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Waterborne polyurethane wood coatings based on rapeseed fatty acid methyl esters
Progress in Organic Coatings 74 (2012) 705–711
Contents lists available at SciVerse ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Waterborne polyurethane wood coatings based on rapeseed fatty acid methyl esters C. Philipp ∗ , S. Eschig Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut (WKI), Bienroder Weg 54 E, 38108 Braunschweig, Germany
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Article history: Received 27 June 2011 Received in revised form 1 September 2011 Accepted 21 September 2011 Available online 15 December 2011 Keywords: Rapeseed oil Fatty acid methyl ester Polyurethane dispersion Dispersion stability
a b s t r a c t Trends in wood coatings are driven to waterborne systems and to renewable resources. Vegetable oils are well known for wood coatings, e.g. alkyds or polyurethane dispersions. In this context, fatty acid methyl esters turn out to be an alternative to technical fatty acids and vegetable oils. High contents of hydrophobic oil-based monomers require a sufficient understanding of the dispersion stability. This study shows the influence of hydrophobic monomers, ionic centers, degree of neutralization, and stirring procedure to the particle size distribution and dispersion stability. Furthermore, the impact of these parameters on the resulting coating film properties was investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The future of wood coatings will be strongly influenced by environmental legislation and economic burdens. The economic implications for research and development priorities are primarily driven by rising raw material costs. With regard to ecological aspects, environmental legislation and a more quality and health oriented consumer, a clear focus will be on waterborne and UV curable coatings [1,2]. In addition, renewable resources will start to replace petrochemical products to increase sustainability in wood coatings. However, renewable resources have been used for coating resins for a long time. In former times, the focus was mainly on vegetable oils (e.g. alkyd resins) and cellulose derivatives (e.g. nitrocellulose lacquer). But the variety of natural resources is manifold [3,4]. Apparently, the focus is still on vegetable oils and fatty acids, as they are available in large amounts and provide suitable chemical functional groups. Their main technical advantages are high hydrophobicity and good chemical resistance, especially black heel resistance [5]. Fig. 1 shows the synthesis route and the potential steps to include vegetable oils and fatty acids in polyurethanes. Fatty acids can be part of the polyester polyols whereas hydroxyl-functional vegetable oils are directly used in polyurethanes [6,7]. Beside fatty acids, fatty acid methyl esters turn out to be an alternative. Two reasons are responsible for this: First, in contrast to fatty acids, the conversion from vegetable oil to fatty acid methyl ester requires no distillation and the product
∗ Corresponding author. E-mail address: [email protected] (C. Philipp). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.09.028
is easier to separate from the side product glycerol, because of no building of soaps. These aspects may make the production less cost-intensive. And secondly, during the polycondensation process, fatty acid methyl esters result in better leaving groups (methanol) with lower boiling point than the leaving group of fatty acids (water), which can shorten the reaction time of the polycondensation. In addition, the biodiesel or fatty acid production lead to raw glycerol as side-product which can be biotechnologically converted to 1,3-propane diol. 1,3-Propane diol is another biobased building-block for polycondensation and polyadditions which leads to a good compromise of hardness and elasticity for the coating film and is expected to replace neopentyl glycol and 1,6-hexane diol partially [8,9]. Beside the coating film properties, the dispersion stability is important. By adding high amounts of extremely hydrophobic monomers (like vegetable oils, fatty acids or fatty acid methyl esters) in the synthesis, the hydrophobicity of the polymer increases. To overcome this incompatibility with the dispersant water, ionic centers are included in the polyurethane chain [10]. The stability of the dispersion increases but the chemical resistance of the coating film declines. The dispersion stability can also be influenced by the degree of neutralization and the stirring procedures. Particle size distribution is a useful precursor to predict dispersion stability. 2. Materials and methods Fatty acid (FA) was provided by Emery Oleochemicals GmbH (Loxstedt, Germany), fatty acid methyl ester (FAME) by Rheinische Bioester GmbH (Neuss, Germany). Both FA and FAME are
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measurements were performed with a Zetasizer Nano ZS instrument by Malvern (Herrenberg, Germany). Pendulum hardness was determined by the method according to König (DIN 53157) and gloss measurement by a glossmeter (micro-TRI-gloss) from BYKGardner GmbH (Geretsried, Germany) according to DIN 67530. 3. Results and discussion
Fig. 1. Synthesis route of oil-modified polyurethane dispersions.
based on rapeseed oil and differ in the natural deviation. The fatty acid composition of FA and FAME is mainly based on oleic acid (FA 75%, FAME 61%); further components are linoleic acid (FA 8–13%, FAME 20%), linolenic acid (FA 2%, FAME 10%) and palmeic acid (FA 2–6%, FAME 5%). Isophorone diisocyanate was provided by Evonik Degussa GmbH (Hanau, Germany) and the hydrophilic HDI-based polyisocyanate hardener Bayhydur® 305 was supplied by Bayer Material Science (Leverkusen, Germany). Other chemicals and solvents, like phthalic anhydride, isophthalic acid, 2-(hydroxymethyl)-2-ethylpropane-1,3-diol, 2,2-dimethyl1,3-propandiol, 2,2-bis(hydroxymethyl)propionic acid, acetone and triethyl amine were purchased from Sigma–Aldrich Chemie GmbH (Steinheim, Germany) and were used without further purification. The synthesis of polyester polyols was carried out by polycondensation using a standard melting process. The use of fatty acids and fatty acid methyl esters require trifunctional monomers to prevent chain stopping. 