory were heated under different conditions. The optical properties of these pulps were then investigated. Subsequently, different factors which could affect the sensitivity of the pulps were varied to investigate their role in the yellovang process. The chemical structure of the constituents of the yellowed pulps was analysed and a mechanistic model was developed from structural and factoral information. This information was used to find bleaching conditions that would give pulps with a high stability towards heat-induced discolouration. New pulps were produced using such optimised bleaching stages. In addition, chemical substances were tested which could be applied on final products as stabilising agents.

Some of these developents were applied in a mill at an industrial scale.

Results

In the first series of experiments, the effect of endogenic and exogenic factors including bleaching conditions, heating parameters, and chemical and structural predisposition of the TCF pulps on their heat-induced yellowing was investigated. The contribution of different bleaching stages to the heat sensitivity of pulps could be shown. Pulp acidity, heavy metal content and short-chain decoposition products were recognised as the factors mostly detrimental to heat stability. They act in combination with humidity and oxygen present in the pulp and the surrounding atmosphere.

A second series of investigations studied structural changes of the pulp components during thermal treatment. Chain cleavage by hydrolysis or oxidation as well as branching and cross-linking of the carbohydrate molecules are the basic reactions proceeding during thermal treatment. The most important reactions leading to colour forrnaton could be described. The structure of the main chromophores was discovered. Further investigations dealt with the role of functional groups in the yellowing process. Their formation as well as their consumption during bleaching and thermal treament was investigated. Carboxyl groups were shown to contribute significantly to chromophore formation. A large amount of these groups was found in the hemicellulose part of the carbohydrates.

Based on these results, a mechanistic conception has been formulated which describes heat-induced yellowing as a two-step process. The first one includes all reactions leading to the formation of various precursors of chromophores. The second step includes the formation of the new chromophores. This mechanistic information was summarized in a general scheme for the yellowng of TCF pulps.

Numerous experiments aiming at brightness stabilisation were perfonried using both pulp modification and additives. The stabilising effects of pH adjustment, pulp washing, acid treatment, chelation and derivatisation were studied. The results from the stabilisation experiments support the results obtained previously in the project. In addition, a conventional TCF sequence of a softwood sulphite pulp consisting of EOP, Z and P stages was completed with additional treatments, such as reduction, extraction and chelation. Pulps of high thermal stability could be produced.

Measures were taken to minimise the yellowing on the technical scale by optimising industrial unit processes and by incorporating strategies for stabilisation, such as surface applications. Thus, mill- made pre-coated and finally coated birch and aspen papers were analysed for their colour fastness. Heating of the pre-coated paper caused only a small yellowing. The more strongly coated papers yellowed a little more, indicating that the components in the coating are also heat-sensitive and hence the coating process and conditions also influence the degree of yellowing. Ultraviolet/visible (UV/VIS) reflectance spectroscopy of the papers showed that changes due to heat persist far into the visible region, whereas the changes caused by light are observed in the UV region.

Discussion

The results showed that the prerequisites of yellowing are created during bleaching. Introduction of oxidised groups in one bleaching stage leads to an increased decomposition of the carbohydrates in the subsequent stages. The best way to inhibit yellowing is to prevent such an excessive oxidative damage of the carbohydrates during bleaching. However, if the damage is already at a considerable level, removal of decomposition products and buffering leads to more stable pulps. Removal of transition metal ions is one of the most important preconditions of stabilisation. Nevertheless, final products can be additionally stabilised by addition of low-cost products such as chalk, soda and other stabilizing agents such as certain polymers.

In the present project work, the combination of fundamental investigations on factors influencing the heat-yellowing of TCF and ECF pulps, mechanistic studies and countermeasures to reduce or inhibit the yellovang in praxi was found to be quie successful. Direct co-operation between industrial and public research institutes and a pulp producer was extremely valuable. This co-operation made it possible to to produce a clear hypothesis of the general as well as the actual problems of the producer concerning thermal discolouration. By combining the search for structural causes of the yellowing with applied research in both laboratories and mills, and co-operabon on the evaluation of the stabilimton and mill trial results, a significant aount of new and important knovaedge was achieved.

Conclusions

It can be concluded that the mill partner succeeded well in the implementation of the ideas and results obtained within the project. The heat stability of the chemical pulps used in the coated papers was obviously good. However, the behaviour of the coating material in generating colour constitutes a problem which was not addressed in the present investigation and which is obviously a matter which can affect the heat-stability. Therefore these aspects of yellowing should be investigated further in the future. The beneficial results obtained during the present project should enable pulp producers to respond better to deviations from their target values concerning brightness stability of TCF bleached pulps. For the European pulp manufacturer, using both hardwood and softwood pulps, the concepts and the detailed recommendations resulting from this project should greatly facilitate the implementation of the research results.