A model study on fatty acid methyl esters as reactive diluents in thermally cured coil coating systems

Progress in Organic Coatings 55 (2006) 382–387

A model study on fatty acid methyl esters as reactive diluents in thermally cured coil coating systems Katarina Johansson, Mats Johansson ∗ Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received 2 November 2005; received in revised form 19 January 2006; accepted 2 February 2006

Abstract A model study on the transesterification reaction between fatty acid methyl ester (FAME), e.g. methyl oleate, methyl linoleate, rape seed methyl ester and different alcohols in thin films have been performed. The purpose was to evaluate the possibility to use fatty acid methyl ester (FAME) as reactive diluent in thermally cured coil coating paints. A reactive diluent must be compatible, act as a diluent, react into the film without affecting the end properties. The transesterification between the methyl ester and hydroxyl functional model compounds was monitored by 1 H NMR and real time IR. The effects addressed in the present study were compatibility, temperature, catalyst, alcohol structure, and fatty acid methyl ester (FAME) structure. Competing factors with the transesterification reaction were shown to be evaporation and side reactions, i.e. oxidation. The structure of the fatty acid methyl ester (FAME) affects the conversion as a higher amount of unsaturations triggers the competing side reaction oxidation. The reaction time and temperature affects both the degree of transesterification conversion, degree of side reactions and the catalyst choice. The present study has shown that a fatty acid methyl ester (FAME) fulfils the reactivity part for a reactive diluent in a thermally cured coating system. © 2006 Elsevier B.V. All rights reserved. Keywords: Fatty acids; Reactive diluents; Coatings; Thermal curing; Transesterification

1. Introduction Organic coatings are applied to all kind of materials and products to enhance the appearance and protect the surface. The coatings often consist of a skeleton of organic binders, solvent, additives and pigments. There are many different methods for applying coatings depending on the properties of the paint and substrate to be coated, examples of techniques are by brush, large rollers, and air assisted spray. A coated product that has obtained an increased interest and market share during the last decades is pre-coated sheet metal (i.e. coil coatings) [1]. Coil coatings [2] are often thermally cured solventborne coatings with a solvent content of over 50%. The paint is applied to a continuous band of sheet metal in a closed system via large rollers producing a coated sheet metal. The coating is then thermally cured at high speed, around 100–200 m/min, in a high temperature convection oven. A typical curing cycle is ∼35 s in a 300 ◦ C convection

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oven to produce a peak metal temperature (PMT) of 230 ◦ C. The solvent is evaporated and incinerated to recycle the energy and the resin system is cured. Both thermoplastic and thermosetting coatings are used as coil coatings. The rapid curing process at a high temperature puts very special demands on the curing chemistry to obtain a good final product. The coated steel sheet is finally cut and formed to various shapes by the customers. Pre-coated sheet metal is attractive for many end-users since they can omit paint shops and thus improve the efficiency and reduce the production cost. The forming process however puts very high demands on the flexibility and adhesion of the coating. The high content of solvent is costly and involves handling of large amounts of environmental unfriendly volatile organic compounds (VOC). Due to an increased environmental pressure and pending legislations to reduce VOC, a reduction of solvent content is highly desirable [3–5]. Introduction of powder coatings and UV curable systems in coil coating processes have been suggested to reduce the VOC content [2,6,7]. Another way to reduce the use of organic solvents is by introducing reactive diluents as has been proposed for several other coating applications [5,8–11].