2-(Hydroxymethyl)-2-ethylpropane1,3-diol (TMP) was chosen as trifunctional monomer to have the same reactivity in all hydroxyl groups. Other building blocks like phthalic anhydride (PSA), isophthalic acid (IPA) and 2,2-dimethyl1,3-propanediol (NPG) were added. The resulting polyester polyols have acid values below 5 mg KOH/g, hydroxyl values in the range of 70 mg KOH/g to 100 mg KOH/g and molar mass Mn 1000 g/mol to 3000 g/mol (measured by GPC with polystyrene calibration) on average. Afterwards the polyester polyols were converted to polyurethane by using a standard acetone process at 70 ◦ C until all isocyanate groups have reacted followed by neutralization and dispersion in water. Comonomers used for polyurethane synthesis were diisocyanates like isophorone diisocyanate (IPDI), ionic center building blocks like 2,2-bis(hydroxymethyl)propionic acid (DMPA) and triethyl amine (TEA) for neutralization. Acid value (AV) determination was performed by titration of acetone diluted samples with 0.3 M potassium hydroxide in methanol and phenolphthalein for indication. For hydroxyl value (OHV) determination the hydroxyl groups were reacted with an excess of acetic anhydride. Specifically, the polymer was diluted in acetic anhydride solution in pyridine (88 g/L) and heated for 90 min at 110 ◦ C with a reflux condenser. The condenser was washed with water and acetone, followed by titration of the excess of hydrolysed acetic anhydride with 1 M potassium hydroxide in water. The OHV can be calculated by the difference to a blind measurement and in consideration of the acid value. Molar mass distribution was performed by GPC measurement with tetrahydrofurane as eluent and with polystyrene-calibration in the range of 162 g/mol to 70,000 g/mol. Three columns SDV 1000A at 40 ◦ C, a variable UV-detector (here: 254 nm), a refractive index detector and the software (WinGPC Unity) were provided by Polymer Standard Service (Mainz, Germany). Particle size and zeta potential
In the first part of this study, the use of fatty acid and fatty acid methyl ester for wood coatings were compared. Afterwards, experiments were performed with fatty acid methyl ester to determine the hydrophobicity and dispersion stability. It is well known and described in the literature [11,12] that the dispersion stability depends on the type and amount of ionic centers in the polymer chain. As other coating film properties like hydrolysis resistance and chemical resistance are negatively influenced by these functional groups, a compromise between dispersion stability and coating film properties has to be arranged. But it is not only the amount of ionic centers in the polymer chain which can influence the particle size distribution and consequently the dispersion stability. In addition, other parameters such as the neutralization and stirring procedure have to be considered. 3.1. Comparison of fatty acids and fatty acid methyl esters In the first part of this study it was planned to describe the relationship between the usage of different oleochemical components at different reaction temperatures and the resulting coating film characteristics. For this reason, four different polyester polyols were synthesized and converted to polyurethane dispersions, waterborne coating resins. The differences between these polyols are the type of the chosen oleochemical components which are fatty acids (FA) on the one hand and fatty acid methyl esters (FAME) on the other hand and the reaction temperatures of 180 ◦ C and 200 ◦ C. The polyesters consist of 1.10 mol isophthalic acid, 0.86 mol 1,3-propanediol, 0.73 mol TMP and 0.73 mol of the corresponding oleochemical component. As fatty acids and fatty acid methyl esters react as monofunctional building blocks, equimolar amounts of TMP were used. Furthermore, the ratio of hydroxyl groups to acid or methyl ester groups was selected to be 1.33 at the beginning of the reaction. This resulted in approximately 41 wt.% vegetable renewables. The polycondensation was carried out following a precise time table in combination with a frequent screening of molar mass distribution and acid value. The compositions of the polyesters are provided in Table 1. All polyester polyols were converted to polyurethane by acetone process performed by the same procedure. Isophorone diisocyanate (IPDI) was used for achieving urethane bonding and 4.65 wt.% dimethylol propionic acid (DMPA) was used to introduce ionic centers to stabilize the aqueous dispersion. The ratio of hydroxyl groups to isocyanate groups was calculated to be 1.25 at the beginning of the polyaddition. Compositions of the polyurethanes are given in Table 2. It was observed that for the reactions at 180 ◦ C the acid value (AV) did not drop to values of 5 mg KOH/g. At the Table 1 Experimental plan for polycondensation. Sample label
FA resp. FAME [g]
Isophthalic acid [g]
Trimethylol propan [g]
1,3-Propane diol [g]
A1 A2 B1 B2
205.86 205.86 214.62 214.62
182.60 182.60 182.60 182.60
97.82 97.82 97.82 97.82
65.36 65.36 65.36 65.36
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C.
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Table 2 Experimental plan for polyaddition. Sample label
Polyester AV [mg KOH/g]
Polyester OHV [mg KOH/g]
Polyester [g]
Isophorone diisocyanate [g]
Dimethylol propionic acid [g]
Triethyl amine [g]
A1 A2 B1 B2
7.1 5.7 7.8 4.4
86 113 105 120
78.66 75.04 76.66 73.86
16.86 20.02 18.80 20.94
4.65 4.95 4.54 5.19
4.50 4.50 4.50 4.50
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C.
Table 3 Physical–chemical characteristics of polyurethane dispersions. Sample label
OH%
pH
Particle size distribution
A1 A2 B1 B2
1.3 1.4 1.7 1.8
7.6 7.3 7.9 7.5
43 nm 46 nm 69 nm 36 nm
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C.
Fig. 2. Change in acid value during polycondensation.
same time the hydroxyl values (OHV) were slightly higher than the hydroxyl values of reactions at 200 ◦ C. This indicates a worse yield in ester bonds at 180 ◦ C compared to reactions performed at 200 ◦ C. In addition, as the reaction time was much longer for reactions at 180 ◦ C than for reactions at 200 ◦ C, side reaction or loss of alcohols during distillation of reaction methanol and reaction water may probably have occurred. Fig. 2 shows the monitoring of acid value during the reaction. The decrease in acid value expresses the progress of the reaction as for each ester bond one acid group or methyl ester group disappears. As expected, the influence of the reaction temperature had a strong influence on the reaction time. It was observed that a reaction performed at 200 ◦ C was two times faster than at 180 ◦ C. For the reactions at 180 ◦ C there was not a large difference in acid value and molar mass average (Mn) observed for FA and FAME based polyester, as shown in Fig. 3. On the other hand, the reactions at 200 ◦ C with fatty acid methyl ester were faster in the beginning compared to the fatty acid based reactions. The acid value dropped down faster and the molar mass average (Mn) of sample D was
Fig. 3. Change in molar mass (Mn) during polycondensation.
