This review analyses and summarises the previous investigations on the oxidation of linseed oil and the self-heating of cotton and other materials impregnated with the oil. It discusses the composition and chemical structure of linseed oil, including its drying properties. The review describes several experimental methods used to test the propensity of the oil to induce spontaneous heating and ignition of lignocellulosic materials soaked with the oil. It covers the thermal ignition of the lignocellulosic substrates impregnated with the oil and it critically evaluates the analytical methods applied to investigate the oxidation reactions of linseed oil. Initiation of radical chains by singlet oxygen ( 1 Δ g ), and their propagation underpin the mechanism of oxidation of linseed oil, leading to the self-heating and formation of volatile organic species and higher molecular weight compounds. The review also discusses the role of metal complexes of cobalt, iron and manganese in catalysing the oxidative drying of linseed oil, summarising some kinetic parameters such as the rate constants of the peroxidation reactions. With respect to fire safety, the classical theory of self-ignition does not account for radical and catalytic reactions and appears to offer limited insights into the autoignition of lignocellulosic materials soaked with linseed oil. New theoretical and numerical treatments of oxidation of such materials need to be developed. The self-ignition induced by linseed oil is predicated on the presence of both a metal catalyst and a lignocellulosic substrate, and the absence of any prior thermal treatment of the oil, which destroys both peroxy radicals and singlet O 2 sensitisers. An overview of peroxyl chemistry included in the article will be useful to those working in areas of fire science, paint drying, indoor air quality, biofuels and lipid oxidation.
Solid phase microextraction (SPME) combined with gas
chromatography–mass spectrometry (GC-MS) served to identify
and quantitate the volatile organic compounds (VOCs) formed in the
degradative oxidation of linseed oil. SPME involved experiments under
nitrogen and oxygen atmosphere, two oil types (raw and boiled), and
two different methods of sampling the head space to differentiate
between the oxidation products and the oil impurities. Further experiments
entailed the oxidation of neat linoleic and linolenic acids to assist
with the development of a detailed reaction mechanism. The majority
of the detected product species originated from the oxidation of linolenic
compounds, which dominate the composition of linseed oil, while 2-propenal,
pentanal, hexanal, 2,4-decadienal, and hexanoic acid, among others,
were released from linoleic compounds. The concentration of hexanal
(7–26), 2-pentenal (25–39), 1-penten-3-ol (3.1–4.8), trans,trans-2,4-heptadienal (33–50), trans,trans-2,4-decadienal (0.7–0.8),
3,5-octadien-2-one (2.5–6.2; relative area), ethanoic acid
(137–195), and hexanoic acid (18–29) increased with
the progress of oxidation; the numbers in the parentheses indicate
initial and final concentration (ppm) of the species in the oil in
6 h experiments.
The pathways of volatile organic compound (VOC) formation
have
been investigated through a computational study, employing the Gaussian
03 suite of programs. We optimized geometries and zero-point vibrational
energies (ZPVEs) at the B3LYP/6-31G(d) level of theory and improved
electronic energies by conducting single-point energy calculations
using the large 6-311++G(3df,3pd) basis set. To describe the predominant
mechanism of the linseed oil oxidation, the following sequence is
proposed: hydrogen abstraction of unsaturated fatty compounds as the
initiation reaction followed by the reaction of allylic-type radicals
with molecular oxygen to form peroxyl radicals and finally intramolecular
rearrangement through four- and five-membered rings. Quantum calculations
identified low-energy pathways following cyclization resulting in
the formation of major products observed, especially aldehydes and
ketones. The overall energy changes taking place through the four-
and five-membered rings were found to be 78 and 93 kJ mol–1 exothermic, respectively. Metal catalysts decompose hydroperoxides
based on the Fenton-like mechanism into alkoxyl and peroxyl
radicals.
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