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A reactive diluent lowers the viscosity of a coating by serving as a solvent in the liquid paint and then is incorporated into the film while curing. A reactive diluent must have low viscosity and a good compatibility to be able to work as a good diluent. To be able to react into the film matrix it must have the right type and amount of reactive sites. If it has too few reactive sites the reactive diluent will work as a plasticizer instead and if the reactive sites are too many it can be too reactive and causes in-can storage problems. A too low reactivity can retard the drying of the film. The volatility must also be low to prevent evaporation before curing [8,11,12]. This last requirement is especially important for high temperature curing systems such as coil coatings. If a reactive diluent could be derived from a renewable resource it would furthermore have positive environmental impact since it would not only reduce the amount of solvent use but also introduce a renewable material into the final film. Vegetable oils and derivatives thereof have a long tradition in coating applications. They have been used, as produced by the plants, e.g. linseed oil, and as components in resin structures, e.g. alkyds [2,13]. Fatty acid methyl esters (FAME) and derivatives thereof have been proposed as reactive diluents in several different coating systems ranging from oxidative drying systems to cationically cured epoxy resins [9,10,14]. Fatty acid derivates have low viscosity and could be derived to meet the demands of reactive diluents [15–17]. These systems however in most cases employ functional groups situated on the carbon chain, e.g. unsaturations in linseed oil, epoxy groups in epoxidized soy bean oil, or hydroxyl groups in castor oil. A drawback with this is that fatty acid methyl esters commonly consist of mixtures of fatty acids derived from different plants and thus also contain fatty acid methyl esters not having the desired functional group, i.e. mainly mono- or unsaturated fatty acids. These less reactive fatty acid methyl esters are either costly to remove or will remain unreacted in the coating system where they affect the coating properties negatively. One functional group that however is present in all fatty acid methyl esters mixtures is the ester acyl moiety. This group is normally reacted in resin synthesis (e.g. alkyds) to produce coating resins but has not been reported to be reactive in the curing process. The present work aims to describe a model study on the use of different fatty acid methyl esters as reactive diluents using the ester group as reactive group in thermally cured coil coating systems. The present study only aims to simulate the reaction between the polyester resin and the reactive diluent. The polyester is normally crosslinked with a melamine resin. Two different alcohols with different structure have been used to simulate the hydroxyl groups in a hydroxyl functional polyester. Initial studies have identified a series of factors that are crucial in order to obtain a working system. These factors are component compatibility (miscibility), rheological performance, component volatility, functional group reactivity, and side reactions (oxidation). The effects that are addressed in the present study are the effect of compatibility, temperature, catalyst, alcohol structure, and fatty acid methyl ester (FAME) structure.

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2. Experimental 2.1. Materials Rape seed methyl ester (RME), was supplied by Statoil Frescati, Stockholm, and the pure methyl oleate (MeO), bp 218 ◦ C, 20 mmHg, and methyl linoleate (MeL), bp 192 ◦ C, 4 mmHg, were purchased from Sigma–Aldrich. RME is a mixture of 66.5% C18:1 (MeO), 15.4% C18:2 (MeL), 6.2% C16:0, 4.5% C18:3, 5.4% other FAMEs (C16:1, C18:0, C20:0, C20:1, C22:0, C22:1, C24:0, and C24:1) and 2% oligomers and other fatty acid derivatives. The first number in C18:1 denotes the number of carbon atoms in the chain and the second the number of double bonds. 1-Octadecanol (mp 60–61 ◦ C) was purchased from Merck and 2-octyl-1-dodecanol (ODOH) from Sigma–Aldrich. The catalyst p-toluene sulfonic acid (PTSA) was purchased from Sigma–Aldrich and dodecylbenzene sulfonic acid (DDBSA), and dibutyltindilaurate (DBTDL) were obtained from Akzo Nobel Nippon Paint AB, Gamleby, Sweden. The substrate was an unprimed hot dipped galvanized steel sheet (HDG) obtained from SSAB Tunnpl˚at AB, Borl¨ange, Sweden. The substrates were cleaned with acetone and ethanol and left to dry at ambient conditions before use. All chemicals were used as received. 2.2. Techniques 1 H NMR were performed on both a 400 MHz Bruker Aspect NMR and a Bruker Avance 400 MHz using CDCl3 as solvent. FTIR and RTIR spectra were obtained on a Perkin Elmer Spectrum 2000 instrument equipped either with a single reflection ATR accessory (Golden Gate) or a heat controlled ATR from Specac Ltd. (Kent, England). The infrared measurements were performed in reflection mode. Thermal cure of films were performed both in a laboratory oven built to simulate coil coating curing manufactured by Tryckluftsteknik i V¨astervik AB, V¨astervik, Sweden (oven 1), and in a convection T6 function line oven from Heraeus instruments (oven 2).