slightly higher than that of sample B. The reason for this observation is the higher reaction velocity vR of the fatty acid methyl ester caused by the lower boiling point of the formed leaving group methanol (65 ◦ C) compared to the boiling point of water (100 ◦ C). As this is an equilibration reaction, reaction velocity vR is higher as long as methanol is formed. The samples were converted to hydroxyl-terminated polyurethanes, neutralized and dispersed in water to obtain an aqueous polyurethane dispersion. The results of the physical– chemical characterization are shown in Table 3. The hydroxyl contents (OH%) of the samples prepared with fatty acid methyl ester were slightly higher than those of the fatty acid based samples. The differences might be caused by side reactions of remaining reaction water in the polyester polyols. The pH value for all samples is in the same range. All polyurethane dispersions are well stabilized as the particle size distribution is very narrow. The appearance of the dispersions is nearly transparent. This is not unusual for polyurethane dispersions for 2 K application as the molecular weight is not as high as for 1 K-polyurethane dispersions. Each polyurethane dispersion was formulated to a master batch and mixed with a water-reducible HDI-based polyisocyanate hardener before application on glass and on wooden beech samples. Pendulum hardness, gloss at 60◦ and 20◦ and appearance were measured and evaluated in relation to the oleochemical component and reaction temperature during the polycondensation. The coating film results are shown in Table 4. All samples display high gloss, which is astonishing for waterborne coatings in general. Gloss determination was performed on samples where the developed resin was used as a top coat on a commercial waterborne 2 K primer. For all measured characteristics (gloss, pendulum hardness and chemical resistance) no influence of reaction temperature or selection of oleochemical component was observed. This part of the study was performed to compare fatty acid and fatty acid methyl ester as well as the reaction condition during polycondensation. The poor pendulum hardness can be increased by changing the polyester and polyurethane composition. Previous work has shown that higher crosslinking by trifunctional monomers and higher OH functionality going along with higher amount of IPDI results in better pendulum hardness. From our results it is concluded that fatty acid methyl ester represent a viable alternative for fatty acids from a technological point of view. The following investigations were performed with
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Table 4 Physical–chemical characteristics of coating film. Sample label
Pendulum hardness (König) 7 d
Gloss 60◦ /20◦ (primer plus topcoat)
Resistance to water (16 h)
Resistance to 48% ethanol (1 h)
A1 A2 B1 B2
17 s 20 s 24 s 27 s
92/78 93/83 93/80 92/74
5 5 5 5
4 4 4 4
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C. Gloss of commercial primer: 92 at 60◦ and 66 at 20◦ ; resistance tests: 1, damage; 5, no effect.
fatty acid methyl esters to analyze specific parameters during the dispersion preparation process with regard to dispersion stability. 3.2. Influence of neutralization degree and stirring procedure on the dispersion stability Polyurethanes are stabilized in water by using an emulsifier. It is common to choose an anionic internal emulsifier, which needs to be neutralized prior to use. But it is not only important to add a minimum amount of neutralization agent. In addition, the order of action and the geometry of the stirring process which can be considered to influence the particle size and the dispersion stability of a polyurethane dispersion are of interest. As a polyurethane dispersion is always prepared by a secondary dispersion method, the critical and essential point during the process is the first time when the polymer gets in contact with the water. Therefore some preliminary tests were performed to determine the order of action and the influence of different stirring forces on the dispersion. Four methods are possible: addition of water to neutralized polymer, addition of neutralized polymer to water, addition of triethyl amine/water to polymer and addition of polymer to triethyl amine/water. It was found that neutralization of the polymer before dispersion in water leads to a more stable dispersion than neutralization during the dispersion step using triethyl amine/water. Furthermore, there is no difference if adding the polymer solution to the water or adding the water to the polymer solution. The stirring force is of less importance as the particle size of the resulting polyurethane dispersion is not influenced in the range of the performed pretests with stirring rates of 500–1500 rpm with a stirrer of 40 mm diameter in a beaker of 80 mm diameter. All following tests were performed by the same procedure: the polymer solution was neutralized and afterwards the water was added with the same stirring geometry and time. Table 5 shows the influence of the degree of neutralization on the particle size distribution. By optimizing these parameters afterwards, the amount of ionic centers can be further reduced without obtaining a negative influence to the dispersion stability. The polyurethane used in this experiment was based on a polyester with 36 wt.% fatty acid methyl ester (0.30 mol isophthalic acid, 0.30 mol phthalic anhydride, 0.40 mol fatty acid methyl ester, 0.59 mol neopentylglycol and 0.40 mol trimethylol propane). The polyurethane contained 72.22 wt.% polyester, 22.74 wt.% IPDI and 5.04 wt.% DMPA, which resulted in an acid value of 25 mg KOH/g.
The degree of neutralization was calculated by molar equivalent of triethyl amine to dimethylol propionic acid. After dispersion in water, acetone and the excess of TEA were removed by vacuum evaporation. It is obvious that the smallest particle size is achieved with a neutralization degree of at least 90% respectively with pH values around 9.0. Lower neutralization degrees lead to extremely higher particle sizes whereas higher neutralization degrees did not influence the particle size significantly. Furthermore, the excess of TEA can easily be removed by vacuum distillation to a pH value of 8.5. The degree of neutralization does not influence the zeta potential which is around −50 mV to −70 mV and though in a range of sufficient anionic stabilization. 3.3. Influence of the amount of ionic centers on the dispersion stability The higher the amount of oleochemicals in the polymer, the more the hydrophobic nature of the polyurethane increases. Hydrophobic polymers are difficult to convert into a stable dispersion without or with small amounts of emulsifier. On the other hand, a high amount of emulsifier negatively influences the surface properties of a coating film. As coating resin compositions with a new monomer are going to be developed, it is necessary to have a deeper look into the balance of surface properties and stabilization. Two different polyesters were synthesized: one with 42 wt.% FAME and the other with 28 wt.% FAME content. Both contain equi-molar amounts of trimethylol propane to have a statistically difunctional monomer (Fig. 4). Isophthalic acid was used as difunctional carboxylic component and 1,3-propane diol was used to bring the ratio of OH/COOH to 1.33. Both polyesters were converted to polyurethane prepolymers as described above and dispersed in water after neutralization with triethyl amine. To analyze the required amount of internal emulsifier DMPA, different polyurethanes were produced. Table 6 gives a visual impression of the samples in this series. The more DMPA was used, the more transparent the dispersion and consequently, the smaller the particle size, which is a good indicator for stable dispersions. In addition, the zeta potential of all optically stable dispersions was around −50 mV to −90 mV which can be explained by a sufficient anionic stabilization. The dispersions with 1.3 wt.% DMPA were stable for 14 days before they settled down. This went along with an increasement of the zeta
Table 5 Influence of degree of neutralization to the particle size distribution and zeta potential. Degree of neutralization 80% 90% 100% 110% 120% 130%
pH value stirring process
pH value after evaporation
Particle size distribution [nm]
Zeta potential [mV]
8.1 8.6 9.4 10.0 10.5 10.7
7.8 8.1 8.4 8.5 8.5 8.7
138 40 40 45 50 42
−73 −51 −53 −52 −51 −69
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Fig. 4. Equi-molar use of fatty acid methylester and TMP lead statistically to a fictive diol.
