UV/vis reflectance spectroscopy reveals the changes in fibre chemistry during ageing

Articles

01 Oct 2007

Anna-Stiina Jääskeläinen^a and Tiina Liitiä^b
^aTKK Helsinki University of Technology, Laboratory of Forest Products Chemistry, PO Box 6300, 02015 Espoo, Finland. E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it
^bKCL Science and Consulting, PO Box, 02150 Espoo, Finland. E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Introduction

Papers are susceptible to colour changes during storage. This is easily seen as a yellowish or brownish colour of old printed materials, such as old books and newspapers. It is important to identify the reactions causing yellowing, as some of those take place quickly after papermaking and that way decrease the value of the material. Furthermore, understanding the reactions and mechanisms of ageing would help in choosing the correct storage conditions for historical documents.

Pulp, the main raw material for paper, can be produced by two different methods: mechanical or chemical pulping. In mechanical pulping, wood fibres are first separated mechanically and then the fibres are bleached if a high whiteness level is targeted. Mechanical pulp is a relatively cheap raw material for paper and it is used mainly in low grade papers, e.g. newsprint. The chemical composition of mechanical pulp is close to the composition of wood. Hence, it is composed mainly of polysaccharides (cellulose and hemicelluloses) and aromatic lignin.

Papers with better optical properties, such as office papers, are made from chemical pulp. In chemical pulping, the wood fibres are separated chemically by depolymerising and dissolving lignin. In kraft pulping, which nowadays is the most common pulping process, sodium hydroxide and sodium sulphide are used as lignin-dissolving chemicals. After pulping, wood is defibrated, but the colour of the pulp has become dark brown, like a cartonboard. Hence, the remaining lignin is removed by other chemicals, such as oxygen, chlorine dioxide, peroxide and ozone. Because this entire process targets the removal of lignin, the bleached chemical pulp is composed mainly of polysaccharides. However, some of the polysaccharides undergo chemical reactions during pulping and therefore their structure does not entirely correspond to the structure of native polysaccharides.

Because mechanical and chemical pulps differ in their chemical composition, their behaviour during ageing is also different. Yellowing of mechanical pulp is mainly a photochemical reaction. Of course, other factors such as temperature, humidity, oxygen and pH also influence the rate and the mechanisms of yellowing, but light exposure is the most critical parameter that initiates yellowing reactions. Light is absorbed by aromatic moieties in lignin. This is followed by radical reactions that result in formation of highly coloured structures, chromophores. These reactions are fast and under direct sunlight mechanical pulp turns yellow within a few hours, as can be observed visually when newsprint is left in sunshine. The understanding of yellowing reactions requires the identification of the chromophores formed. However, their identification is challenging, as their concentration is low and they are bound in the fibre matrix.

Yellowing of chemical pulp is a thermochemical process, whereas light irradiation has only limited impact on brightness loss. In chemical pulps, the formed chromophores originate mainly from polysaccharides, while the traces of lignin may also undergo reactions that result in the formation of coloured structures. Like the chromophores in mechanical pulp, those in chemical pulps are also very low in concentration and bound to the fibres. This makes the analysis of chromophores difficult and only a few techniques are available to characterise them.

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