Making wood durable. A sustainable approachwith linseed oil

Abstract: Linseed oil has been and is used for vast number of applications, such as in food and paint industry, and wood preservation. It is a good environmental choice, as it originates from renewable sources. Linseed oil is mainly a mixture of triglyceride of fatty acids, both saturated and mono- or polyunsaturated, which allows the oil to oxidize. The oxidation occurs via an auto-oxidation mechanism with the carbon-carbon double bonds and oxygen from the air, reacts to form a polymer. Herein, four different linseed oils (three commercial ones and one industrially available) were analyzed to obtain a better understanding of why different oils provide different protection of wooden materials. This was done by a study of the unoxidized oil, followed by an oxidation time-resolved study of oxidized oil films. The analysis was done by nuclear magnetic resonance, gas chromatography - mass spectrometry and/or inductively coupled plasma atomic emission spectroscopy. This study provided the fatty acid profile of the oils, which were similar for all oils. The unoxidized oils contained some metals ions which probably originate from additives. Aluminum, cobalt, iron, manganese, and zinc was detected in some of the oils at concentrations up to 135 mg/L, but only manganese was detected in all oils and its concentration was much higher than all other metals together. The time-resolved oxidation study had some problems with the solubility of the formed polymers. Several solvents were examined, such as dimethylsulfoxide, alkaline alcohol solutions and toluene, before chloroform-d was chosen as solvent. Though, chloroform-d was not a perfect solvent; it was capable to solve a fraction of the sample, but the fraction decreased with oxidation time. After fifteen days of oxidation, only a few percent of the sample could be dissolved, but for short oxidation times (<48 h) the majority of the samples were dissolved. The oils were analyzed after thirteen different oxidation times. Some structural changes appeared, for example loss of unsaturated protons and some oxidation products arose, such as peroxides and aldyhydes. The diffusion coefficient decreased over the first 3-4 days of oxidation, as expected when the polymerization progressed. After a week of oxidation, the diffusion coefficient increased again, this could possibly be explained by the solubility problem for the large polymer formed. Contrary, at shorter oxidation times this method probably could still be used, as the majority of the sample was dissolved. However, the solubility problem made it impossible to conclude anything about the oxidation rate at longer oxidation times and thus prevented any ranking of the oils.