Table 6 Polyurethane dispersions with different amounts of DMPA. 5.0% DMPA
3.7% DMPA
2.5% DMPA
1.9% DMPA
1.3% DMPA
45 nm −47 mV 207 mPa s
42 nm −56 mV 214 mPa s
54 nm −57 mV 214 mPa s
76 nm −90 mV 57 mPa s
364 nm −2 mV 6 mPa s
35 nm −63 mV 190 mPa s
51 nm −58 mV 280 mPa s
66 nm −60 mV 87 mPa s
143 nm −57 mV 47 mPa s
233 nm −66 mV 5 mPa s
Series A(42% FAME)
Particle size Zeta potential Viscosity
Series B(28% FAME)
Particle size Zeta potential Viscosity
potential in case of series A. However, for series B the dispersion settled down although the zeta potential was in the expected range. Maybe other reasons like van der Waals’ forces are responsible for this observation as the particle size was increased to relatively high values. As described by Nanda and Wicks [10] an influence of the amount of DMPA on the viscosity of the polyurethane dispersion was also observed in this study. The higher the amount of DMPA and the lower the particle size, the higher was the viscosity. However, the dispersions with less FAME did not need less DMPA compared to those with more FAME, which was expected due to the lower amount of hydrophobic component. It is assumed that the role of
TMP might be more important than the role of the hydrophobic FAME. More TMP increases the risk of crosslinking during an early stage in the synthesis and may influence the steric performance of the polymer. However, this might cause a more suitable distribution of the DMPA and though lead to better stabilized particles with smaller particle sizes. The coating resins were applied on glass plates to analyze the surface properties of the dried coating film. Again, the influence of the amount of DMPA was observed. Figs. 5 and 6 show pictures of the transparent films. When sunlight appears on the surface, micro-structures were observed in a specific angle and projected
Fig. 5. Surface structure depending on the amount of DMPA in series A (left to right 5.0 wt.%; 3.7 wt.%; 2.5 wt.%; 1.9 wt.%; 1.3 wt.%), lower line pictures = 10 mm edge length.
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Fig. 6. Surface structure depending on the amount of DMPA in series B (left to right 5.0 wt.%; 3.7 wt.%; 2.5 wt.%; 1.9 wt.%; 1.3 wt.%), lower line pictures = 10 mm edge length.
to a white paper in the distance of 15 mm to visualize them in the photos. For both figures (Figs. 5 and 6) the first line of pictures was arranged with a 10 mm standard, the second line is an enlargement of the original pictures with an edge length of 10 mm to illustrate the differences. In both series, polyurethane dispersions with 1.3 wt.% DMPA show the worst film formation with inhomogeneous structures which may be caused by the relatively large particle sizes in the dispersion. For series A (large amount of FAME), the structures in the coating film are smaller dimensioned with increasing amount of DMPA, but they are still optically visible. In contrast, there are no structures visible in the coating films of series B (low amount of FAME) up to a DMPA amount of 1.9 wt.%. This leads to the assumption that the structures are caused by the higher amount of hydrophobic component in series A. It has to be mentioned that the particle size is not sufficient to predict the film-forming properties. All films show sufficient water and ethanol (48 vol.%) resistance. Only the samples with the highest amount of DMPA show some minor defects as they were swollen by ethanol (48 vol.%) which was reversible. From these investigations it seems to be more a visual appearance issue than a loss in coating film properties, which means that it was successful to decrease the amount of ionic emulsifiers in polyurethane dispersions containing hydrophobic building blocks down to 1.9 wt.%. The coating films of both series are going to be analyzed regarding further characteristics which can verify the observed impressions. 4. Conclusions Although fatty acid modified polyurethane dispersions are well known, fatty acid methyl esters are quite new as a raw material for coating resins. The chemical difference between these two raw materials is the carboxylic group and methyl ester group, which have no influence on the resulting polyester polyol, because both groups lead to ester bonds. However, the reaction velocity at the beginning can be accelerated by using fatty acid methyl esters, because of the better leaving group in case of fatty acid methyl ester. Different kinetics may cause different polyester structures as well. Another reason why fatty acids have been more favoured in coating resins than fatty acid methyl esters is their fatty acid composition, which often comprises a high amount of unsaturation. High amounts of unsaturation, especially linolic (18:2) and linolenic acid (18:3), can additionally be cured by oxidation (alkyds). Fatty acids methyl esters, commercially produced for biodiesel, are often based
on lower amounts of unsaturation such as rapeseed oil or palm oil. The dominant fatty acid in these fatty acid compositions is oleic acid (18:1). The lower level of unsaturation may be an advantage and lead to less yellowing. The study shows that fatty acid methyl esters are suitable to substitute fatty acids in polyester polyurethane coatings. The reaction time for polycondensation can be shortened. Furthermore, the neutralization and stirring process was optimized and DMPA was reduced to a minimum amount. It was shown that even at a lower content of emulsifier; stable dispersions and film formation are possible. It is confirmed that the lower the content of ionic emulsifier, the higher is the chemical resistance and hydrophobicity. Regarding the dispersion stability and particle size distribution, there was no influence of the amount of hydrophobic components observed. However, the visual appearance of the coating films is improved with increasing amount of DMPA used and even further improved with low amounts of hydrophobic components. At present, the fatty acid methyl esters were used as monofunctional building blocks and require trifunctional building blocks to go along with them. But rapeseed oil-based fatty acid methyl esters provide further possibilities for modification. Ongoing investigations are focusing on modification at the double bond and on the ester functionality. It is our objective to obtain new bifunctional monomers for polycondensation. As rapeseed fatty acid methyl ester contains mainly oleic acid, it is easier to obtain bifunctional monomers than to use triglycerides or highly unsaturated fatty acids, which lead to highly functional monomers and promote networks instead of polymer chains. In conclusion, fatty acid methyl esters offer a great opportunity to increase bio-based raw material use in coatings and to improve properties like chemical resistance and hydrophobicity at the same time. Acknowledgements The authors would like to thank their colleagues Sandra Hofmeister, Simon Niegebar, Carsten Hoyer and Antonia Mücke for their kind help in the experiments, and the European Commission for financial support. References [1] Frost and Sullivan, Western European Industrial Wood Coatings Markets (M07C-39), February 2007.
C. Philipp, S. Eschig / Progress in Organic Coatings 74 (2012) 705–711 [2] [3] [4] [5]
Frost and Sullivan, Wood Coatings-Technology Developments (D16C), 2008. J.T.P. Derksen, F.P. Cuperus, P. Kolster, Prog. Org. Coat. 27 (1996) 45–53. S. Friebel, O. Deppe, Chem. Ing. Tech. 81 (2009) 11. R. Tennebroek, J. Geurts, A. Overbeek, A. Harmsen, Surf. Coat. Int. Part B: Coat. Transact. 83 (2000) 13–19. [6] Y. Lu, R.C. Larock, Biomacromolecules 9 (2008) 3332–3340. [7] V.D. Athawale, J. Am. Oil Chem. Soc. (2010), doi:10.1007/s11746-010-1668-9.