2.3. Test series The composition of each evaluated formulation is presented in Table 1. The formulations were made with a FAME/alcohol molar ratio of 1:1 and the catalyst amount added was 3 wt.%. Each formulation was cured in both ovens 1 and 2 whereas the experiments will be named 1 for oven 1, and 2 for oven 2. The experiment B2 consist of a RME/octadecanol mixture in functional group ratio 1:1 with 3 wt.% DDBSA cured in oven 2. 2.4. Procedures Real time infrared measurements (RTIR): for the RTIR measurements, the heat controller was set to desired temperature (110, 130, 150, or 170 ◦ C) and the system left to equilibrate for 10 min. Two drops (∼75 ␮l) of sample was then placed on the ATR crystal and the measurements were immediately started. Spectra were recorded every 35 s over a period of 60 min using

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Table 1 The composition of the formulations evaluated in the studies Experiment

FAME

Alcohol

Catalyst

A B C D E F G H I J K L M

RME RME RME RME RME MeO MeO MeO MeO MeL MeL MeL MeL

1-Octadecanol 1-Octadecanol 1-Octadecanol ODOH ODOH 1-Octadecanol 1-Octadecanol ODOH ODOH 1-Octadecanol 1-Octadecanol ODOH ODOH

PTSA DDBSA DBTDL DDBSA DBTDL DDBSA DBTDL DDBSA DBTDL DDBSA DBTDL DDBSA DBTDL

TimeBase® software from Perkin Elmer. The reaction was followed by monitoring the shift of the carbonyl peak from 1744 to 1740 cm−1 . 2.4.1. Reaction in thin film For samples B–C, F–G, and J–K containing 1-octadecanol (mp 60–61 ◦ C) the substrates were pre-heated to 68 ◦ C in order to avoid crystallization. Films reacted in oven 1 were applied with a thickness of 60 ␮m wet film on cleaned and dried substrates. The films were reacted at 330 ◦ C for 55 and 50 s, respectively, for the ambient and pre-heated substrate until the PMT reached approximately 230 ◦ C as determined with temperatureindicators attached to the steel substrate. The samples were then air-cooled at ambient conditions. 1 H NMR samples were taken from the reacted films. Films reacted in oven 2 were also applied with a 60 ␮m applicator on cleaned and dried substrates and then reacted for up to 60 min at, respectively, 110, 130, and 150 ◦ C. Approximately 10 1 H NMR samples were taken at regular interval during the reaction period. 2.4.2. Reaction in bulk Thermal reactions in bulk in inert atmosphere were also conducted for comparison. Mixtures of 0.5 g of formulations B–M were prepared in a round bottled flask placed and left under argon (Ar) flow for about 30 min while stirring with a magnetic stirrer. The temperature was set to 68 ◦ C for samples B–C, F–G, and J–K containing 1-octadecanol (mp 60–61 ◦ C) and room temperature for the others. The temperature of all samples was then raised to 130 ◦ C and the catalyst was added. The system was left to react for 15 min under Ar flow. The mixtures were then analyzed with 1 H NMR. 3. Result and discussion The coil coating process is a rapid and continuous process with high demands on the coating system. The coating is cured at high speed at high temperature demanding that the cure must be fast and controlled. Added reactive diluent also must be able to react during the seconds it takes to pass the oven. Solvents and other volatiles must also evaporate at a controlled rate to avoid

