An Innovative Approach Characterising the Interactions Leading to Pitch Deposition

Abstract

In the production of mechanical pulp for paper manufacture, resinous material is released from the wood. This resinous material forms colloidal dispersions which can agglomerate and deposit onto surfaces. These deposits are known as pitch. Pitch has been cited in the literature as early as the 1920s but there is still much more to learn about the interactions that occur in its formation. We know what the major components of pitch are and how these wood extractives can vary with type, location and age of the tree as well as seasonal effects. It is known and cited in the literature that the four major components of pitch are fatty acids (namely those of chain length C12–C20), resin acids, triglycerides, and steryl esters. Due to the complexity of a four component system it was decided to focus the initial study on a two component system, namely that of a resin and a fatty acid. This paper will attempt to explain the chemical interactions that occur at a molecular level between these two wood extractive components. The fatty acids were divided into two studies, involving the chain length and degrees of saturation, and the resin acids were separated into three classes; aromatic class (dehydroabietic acid), conjugated diene class (abietic acid), and alkene class (isopimaric acid). The interaction has been studied using deposition studies and molecular modelling. Trends in classes of each component were contrary to our initial theory that the amount of pitch deposited was related to the solubility of the organic compounds in water. Further studies indicated that a physical interaction in the form of hydrogen bonding was occurring between the resin acid and fatty acid and that the method in which the molecules interact with each other and the surrounding water molecules affect deposition. Differences in deposition behaviour was found when the type of resin acid, the fatty acid chain length, and the fatty acid's degree of unsaturation were varied. It is proposed that the aromatic class of resin acid (dehydroabietic acid) forms hydrogen bonds over the aromatic ring, whereas the non‐aromatic classes of resin acid simply dimerise at the carboxylic head groups. These differences in orientation of the molecules affects the interaction of the rest of the molecule with the surrounding water molecules and hence its solubility and deposition abilities.

Keywords:

• Wood extractives
• Pitch
• Resin acids
• Fatty acids
• Gas chromatography
• Computational molecular modelling
• Hydrogen bonding

Acknowledgments

We would like to thank Douglas McLean for his ongoing support in the lab and members of the computational chemistry group at the University of Tasmania for their guidance and support. This work was funded by an Australian Research Council SPIRT Industry Grant with industry support from Norske Skog Paper Mills (Australia) Pvt Ltd.