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[8] S. Friebel, C. Philipp, S. Eschig, U. Baumann ,White Biotechnology for Wood Coatings Industrié at PRA’s 6th Woodcoatings Congress in Amsterdam, The Netherlands, Paper 2, 2008. [9] S. Friebel, C. Philipp, Farbe & Lack (2010) 10. [10] A.K. Nanda, D.A. Wicks, Polymer 47 (2006) 1805–1811. [11] A. Patel, C. Patel, M. Patel, A. Dighe, Prog. Org. Coat. 67 (2010) 255–263. [12] G.N. Manvi, R.N. Jagtap, J. Dispersion Sci. Technol. 31 (2010) 1376–1382.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Waterborne polyurethane wood coatings based on rapeseed fatty acid methyl esters C. Philipp ∗ , S. Eschig Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut (WKI), Bienroder Weg 54 E, 38108 Braunschweig, Germany
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Article history: Received 27 June 2011 Received in revised form 1 September 2011 Accepted 21 September 2011 Available online 15 December 2011 Keywords: Rapeseed oil Fatty acid methyl ester Polyurethane dispersion Dispersion stability
a b s t r a c t Trends in wood coatings are driven to waterborne systems and to renewable resources. Vegetable oils are well known for wood coatings, e.g. alkyds or polyurethane dispersions. In this context, fatty acid methyl esters turn out to be an alternative to technical fatty acids and vegetable oils. High contents of hydrophobic oil-based monomers require a sufficient understanding of the dispersion stability. This study shows the influence of hydrophobic monomers, ionic centers, degree of neutralization, and stirring procedure to the particle size distribution and dispersion stability. Furthermore, the impact of these parameters on the resulting coating film properties was investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The future of wood coatings will be strongly influenced by environmental legislation and economic burdens. The economic implications for research and development priorities are primarily driven by rising raw material costs. With regard to ecological aspects, environmental legislation and a more quality and health oriented consumer, a clear focus will be on waterborne and UV curable coatings [1,2]. In addition, renewable resources will start to replace petrochemical products to increase sustainability in wood coatings. However, renewable resources have been used for coating resins for a long time. In former times, the focus was mainly on vegetable oils (e.g. alkyd resins) and cellulose derivatives (e.g. nitrocellulose lacquer). But the variety of natural resources is manifold [3,4]. Apparently, the focus is still on vegetable oils and fatty acids, as they are available in large amounts and provide suitable chemical functional groups. Their main technical advantages are high hydrophobicity and good chemical resistance, especially black heel resistance [5]. Fig. 1 shows the synthesis route and the potential steps to include vegetable oils and fatty acids in polyurethanes. Fatty acids can be part of the polyester polyols whereas hydroxyl-functional vegetable oils are directly used in polyurethanes [6,7]. Beside fatty acids, fatty acid methyl esters turn out to be an alternative. Two reasons are responsible for this: First, in contrast to fatty acids, the conversion from vegetable oil to fatty acid methyl ester requires no distillation and the product
∗ Corresponding author. E-mail address: [email protected] (C. Philipp). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.09.028
is easier to separate from the side product glycerol, because of no building of soaps. These aspects may make the production less cost-intensive. And secondly, during the polycondensation process, fatty acid methyl esters result in better leaving groups (methanol) with lower boiling point than the leaving group of fatty acids (water), which can shorten the reaction time of the polycondensation. In addition, the biodiesel or fatty acid production lead to raw glycerol as side-product which can be biotechnologically converted to 1,3-propane diol. 1,3-Propane diol is another biobased building-block for polycondensation and polyadditions which leads to a good compromise of hardness and elasticity for the coating film and is expected to replace neopentyl glycol and 1,6-hexane diol partially [8,9]. Beside the coating film properties, the dispersion stability is important. By adding high amounts of extremely hydrophobic monomers (like vegetable oils, fatty acids or fatty acid methyl esters) in the synthesis, the hydrophobicity of the polymer increases. To overcome this incompatibility with the dispersant water, ionic centers are included in the polyurethane chain [10]. The stability of the dispersion increases but the chemical resistance of the coating film declines. The dispersion stability can also be influenced by the degree of neutralization and the stirring procedures. Particle size distribution is a useful precursor to predict dispersion stability. 2. Materials and methods Fatty acid (FA) was provided by Emery Oleochemicals GmbH (Loxstedt, Germany), fatty acid methyl ester (FAME) by Rheinische Bioester GmbH (Neuss, Germany). Both FA and FAME are
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measurements were performed with a Zetasizer Nano ZS instrument by Malvern (Herrenberg, Germany). Pendulum hardness was determined by the method according to König (DIN 53157) and gloss measurement by a glossmeter (micro-TRI-gloss) from BYKGardner GmbH (Geretsried, Germany) according to DIN 67530. 3. Results and discussion
Fig. 1. Synthesis route of oil-modified polyurethane dispersions.
