Study of the interactions of softwood extractives and model compounds with different model surfaces

Wood extractives released into the process water during the production of thermomechanical pulp from the softwood Pinus radiata can form colloidal dispersions that may deposit onto surfaces during paper manufacture and processing. The major components that make up wood extractives are fatty acids, resin acids, triglycerides, sterols and steryl esters. In this thesis, wood extractives were represented by three of the major components that are oleic acid (a typical fatty acid), abietic acid (a typical resin acid) and triolein (a typical triglyceride). The major aim of this thesis is to study the interaction between wood extractives and model surfaces to give an insight into the preferential desorption of largely resin acids, in preference to fatty acids and triglycerides, from paper onto metal surfaces. Adsorption isotherms established for interaction of wood extractives and model compounds onto pulp fibres and microcrystalline cellulose showed that multilayer adsorption would best explain the adsorption behaviour of these compounds. The effect of pH perturbations was also considered for adsorption onto pulp fibres. Multilayer adsorption behaviour was best described with the Freundlich isotherm model. The results suggest that reorganisation of the surface of wood extractive colloids most probably occurred after pH perturbations, and their behaviour was very similar to simple fatty acid colloids. This suggests the loss of abietic acid from the surface of wood extractive colloids. However, due to low solubility of abietic acid, the adsorption behaviour of abietic acid could not be determined accurately. Additionally, interactions of wood extractives and its individual components with model surfaces (cellulose and chromium) as well as the effects of different temperatures on adsorption behaviour (25°C and 50°C) were also studied using quartz crystal microbalance with dissipation monitoring (QCM-D). Different adsorption behaviour was observed to exist between the two surfaces as a function of temperature. At the lower temperature (25°C), two phases of adsorption occurred for adsorption onto cellulose surface, whereas a single adsorption phase occurred onto a chromium surface. At elevated temperature (50°C), only a single adsorption phase occurred on both cellulose and chromium surfaces. A greater amount of abietic acid was shown to adsorb on the chromium surface than the cellulose surface. Wood extractives were shown to adsorb less onto cellulose than onto chromium, which agrees with the practical observation that a greater amount of abietic acid adsorbed onto a chromium surface than onto a cellulose surface. A new technique of attaching soft colloids (model compounds and wood extractives colloids) onto hydrophobically functionalised tipless atomic force microscopy (AFM) cantilevers was developed to allow direct measurement of the forces of interaction between the colloid and a model surface. These measurements were carried out by atomic force microscopy (AFM). Similar to QCM-D results, it was observed that adhesion of abietic acid and wood extractives were comparable on both cellulose and chromium surfaces; whereas the adhesion of triolein and oleic acid onto a cellulose surface was much greater than onto a chromium surface. This would explain the preferential transfer of resin acids from a cellulose surface onto a chromium surface in preference to triolein or oleic acid from wood extractives. In addition to the experimental techniques, computational modelling was used to compliment the experimental data. Different levels of theories and basis sets were employed to model the interaction of the model compounds and model surfaces. However, computational modelling with ab initio theories was shown to be unsuitable for quantitative modelling of these interactions. Thus different modelling programs and methods may be needed to successfully model the interaction of model compounds with cellulose or chromium surfaces.