defects. The compatibility between the diluent and the other components is also an important factor since this will effect both the film formation and the chemical reactions. Fatty acid methyl esters have previously been shown to exhibit a low enough viscosity to make them suitable as diluents [17]. The boiling point is also high which indicates that they can be used in high temperature curing systems although some amount of evaporation can be foreseen. 3.1. Compatibility The components must be compatible throughout the entire cure to obtain good films. If not, they will phase separate making it impossible to obtain a homogenous film which inhibits the transesterification reaction. Vegetable oil derivatives are very un-polar molecules that demand similar un-polar resins and catalysts to be able to obtain good miscibility. In the present work, fatty acid methyl esters derived from vegetable oils have been chosen as reactive diluent since they have a fair compatibility with a polyester/melamine resin system. The compatibility issue could be seen when trying to make mixture A when using PTSA as catalyst. The solubility of PTSA was limited in this case due to the polarity difference between the catalyst and monomer mixture. By changing to more un-polar catalysts like DDBSA and DBTDL (mixtures B and C), the miscibility problem was solved and homogenous mixtures were obtained. 3.2. Transesterification studies 3.2.1. Experimental techniques The reactive diluent FAME is proposed to transesterify with the hydroxyl groups in the polyester resin in a coil coating system. The stoichiometry between the hydroxyl groups in the polyester resin and the crosslinker (usually melamine) as well as the functionality of the polyester must of course be adjusted to allow the melamine crosslinking to occur. The relative reaction rate between the crosslinking reaction (polyester/melamine) and the transesterification reaction (polyester/FAME) is not evaluated in the present work but will be described in future papers. The possibility to cure a full system has been previously described but without full details on this reaction rate comparison [18]. In the performed model studies the polyester has been replaced with monoalcohols to make the evaluation of the reaction kinetics easier. The reactions were studied both with RTIR (in situ) and NMR (samples collected after different time periods). RTIR measurements show that the carbonyl peak (1744 cm−1 ) shift to lower wavelengths (1740 cm−1 ) as the methyl esters form 1-octadecyl and 2-octyl-dodecyl esters, respectively, see Fig. 1. 1 H NMR spectra, Fig. 2, confirm the transesterification reaction as a new peak at 4 ppm (–CH2 CH2 OCO–) from the new ester appears and the integrals from the reactants at 3.6 ppm (FAME CH3 OCO–), 3.6 ppm (octadecanol –CH2 CH2 OH), and 3.5 ppm (ODOH–CHCH2 OH) decreases. The NMR data can furthermore be used to estimate the amount of oxidation that occurs as a side reaction. NMR spectra show that the FAMEs used in the study oxidize as the integrals from the unsaturations

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Table 2 Conversion for samples B–M reacted in bulk and in oven 2 at 130 ◦ C Sample no.

Bulk (%)

Thin film reacted in oven 2 (%)

B C D E F G H I J K L M

81 55 89 50 79 88 91 89 79 80 84 75

73 38 62 44 56 52 80 60 52 36 13 16

Fig. 1. RTIR shift of the carbonyl peak at approximately 1740 cm−1 during 60 min for mixture J at130 ◦ C.

at 5.3 ppm (–CH CH–) and 2.7 ppm (–CH CHCH2 CH CH–) decrease when the reaction is performed in air. The oxidation reaction process is very complex and numerous reactions occur simultaneously making it difficult to get an exact value of the amount of oxidation [19,20]. The decrease of the signal at 5.3 ppm relative to the protons on the carbon beside the acyl group at 2.3 ppm has in the present study been used as a relative measure of the oxidation although this only describes a few parts of the overall oxidation process. The oxidation is considered as a competing side reaction with the transesterification since an oxidized FAME to some extent is cleaved in smaller molecules that evaporate [21]. The bulk polymerizations of mixtures B–M performed under inert conditions confirm that this is true as the transesterification conversion for these samples are higher in all cases compared to the corresponding mixtures reacted as films in air, Table 2. The results in this comparison furthermore support the conclusions drawn in Sections 3.2.3–3.2.5 concerning the effect of choice of catalyst, FAME and alcohol structure. 3.2.2. Effect of temperature The steel surface temperature reaches 230 ◦ C in a very short time period in the coil coating process. This makes the polymerization kinetics very difficult to study under real conditions. Therefore, parts of this study have been performed at the lower reaction temperatures, 130, 150, and 170 ◦ C to follow the reac-

Fig. 2. 1 H NMR spectra mixture F2 after 30 min at 130 ◦ C.

tion in complement to systems cured at real conditions. The transesterification reaction was followed with RTIR at these low temperatures as mentioned in previous section. The normalized absorbance of fully shifted carbonyl peak (1740 cm−1 ) for mixture F is plotted against time in Fig. 3. The diagram shows that time to fully shifted carbonyl peak decreases with increased temperature, i.e. the reaction proceeds faster at higher temperature. For these low temperatures, is the reaction time to full conversion (∼5 min at 170 ◦ C) very long compared to the reaction time in the industrial process (∼37 s), however by extrapolating the curves to the industrially obtained peak metal temperature of ∼230 ◦ C it is clearly seen that full conversion could be obtained within the desired time frame. It also should be noted, that evaporation of the monomers could be detected at 170 ◦ C. 3.2.3. Effect of FAME structure A reactive diluent should work as a diluent and must have a low enough viscosity. For a specific FAME to meet that demand it must contain one or several unsaturations, otherwise it will crystallize at ambient conditions. The three evaluated FAMEs has, respectively, one (MeO), two (MeL) and a mixture of one and two unsaturations (RME). The transesterification conversion increases with decreased amount of unsaturation in the different FAME, i.e. MeO in F–I has the highest conversion and MeL in

Fig. 3. Normalized absorbance for fully shifted carbonyl peak vs. time for mixture F at 130, 150, and 170 ◦ C measured with RTIR.