based on rapeseed oil and differ in the natural deviation. The fatty acid composition of FA and FAME is mainly based on oleic acid (FA 75%, FAME 61%); further components are linoleic acid (FA 8–13%, FAME 20%), linolenic acid (FA 2%, FAME 10%) and palmeic acid (FA 2–6%, FAME 5%). Isophorone diisocyanate was provided by Evonik Degussa GmbH (Hanau, Germany) and the hydrophilic HDI-based polyisocyanate hardener Bayhydur® 305 was supplied by Bayer Material Science (Leverkusen, Germany). Other chemicals and solvents, like phthalic anhydride, isophthalic acid, 2-(hydroxymethyl)-2-ethylpropane-1,3-diol, 2,2-dimethyl1,3-propandiol, 2,2-bis(hydroxymethyl)propionic acid, acetone and triethyl amine were purchased from Sigma–Aldrich Chemie GmbH (Steinheim, Germany) and were used without further purification. The synthesis of polyester polyols was carried out by polycondensation using a standard melting process. The use of fatty acids and fatty acid methyl esters require trifunctional monomers to prevent chain stopping. 2-(Hydroxymethyl)-2-ethylpropane1,3-diol (TMP) was chosen as trifunctional monomer to have the same reactivity in all hydroxyl groups. Other building blocks like phthalic anhydride (PSA), isophthalic acid (IPA) and 2,2-dimethyl1,3-propanediol (NPG) were added. The resulting polyester polyols have acid values below 5 mg KOH/g, hydroxyl values in the range of 70 mg KOH/g to 100 mg KOH/g and molar mass Mn 1000 g/mol to 3000 g/mol (measured by GPC with polystyrene calibration) on average. Afterwards the polyester polyols were converted to polyurethane by using a standard acetone process at 70 ◦ C until all isocyanate groups have reacted followed by neutralization and dispersion in water. Comonomers used for polyurethane synthesis were diisocyanates like isophorone diisocyanate (IPDI), ionic center building blocks like 2,2-bis(hydroxymethyl)propionic acid (DMPA) and triethyl amine (TEA) for neutralization. Acid value (AV) determination was performed by titration of acetone diluted samples with 0.3 M potassium hydroxide in methanol and phenolphthalein for indication. For hydroxyl value (OHV) determination the hydroxyl groups were reacted with an excess of acetic anhydride. Specifically, the polymer was diluted in acetic anhydride solution in pyridine (88 g/L) and heated for 90 min at 110 ◦ C with a reflux condenser. The condenser was washed with water and acetone, followed by titration of the excess of hydrolysed acetic anhydride with 1 M potassium hydroxide in water. The OHV can be calculated by the difference to a blind measurement and in consideration of the acid value. Molar mass distribution was performed by GPC measurement with tetrahydrofurane as eluent and with polystyrene-calibration in the range of 162 g/mol to 70,000 g/mol. Three columns SDV 1000A at 40 ◦ C, a variable UV-detector (here: 254 nm), a refractive index detector and the software (WinGPC Unity) were provided by Polymer Standard Service (Mainz, Germany). Particle size and zeta potential
In the first part of this study, the use of fatty acid and fatty acid methyl ester for wood coatings were compared. Afterwards, experiments were performed with fatty acid methyl ester to determine the hydrophobicity and dispersion stability. It is well known and described in the literature [11,12] that the dispersion stability depends on the type and amount of ionic centers in the polymer chain. As other coating film properties like hydrolysis resistance and chemical resistance are negatively influenced by these functional groups, a compromise between dispersion stability and coating film properties has to be arranged. But it is not only the amount of ionic centers in the polymer chain which can influence the particle size distribution and consequently the dispersion stability. In addition, other parameters such as the neutralization and stirring procedure have to be considered. 3.1. Comparison of fatty acids and fatty acid methyl esters In the first part of this study it was planned to describe the relationship between the usage of different oleochemical components at different reaction temperatures and the resulting coating film characteristics. For this reason, four different polyester polyols were synthesized and converted to polyurethane dispersions, waterborne coating resins. The differences between these polyols are the type of the chosen oleochemical components which are fatty acids (FA) on the one hand and fatty acid methyl esters (FAME) on the other hand and the reaction temperatures of 180 ◦ C and 200 ◦ C. The polyesters consist of 1.10 mol isophthalic acid, 0.86 mol 1,3-propanediol, 0.73 mol TMP and 0.73 mol of the corresponding oleochemical component. As fatty acids and fatty acid methyl esters react as monofunctional building blocks, equimolar amounts of TMP were used. Furthermore, the ratio of hydroxyl groups to acid or methyl ester groups was selected to be 1.33 at the beginning of the reaction. This resulted in approximately 41 wt.% vegetable renewables. The polycondensation was carried out following a precise time table in combination with a frequent screening of molar mass distribution and acid value. The compositions of the polyesters are provided in Table 1. All polyester polyols were converted to polyurethane by acetone process performed by the same procedure. Isophorone diisocyanate (IPDI) was used for achieving urethane bonding and 4.65 wt.% dimethylol propionic acid (DMPA) was used to introduce ionic centers to stabilize the aqueous dispersion. The ratio of hydroxyl groups to isocyanate groups was calculated to be 1.25 at the beginning of the polyaddition. Compositions of the polyurethanes are given in Table 2. It was observed that for the reactions at 180 ◦ C the acid value (AV) did not drop to values of 5 mg KOH/g. At the Table 1 Experimental plan for polycondensation. Sample label
FA resp. FAME [g]
Isophthalic acid [g]
Trimethylol propan [g]
1,3-Propane diol [g]
A1 A2 B1 B2
205.86 205.86 214.62 214.62
182.60 182.60 182.60 182.60
97.82 97.82 97.82 97.82
65.36 65.36 65.36 65.36
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C.
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Table 2 Experimental plan for polyaddition. Sample label
Polyester AV [mg KOH/g]
Polyester OHV [mg KOH/g]
Polyester [g]
Isophorone diisocyanate [g]
Dimethylol propionic acid [g]
Triethyl amine [g]
A1 A2 B1 B2
7.1 5.7 7.8 4.4
86 113 105 120
78.66 75.04 76.66 73.86
16.86 20.02 18.80 20.94
4.65 4.95 4.54 5.19
4.50 4.50 4.50 4.50
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C.
Table 3 Physical–chemical characteristics of polyurethane dispersions. Sample label
OH%
pH
Particle size distribution
A1 A2 B1 B2
1.3 1.4 1.7 1.8
7.6 7.3 7.9 7.5
43 nm 46 nm 69 nm 36 nm
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C.
Fig. 2. Change in acid value during polycondensation.
same time the hydroxyl values (OHV) were slightly higher than the hydroxyl values of reactions at 200 ◦ C. This indicates a worse yield in ester bonds at 180 ◦ C compared to reactions performed at 200 ◦ C. In addition, as the reaction time was much longer for reactions at 180 ◦ C than for reactions at 200 ◦ C, side reaction or loss of alcohols during distillation of reaction methanol and reaction water may probably have occurred. Fig. 2 shows the monitoring of acid value during the reaction. The decrease in acid value expresses the progress of the reaction as for each ester bond one acid group or methyl ester group disappears. As expected, the influence of the reaction temperature had a strong influence on the reaction time. It was observed that a reaction performed at 200 ◦ C was two times faster than at 180 ◦ C. For the reactions at 180 ◦ C there was not a large difference in acid value and molar mass average (Mn) observed for FA and FAME based polyester, as shown in Fig. 3. On the other hand, the reactions at 200 ◦ C with fatty acid methyl ester were faster in the beginning compared to the fatty acid based reactions. The acid value dropped down faster and the molar mass average (Mn) of sample D was
Fig. 3. Change in molar mass (Mn) during polycondensation.
