Low temperature oxidation of linseed oil: a review

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 (Δg), 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 O2 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. Introduction Since the 15 century, linseed oil has been extensively used in varnishes and oil-based house paints (Lazzari & Chiantore 1999). It has also been applied for treating wood and in manufacturing of linoleum, a floor covering made from mixture of natural materials, such as wood, calcium, vegetable pigments and resin. Nowadays, it is also utilised in industrial lubricants, for the treatment of leather products, for bicycle maintenance, as well as rust inhibitor. Many studies have focussed on improving the drying performance of this oil and reducing the hazardous properties related to its application. A herbaceous plant, Linum usitatissimum, linseed (also called flax) produces seeds which are oval and flattened in shape, 4–6 cm long, pale to dark brown and shiny. The oil prepared by crushing the seeds finds applications in formulating the so-called drying alkyd * Correspondence: [email protected] Process Safety and Environmental Protection Group, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia Full list of author information is available at the end of the article © 2012 Juita et al.; licensee Springer. This is an Attribution License (http://creativecommons.or in any medium, provided the original work is p paints, which exhibit drying and hardening properties when exposed to air. This occurs as a consequence of high content of glycerol esters (also known as glycerides or triacylglycerols) of linolenic acid in linseed oil, an important component of the drying alkyd paints, with the unsaturated bonds in the acids undergoing the oxidation reactions. The non-drying alkyd resins are devoid of esters of unsaturated fatty acid and display no curing behaviour. The properties of drying alkyd paints vary depending on the type and amount of unsaturated fatty acid employed in preparing the paint formulations (Oyman et al. 2005a). The primary remaining components of both the drying and non-drying alkyd resins include alcohols (polyols), such as pentaerythritol or glycerol, and dicarboxylic acid anhydrides, such as phthalic and maleic anhydrides. The term alkyd derives from the original acronym alcid, conveying the chemical meaning of polyesters (van Gorkum & Bouwman 2005). There are four varieties of linseed oil sold in the market, including raw, boiled, stand and refined linseed oils. Raw Open Access article distributed under the terms of the Creative Commons g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction roperly cited. Juita et al. Fire Science Reviews 2012, 1:3 Page 2 of 36 http://www.firesciencereviews.com/content/1/1/3 linseed oil refers to pure oil with no additional treatment and with no additives, while boiled linseed oil is produced by adding a mixture of hydrocarbon solvents and metallic dryers to speed its drying time. Boiled oil is a well known trade name, even though the process does not involve boiling of raw oil, whereas the stand linseed oil is processed by heating the oil to about 300°C, over a few days in the absence of oxygen. During this process, polymerisation reaction occurs, increasing the oil’s viscosity. It means that stand or polymerised oil has been boiled to make it unreactive and more viscous. The production of refined linseed oil involves the alkali treatment following the pressing process, improving the colour of this oil (paler and clearer). It is utilised as a medium to increase gloss and transparency of paint colours. The drying rate of linseed oil is too slow for convenient applications, necessitating the addition of metallic salts (drying agents) to accelerate the drying process (Mallégol et al. 2000). Unfortunately, in the presence of lignocellulosic fuels, such as cotton fibres, these agents may induce the fuel’s self-heating and autoignition. This dangerous side effect of the oil has been known to fire investigators for almost 200 years (Abraham 1996). Several cases of fire have been reported; in particular those induced by improperly disposed rags soaked with linseed oil. Typical ignition scenarios involve waste baskets filled with disposed cotton rags used to clean paint brushes. Two recent cases of fire ignited by oily rags in California and Illinois, have been reported, one case occurred in the plant section where wooden cabinets have just been stained and finished, while the other was caused by a pile of oily rags in the storage area which had been used to treat the refinish deck. There were no injuries in either case, however they suffered estimated loss of $200,000 and $2,000, respectively. This substantial difference in losses resulted from the operation of sprinklers, in the case of the fire in Illinois (Evarts 2011). The US Fire Administration’s National Fire Incident Reporting System (NFIRS) and National Fire Protection Association (NFPA), which provide the average annual data of fires for 2005–2009, give account that the spontaneous heating of oily rags comprised 22% of fires ignited by spontaneous heating or chemical reaction. In 14,070 cases of fires caused by spontaneous heating or chemical reaction, there were 7 civilian deaths and 92 civilian injuries reported, with corresponding direct property damage of $143 millions (Evarts 2011). This review summarises the previous research studies dealing with oxidation of linseed oil itself and spontaneous ignition of lignocellulosic materials wetted with linseed oil. The review commences with the discussion of linseed oil composition, structure and oxidation characteristics. Several test methods for examining the oxidation and self-heating processes are then described, with application to the considered material. This is followed by illustrations of the effect of several transitional metals on the oxidation process. Subsequently, we discuss the chemistry involved in the oxidation reaction and the reaction pathways suggested in the literature, as well as the reported kinetic parameters. Finally, this review identifies the gaps in knowledge and proposes further investigation to gain improved fundamental understanding of the oxidation of linseed oil. Characteristics of linseed oil Composition and structure of linseed oil As linseed is grown in several geographical areas, including Europe, North and South America (especially Argentina) and Asia (especially India), the linseed oil pressed from the seeds displays natural variation in composition that reflects the growing, agronomic and environmental conditions (Gunstone 1996). In particular, climate affects the abundance of the unsaturated fatty acid in the oil; the colder the climate, the higher the iodine value of oil or the degree of its unsaturation (Fjällström et al. 2002). Iodine value is normally expressed in terms of grams of iodine added per 100 grams of oil. The oil consists almost exclusively of the esters of glycerol (C3 alcohol with three hydroxyl groups, one on each carbon atom) and five fatty acids, two of them saturated, C16 palmitic and C18 stearic, and three unsaturated, oleic, linoleic and linolenic, exhibiting one, two and three double bonds, respectively, as illustrated in Figure 1. The two and three double bonds are non-conjugated, being separated from each other by CH2 groups. The minor components that may also be present are monoacylglycerols (monoesters of glycerol), diacylglycerols (diesters of glycerol) and free fatty acids (Gunstone 1996). Although linseed oil contains around 60% of linolenic acid, the most unsaturated fatty acid, this acid occurs only in small amounts, usually below 1% in other oil types. The notable exceptions are soybean and rape oil, with 8 and 7% linolenic content, respectively (Gunstone 1996). This high level of linolenic acid in linseed oil affects the drying property of the oil, making it particularly suitable in formulations of drying alkyd paints. Table 1 summarises the composition of various natural oil; the symbols 16:0, 18:0, 18:1, 18:2 and 18:3 reflect the number of carbon atoms and (after the colon) the number of double bonds. They correspond to palmitic, stearic, oleic, linoleic and linolenic acids, respectively. The natural unsaturated fatty acids exhibit one or more carbon double bonds with cis configuration starting at the location of the ninth carbon atom (Roberfroid & Calderon 1995). Carbon-carbon double bonds in unsaturated fatty acids induce the chain to deviate by an angle of about 40°, requiring more space to accommodate the chain conformation (Jennings 1981). The chemical