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RME mixture contains small fraction volatiles, e.g. methyl laurate, affecting the measurements. The absolute values should therefore be considered having an accuracy of ±10% in this case. The slope should however be much more accurate after the initial 5 min of reaction.

Fig. 4. Conversion vs. reaction time for mixtures B2, D2, and F2 at 110 ◦ C.

Fig. 5. Degree of oxidation vs. reaction time for mixtures B2, D2, and F2 at 110 ◦ C.

J–M the lowest. This is due to the higher degree of oxidation of compounds with two unsaturations and the earlier mentioned fact that the oxidation is a competing factor with the transestrification. It should be noted that the oxidation reaction depends on the presence of oxygen, temperature, and reaction time. The data presented in Figs. 4 and 5 show data obtained at much lower temperatures and longer reaction times compared to the reaction conditions in the coil coating process. The reason for the B2 curve in Fig. 5 starting with negative values is that the

3.2.4. Effect of catalyst The choice of catalyst has been shown to be an important factor and also difficult to evaluate since the catalytic effect change due to the cure conditions. The increase in rate with temperature greatly differs between the catalysts. The industrial curing condition in high temperature convection ovens (oven 1) creates a certain heating rate for the coating. The overall effectivity of the different catalysts depends on the catalytic activity at different temperature and subsequently on the temperature ramping profile of the coating. The heating ramp depends on the air flow in the oven, the substrate thickness, and oven temperature which furthermore also affect the evaporation. The efficiency of the two catalysts studied are similar for MeO (F2–J2) between 110 and 150 ◦ C. DDBSA is a more efficient catalyst for RME (B2 and D2) and MeL (J2 and L2) compared to DBTDL (C2, E2, K2, and M2) within this temperature range. Fig. 6 shows conversion versus time for mixture B2 (left diagram) and C2 (right diagram), respectively, at 110, 130, and 150 ◦ C. It is seen comparing these figures that DDBSA is a more efficient catalyst compared to DBTDL within this temperature. This can be related to different degree of oxidation which is higher for mixture C2 than B2 (data not shown). The catalytic efficiency is oppositely greater for DBTDL than DDBSA for the samples cured in oven 1 (B1–M1). One explanation for this is that the temperature dependence can differ greatly for the different catalysts [22]. An increase in temperature may result in a larger rate increase for DBTDL than DDBSA, i.e. the Arrhenius factor and activation energy differ. Another explanation could be the fact that the cure time is so short (50–55 s) not allowing much oxidation to occur. This should mean that the oxidation has less impact on the transesterification at industrial curing conditions. The different results obtained at different curing conditions clearly show that a proper choice of catalyst system is crucial to obtain a maximum conversion.

Fig. 6. Conversion vs. time on the left side for mixture B2 (DDBSA catalyst) and on the right side for mixture C2 (DBTDL catalyst) at, respectively, 110, 130 and 150 ◦ C.

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the same extent as the more unsaturated structures. Choice of transesterification catalyst can matter for the oxidation level and consequently for the transesterification conversion. Heating rates and final curing temperature has a large impact on the reactivity for different catalyst. Efficient catalysts at low temperatures have shown not to be as efficient at high temperatures. Acknowledgements

Fig. 7. Conversion vs. time for B2, D2, F2, H2, J2, and L2 at 130 ◦ C.