slightly higher than that of sample B. The reason for this observation is the higher reaction velocity vR of the fatty acid methyl ester caused by the lower boiling point of the formed leaving group methanol (65 ◦ C) compared to the boiling point of water (100 ◦ C). As this is an equilibration reaction, reaction velocity vR is higher as long as methanol is formed. The samples were converted to hydroxyl-terminated polyurethanes, neutralized and dispersed in water to obtain an aqueous polyurethane dispersion. The results of the physical– chemical characterization are shown in Table 3. The hydroxyl contents (OH%) of the samples prepared with fatty acid methyl ester were slightly higher than those of the fatty acid based samples. The differences might be caused by side reactions of remaining reaction water in the polyester polyols. The pH value for all samples is in the same range. All polyurethane dispersions are well stabilized as the particle size distribution is very narrow. The appearance of the dispersions is nearly transparent. This is not unusual for polyurethane dispersions for 2 K application as the molecular weight is not as high as for 1 K-polyurethane dispersions. Each polyurethane dispersion was formulated to a master batch and mixed with a water-reducible HDI-based polyisocyanate hardener before application on glass and on wooden beech samples. Pendulum hardness, gloss at 60◦ and 20◦ and appearance were measured and evaluated in relation to the oleochemical component and reaction temperature during the polycondensation. The coating film results are shown in Table 4. All samples display high gloss, which is astonishing for waterborne coatings in general. Gloss determination was performed on samples where the developed resin was used as a top coat on a commercial waterborne 2 K primer. For all measured characteristics (gloss, pendulum hardness and chemical resistance) no influence of reaction temperature or selection of oleochemical component was observed. This part of the study was performed to compare fatty acid and fatty acid methyl ester as well as the reaction condition during polycondensation. The poor pendulum hardness can be increased by changing the polyester and polyurethane composition. Previous work has shown that higher crosslinking by trifunctional monomers and higher OH functionality going along with higher amount of IPDI results in better pendulum hardness. From our results it is concluded that fatty acid methyl ester represent a viable alternative for fatty acids from a technological point of view. The following investigations were performed with
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Table 4 Physical–chemical characteristics of coating film. Sample label
Pendulum hardness (König) 7 d
Gloss 60◦ /20◦ (primer plus topcoat)
Resistance to water (16 h)
Resistance to 48% ethanol (1 h)
A1 A2 B1 B2
17 s 20 s 24 s 27 s
92/78 93/83 93/80 92/74
5 5 5 5
4 4 4 4
A contains fatty acids, B contains fatty acid methyl ester. Index 1: polycondensation at 180 ◦ C, index 2 polycondensation at 200 ◦ C. Gloss of commercial primer: 92 at 60◦ and 66 at 20◦ ; resistance tests: 1, damage; 5, no effect.
fatty acid methyl esters to analyze specific parameters during the dispersion preparation process with regard to dispersion stability. 3.2. Influence of neutralization degree and stirring procedure on the dispersion stability Polyurethanes are stabilized in water by using an emulsifier. It is common to choose an anionic internal emulsifier, which needs to be neutralized prior to use. But it is not only important to add a minimum amount of neutralization agent. In addition, the order of action and the geometry of the stirring process which can be considered to influence the particle size and the dispersion stability of a polyurethane dispersion are of interest. As a polyurethane dispersion is always prepared by a secondary dispersion method, the critical and essential point during the process is the first time when the polymer gets in contact with the water. Therefore some preliminary tests were performed to determine the order of action and the influence of different stirring forces on the dispersion. Four methods are possible: addition of water to neutralized polymer, addition of neutralized polymer to water, addition of triethyl amine/water to polymer and addition of polymer to triethyl amine/water. It was found that neutralization of the polymer before dispersion in water leads to a more stable dispersion than neutralization during the dispersion step using triethyl amine/water. Furthermore, there is no difference if adding the polymer solution to the water or adding the water to the polymer solution. The stirring force is of less importance as the particle size of the resulting polyurethane dispersion is not influenced in the range of the performed pretests with stirring rates of 500–1500 rpm with a stirrer of 40 mm diameter in a beaker of 80 mm diameter. All following tests were performed by the same procedure: the polymer solution was neutralized and afterwards the water was added with the same stirring geometry and time. Table 5 shows the influence of the degree of neutralization on the particle size distribution. By optimizing these parameters afterwards, the amount of ionic centers can be further reduced without obtaining a negative influence to the dispersion stability. The polyurethane used in this experiment was based on a polyester with 36 wt.% fatty acid methyl ester (0.30 mol isophthalic acid, 0.30 mol phthalic anhydride, 0.40 mol fatty acid methyl ester, 0.59 mol neopentylglycol and 0.40 mol trimethylol propane). The polyurethane contained 72.22 wt.% polyester, 22.74 wt.% IPDI and 5.04 wt.% DMPA, which resulted in an acid value of 25 mg KOH/g.
The degree of neutralization was calculated by molar equivalent of triethyl amine to dimethylol propionic acid. After dispersion in water, acetone and the excess of TEA were removed by vacuum evaporation. It is obvious that the smallest particle size is achieved with a neutralization degree of at least 90% respectively with pH values around 9.0. Lower neutralization degrees lead to extremely higher particle sizes whereas higher neutralization degrees did not influence the particle size significantly. Furthermore, the excess of TEA can easily be removed by vacuum distillation to a pH value of 8.5. The degree of neutralization does not influence the zeta potential which is around −50 mV to −70 mV and though in a range of sufficient anionic stabilization. 3.3. Influence of the amount of ionic centers on the dispersion stability The higher the amount of oleochemicals in the polymer, the more the hydrophobic nature of the polyurethane increases. Hydrophobic polymers are difficult to convert into a stable dispersion without or with small amounts of emulsifier. On the other hand, a high amount of emulsifier negatively influences the surface properties of a coating film. As coating resin compositions with a new monomer are going to be developed, it is necessary to have a deeper look into the balance of surface properties and stabilization. Two different polyesters were synthesized: one with 42 wt.% FAME and the other with 28 wt.% FAME content. Both contain equi-molar amounts of trimethylol propane to have a statistically difunctional monomer (Fig. 4). Isophthalic acid was used as difunctional carboxylic component and 1,3-propane diol was used to bring the ratio of OH/COOH to 1.33. Both polyesters were converted to polyurethane prepolymers as described above and dispersed in water after neutralization with triethyl amine. To analyze the required amount of internal emulsifier DMPA, different polyurethanes were produced. Table 6 gives a visual impression of the samples in this series. The more DMPA was used, the more transparent the dispersion and consequently, the smaller the particle size, which is a good indicator for stable dispersions. In addition, the zeta potential of all optically stable dispersions was around −50 mV to −90 mV which can be explained by a sufficient anionic stabilization. The dispersions with 1.3 wt.% DMPA were stable for 14 days before they settled down. This went along with an increasement of the zeta
Table 5 Influence of degree of neutralization to the particle size distribution and zeta potential. Degree of neutralization 80% 90% 100% 110% 120% 130%
pH value stirring process
pH value after evaporation
Particle size distribution [nm]
Zeta potential [mV]
8.1 8.6 9.4 10.0 10.5 10.7
7.8 8.1 8.4 8.5 8.5 8.7
138 40 40 45 50 42
−73 −51 −53 −52 −51 −69
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Fig. 4. Equi-molar use of fatty acid methylester and TMP lead statistically to a fictive diol.
