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Fixation chemistry of amine-copper preservatives Jiang, Xiao

Abstract

The principal objective of this thesis was to study the fixation mechanism of amine-copper preservatives, with an additional focus on the active component HDO (N-cyclohexyl- diazeniumdioxide) in Wolman CX™, to determine its influences on the fixation of the copper. The degree of fixation was assessed first by a leaching study. The depletion of copper and amine from copper-amine and copper-HDO amine treated Scots pine (Pinus sylvestris) blocks was investigated. For copper-amine treated blocks, the greatest leaching resistance was found for copper-monoethanolamine and copper-polyimine treatments, with more than 80% (water leaching) and 50% (buffer leaching) copper retained. Copper ethylenediamine showed the greatest leachability. The inclusion of HDO greatly improved the leaching resistance of the treated blocks to citrate buffer solution, with the copper-HDO diamine system having the highest leaching resistance. The formation of very stable Cu(HDO)₂ (bis-(N-cyclohexyldiazeniumdioxy)-copper) precipitate accounted for the enhanced leachability. It was found that about 25-40% of the amine reacted with wood irreversibly and that this was independent of the amine-copper reaction. The residual amine to copper mole ratio retained confirmed the formation of a copper-amine complex in wood. However, whether copper is present as copper(amine).wood or copper(amine)₂ wood could not be determined from the leaching study. The fixation reactions which took place between amine, copper-amine, copper- HDO amine and wood were explored using Fourier Transformed Infrared Spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR). The FTIR spectra revealed that the carboxylic acid and phenolic hydroxyl groups in wood react with both amine solvent and copper to form stable complexes. The FTIR spectra suggested that amine acid salts could be formed and this was supported by the XPS, which also identified that a second amine reaction occurred with phenolic hydroxyl groups in wood. The type of amine copper complexes formed in wood were identified by XPS and ESR. It was found that in copper-amine treated wood, Cu(amine)₂ wood was the dominant copper complex. However, in copper-HDO amine treated wood, copper was preferentially formed as Cu(HDO)₂ precipitate. The fixation mechanism of copper and amine in wood was also studied individually through the reactions of vanillin (a lignin model compound), organosolv lignin, Scots pine holocellulose, pectin and extractives with copper amine and copper- HDO amine solution. The reddish copper complex formed from the reaction of vanillin and copper ethylenediamine sulphate was identified by FTIR, ESR and X-ray single crystallography as Cu(vanillin)₂(H₂0)₂. In this complex, the methoxyl and water oxygen atoms were coordinated to copper, together with the phenolic oxygen atoms forming a six member coordinated complex. This study suggested that a similar reaction between ethylenediamine copper solution and lignin in wood could take place to form a stable lignin-copper complex without nitrogen present. However, based on the leaching study, XPS and ESR spectra, such reaction would be considered to be a minor reaction in the fixation mechanism of copper amine treated wood. The reactions of copper-amine or copper-HDO amine with organosolv lignin, holocellulose, and extractives confirmed that these three wood components took an important role in the fixation of copper and amine in wood. The carboxylic acid group in hemicellulose, and the phenolic hydroxyl group in lignin and extractives were the major reactive sites for copper and amine. The distribution and location of copper in copper amine and copper-HDO amine treated wood was studied using light visible microscopy and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX). From the light microscopic study of thin sections of southern pine (Pinus sp.) and Douglas-fir (Pseudotsuga menziesii), it was found that copper was concentrated in the highly lignified cell corners and compound middle lamella, due to the copper-lignin fixation reaction. For copper-HDO amine treatment, copper was principally observed to deposit on the surface of the cell walls. This implied the preferential formation of a copper precipitate in wood. The observation of crystalline copper solids in ray cells and pits in CuHDO-en treated samples suggested by SEM-EDX the formation of Cu(HDO)₂ precipitate in wood. No such deposits were found in copper-amine treated samples.

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