3.2.5. Effect of alcohol structure Different alcohols are often used as monomers in polyester resins for coil coating depending on the property demands of the final product, i.e. is neopentyl glycol used to obtain better hydrolytic stability compared to primary alcohols. In the present study, a neopentyl-like alcohol and a primary alcohol have been evaluated as model monomers. The transesterification rate is in the same range for the both alcohols when reacted with RME (B2–E2) and MeO (F2–I2). ODOH however reacts significantly slower with MeL, L2–M2 compared to J2–K2 as seen in Fig. 7. A significantly higher amount of oxidation is also seen for systems L2–M2. This indicates that also the alcohol is susceptible to oxidation due to the presence of tertiary hydrogens which can be abstracted in an autooxidation process. This should however not be a problem in conventional resin systems since the corresponding diol, neopentylalcohol, does not have any tertiary hydrogens. The results do however show that great care must be taken to not introduce monomers that are easily oxidized when using linoleate esters as diluents. 4. Conclusions The results show that FAME can be used as a reactive diluent in a thermally cured coil coating system. The evaluated RME works as a diluent lowering the viscosity and are transesterified with hydroxyl functional groups in the polyester at a high enough rate. Model studies of transesterification reactivity between different FAMEs and alcohols have confirmed that the transesterification reaction takes place at the reaction rate needed in the intended application. Both evaporation and oxidation of FAME is competing with the transesterification reaction. Component compatibility has shown to be one of the most important factors to obtain a working system. The FAME structure is also of importance as the transesterification conversion is greater with more saturated FAME structures that are not oxidized to

The Swedish Agency for Innovation Systems (VINNOVA), Grant P23943-1 A; SSAB Tunnpl˚at AB; Svenska Lantm¨annen; and Akzo Nobel Nippon Paint AB are acknowledged for financial support. Per-Erik Sundell and Tina Bergman at SSAB Tunnpl˚at AB; Martin Svensson at Svenska Lantm¨annen; and Tomas Deltin and Glenn Svensson at Akzo Nobel Nippon Paint AB are thanked for valuable discussions. References [1] A.H. Tullo, C&EN 82 (2004) 25. [2] Z.W. Wicks Jr., F.N. Jones, P.S. Pappas, Organic Coatings: Science and Technology, 2nd ed., Wiley, New York, US, 1999. [3] E.D. Casebolt, B.E. Mote, D.L. Trumbo, Prog. Org. Coat. 44 (2002) 147. [4] M. de Meijer, Prog. Org. Coat. 43 (2001) 217. [5] J. Lindeboom, Prog. Org. Coat. 34 (1998) 147. [6] T.A. Misev, R. van der Linde, Prog. Org. Coat. 34 (1998) 160. [7] G. Toulemonde, G. Clausen, S. Vigneron, Proceedings of the Second International Symposium on Characterization and Control of Odours and VOC in the Process Industries, Louvain-la-Neuve, Belgium, 1993, p. 239. [8] W.J. Muizebelt, J.C. Hubert, M.W.F. Nielen, et al., Prog. Org. Coat. 40 (2000) 121. [9] P. Muturi, D. Wang, S. Dirlikov, Prog. Org. Coat. 25 (1994) 85. [10] C. Stenberg, M. Svensson, E. Wallstr¨om, M. Johansson, Surf. Coat. Int. Part B: Coat. Trans. 88 (2005) 1. [11] K.H. Zabel, R.P. Klaasen, W.J. Muizebelt, et al., Prog. Org. Coat. 35 (1999) 255. [12] J. Samuelsson, Ph.D. Thesis, Department of Polymer Technology, Royal Institute of Technology, Stockholm, Sweden, 2004. [13] C.H. Hare, Protective Coating; Fundamentals of Chemistry and Composition, SSPC: The Society for Protective Coatings, Pittsburgh, US, 1998. [14] J. Samuelsson, P.-E. Sundell, M. Johansson, Prog. Org. Coat. 50 (2004) 193. [15] G.H. Hutchinson, Surf. Coat. Int. 85 (2002) 1. [16] J.T.P. Derksen, F.P. Cuperus, P. Kolster, Ind. Crops Prod. 3 (1995) 225. [17] J. Samuelsson, M. Johansson, J. Am. Oil Chem. Soc. 78 (2001) 1191. [18] M.K.G. Johansson, M. Svensson, P.-E. Sundell, Svenska Lantm¨annen Ek. F¨or., Swed., WO Patent WO2005/052070 (2005). [19] W.J. Muizebelt, M.W.F. Nielen, J. Mass Spectrom. 31 (1996) 545. [20] Z.O. Oyman, Ph.D. Thesis, Materials and Interface Chemistry, Eindhoven University of Technology, Eindhoven, Netherlands, 2005. [21] C. Stenberg, M. Svensson, M. Johansson, Ind. Crops Prod. 21 (2005) 263. [22] P. Sykes, A Guidebook to Mechanism in Organic Chemistry, 5th ed., Longman Groups Ltd., New York, US, 1981.