- Title
- Low temperature oxidation of linseed oil
- Creator
- Juita,
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2011
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis presents a series of investigations into the complex phenomena of low temperature oxidation of linseed (flaxseed) oil designed to explain the oil’s potential to engender spontaneous ignition. Varieties of linseed oil commercially available have found numerous applications, especially, in painting and in wood treatment. The addition of transition metal dryers to the oil is a well established practice as it improves the drying rate of the paint. However, the presence of metal salts often induces self-heating and spontaneous ignition, when linseed oil is applied to lignocellulosic materials, such as cotton or timber. This phenomenon is also responsible for the emission of volatile organic compounds (VOC), causing health problems to building occupants. The thesis studies the effect of transition metal salts on the rate of oxidation and self-heating of linseed oil, examines the gaseous products and VOC, develops the reaction mechanisms through the quantum chemical calculations and models important intermediates arising in the oxidation process. We constructed two experimental rigs: (i) a flow reactor for the measurement of gaseous oxidation products and for performing the ignition experiments; and, (ii) a batch reactor system for the determination of peroxide and unsaturation values, and headspace analysis of VOC. Fourier transform infrared (FTIR) and gas chromatographic – mass spectrometric (GC-MS) analyses indicated the presence of carbon oxides, ethane, ethylene, acetaldehyde, propionaldehyde, acrolein, crotonaldehyde and formic acid in the gaseous products. VOC products comprised aldehydes, ketones, alcohols, carboxylic acids and furans such as hexanal, 2-pentenal, 1-penten-3-ol, 2,4-heptadienal, 2,4-decadienal, 3,5-octadien-2-one, ethanoic acid, hexanoic acid, 2-pentyl-furan. The majority of the species arose from the oxidation of linolenic glycerides, the most abundant esters of linolenic acid and glycerol, present at approximately 55 % in linseed oil, while other species such as 2-propenal, pentanal, hexanal, 2,4-decadienal and hexanoic acid formed from linoleic compounds. Moreover, we detected cross-linking or polymerised products in the oil following the oxidation process. This phenomenon is exploited in practice in paint drying. Salts of cobalt(II) are the most effective catalysts among other transition metal salts, followed by manganese(II) and iron(II), indicated by the highest emission of oxidation products. Cobalt displays the highest activity for the decomposition of peroxides that form during oxidation of linseed oil, into reactive radicals, leading to the exponential increase in the oxidation rate. This is reflected in cobalt’s being frequently implicated in ignition of lignocellulosic materials impregnated with the oil. Nitrates augment the rate of decomposition of peroxides, as oppose to chlorides, which slow it down. As expected, higher temperatures increased the rate of peroxide decomposition. The overall activation energy of peroxide formation corresponds to 71 ± 1 kJ mol⁻¹. FTIR analysis of oil film demonstrated the progressive decrease in the concentration of cis non-conjugated double bonds, formation of trans conjugated double bonds, appearance of hydroxyl groups and the broadening of the carbonyl peak in the oxidation process, suggesting the consumption of unsaturated compounds in the oil and the formation of alcohols, aldehydes, ketones, carboxylic acids and esters. The overall rate of disappearance of double bonds follows first order kinetics with a rate constant of 0.030 ± 0.007 h⁻¹ at 80 °C. Boiled linseed oil is the most reactive oil studied, followed by raw and refined linseed oils, with stand linseed oil as the least reactive. A very high reactivity of boiled linseed oil arises as a consequence of the addition of cobalt dryers, with the presence of cobalt confirmed by means of the inductively coupled plasma – optical emission spectroscopic (ICP-OES) analysis. Experiments with linseed oil applied to reactive (cotton) and unreactive (glass wool) supports have demonstrated the role of linseed oil in inducing the self heating of the reactive supports. Much more gaseous products formed in experiments with reactive than with unreactive supports. The pathways of the oxidation of linseed oil comprise hydrogen abstraction from species of unsaturated fatty acids as the initiation reaction, followed by the interaction of allylic-type radicals with molecular oxygen to form peroxyl radicals, cyclisation reactions proceeding by intramolecular rearrangement through four and five-membered rings, and finally bond scission leading to the product formation. Quantum chemical calculations identified low energy cyclisation pathways resulting in the formation of major products, especially aldehydes and ketones. We calculated the overall exothermicity of the decomposition of the four and five membered rings as 78 and 93 kJ mol⁻¹, respectively. The results of this thesis will find practical applications in formulating more accurate models of autoignition of lignocellulosic substrates impregnated with oils, the latter consisting of glycerol esters of carboxylic acids containing cis non-conjugated double bonds, such as fish oil and a number of vegetable oils with high linoleic content. The detailed measurements of the evolving VOC provide data for quantitative risk assessment of drying of paints based on linseed oil. The study on the catalytic effect of transition metal salts could be useful in applications ranging from catalysis to safety, considering that transition metals are common catalysts and the presence of transition metals can make chemical systems prone to run-away reactions. Finally, the knowledge of gaseous organic species produced in early stages of the self-heating process may be deployed in developing detection systems for warehouses storing large quantities of reactive oils.
- Subject
- linseed oil; oxidation
- Identifier
- http://hdl.handle.net/1959.13/928210
- Identifier
- uon:10360
- Rights
- Copyright 2011 Juita
- Language
- eng
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