Table 6 Polyurethane dispersions with different amounts of DMPA. 5.0% DMPA
3.7% DMPA
2.5% DMPA
1.9% DMPA
1.3% DMPA
45 nm −47 mV 207 mPa s
42 nm −56 mV 214 mPa s
54 nm −57 mV 214 mPa s
76 nm −90 mV 57 mPa s
364 nm −2 mV 6 mPa s
35 nm −63 mV 190 mPa s
51 nm −58 mV 280 mPa s
66 nm −60 mV 87 mPa s
143 nm −57 mV 47 mPa s
233 nm −66 mV 5 mPa s
Series A(42% FAME)
Particle size Zeta potential Viscosity
Series B(28% FAME)
Particle size Zeta potential Viscosity
potential in case of series A. However, for series B the dispersion settled down although the zeta potential was in the expected range. Maybe other reasons like van der Waals’ forces are responsible for this observation as the particle size was increased to relatively high values. As described by Nanda and Wicks [10] an influence of the amount of DMPA on the viscosity of the polyurethane dispersion was also observed in this study. The higher the amount of DMPA and the lower the particle size, the higher was the viscosity. However, the dispersions with less FAME did not need less DMPA compared to those with more FAME, which was expected due to the lower amount of hydrophobic component. It is assumed that the role of
TMP might be more important than the role of the hydrophobic FAME. More TMP increases the risk of crosslinking during an early stage in the synthesis and may influence the steric performance of the polymer. However, this might cause a more suitable distribution of the DMPA and though lead to better stabilized particles with smaller particle sizes. The coating resins were applied on glass plates to analyze the surface properties of the dried coating film. Again, the influence of the amount of DMPA was observed. Figs. 5 and 6 show pictures of the transparent films. When sunlight appears on the surface, micro-structures were observed in a specific angle and projected
Fig. 5. Surface structure depending on the amount of DMPA in series A (left to right 5.0 wt.%; 3.7 wt.%; 2.5 wt.%; 1.9 wt.%; 1.3 wt.%), lower line pictures = 10 mm edge length.
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Fig. 6. Surface structure depending on the amount of DMPA in series B (left to right 5.0 wt.%; 3.7 wt.%; 2.5 wt.%; 1.9 wt.%; 1.3 wt.%), lower line pictures = 10 mm edge length.
to a white paper in the distance of 15 mm to visualize them in the photos. For both figures (Figs. 5 and 6) the first line of pictures was arranged with a 10 mm standard, the second line is an enlargement of the original pictures with an edge length of 10 mm to illustrate the differences. In both series, polyurethane dispersions with 1.3 wt.% DMPA show the worst film formation with inhomogeneous structures which may be caused by the relatively large particle sizes in the dispersion. For series A (large amount of FAME), the structures in the coating film are smaller dimensioned with increasing amount of DMPA, but they are still optically visible. In contrast, there are no structures visible in the coating films of series B (low amount of FAME) up to a DMPA amount of 1.9 wt.%. This leads to the assumption that the structures are caused by the higher amount of hydrophobic component in series A. It has to be mentioned that the particle size is not sufficient to predict the film-forming properties. All films show sufficient water and ethanol (48 vol.%) resistance. Only the samples with the highest amount of DMPA show some minor defects as they were swollen by ethanol (48 vol.%) which was reversible. From these investigations it seems to be more a visual appearance issue than a loss in coating film properties, which means that it was successful to decrease the amount of ionic emulsifiers in polyurethane dispersions containing hydrophobic building blocks down to 1.9 wt.%. The coating films of both series are going to be analyzed regarding further characteristics which can verify the observed impressions. 4. Conclusions Although fatty acid modified polyurethane dispersions are well known, fatty acid methyl esters are quite new as a raw material for coating resins. The chemical difference between these two raw materials is the carboxylic group and methyl ester group, which have no influence on the resulting polyester polyol, because both groups lead to ester bonds. However, the reaction velocity at the beginning can be accelerated by using fatty acid methyl esters, because of the better leaving group in case of fatty acid methyl ester. Different kinetics may cause different polyester structures as well. Another reason why fatty acids have been more favoured in coating resins than fatty acid methyl esters is their fatty acid composition, which often comprises a high amount of unsaturation. High amounts of unsaturation, especially linolic (18:2) and linolenic acid (18:3), can additionally be cured by oxidation (alkyds). Fatty acids methyl esters, commercially produced for biodiesel, are often based
on lower amounts of unsaturation such as rapeseed oil or palm oil. The dominant fatty acid in these fatty acid compositions is oleic acid (18:1). The lower level of unsaturation may be an advantage and lead to less yellowing. The study shows that fatty acid methyl esters are suitable to substitute fatty acids in polyester polyurethane coatings. The reaction time for polycondensation can be shortened. Furthermore, the neutralization and stirring process was optimized and DMPA was reduced to a minimum amount. It was shown that even at a lower content of emulsifier; stable dispersions and film formation are possible. It is confirmed that the lower the content of ionic emulsifier, the higher is the chemical resistance and hydrophobicity. Regarding the dispersion stability and particle size distribution, there was no influence of the amount of hydrophobic components observed. However, the visual appearance of the coating films is improved with increasing amount of DMPA used and even further improved with low amounts of hydrophobic components. At present, the fatty acid methyl esters were used as monofunctional building blocks and require trifunctional building blocks to go along with them. But rapeseed oil-based fatty acid methyl esters provide further possibilities for modification. Ongoing investigations are focusing on modification at the double bond and on the ester functionality. It is our objective to obtain new bifunctional monomers for polycondensation. As rapeseed fatty acid methyl ester contains mainly oleic acid, it is easier to obtain bifunctional monomers than to use triglycerides or highly unsaturated fatty acids, which lead to highly functional monomers and promote networks instead of polymer chains. In conclusion, fatty acid methyl esters offer a great opportunity to increase bio-based raw material use in coatings and to improve properties like chemical resistance and hydrophobicity at the same time. Acknowledgements The authors would like to thank their colleagues Sandra Hofmeister, Simon Niegebar, Carsten Hoyer and Antonia Mücke for their kind help in the experiments, and the European Commission for financial support. References [1] Frost and Sullivan, Western European Industrial Wood Coatings Markets (M07C-39), February 2007.
C. Philipp, S. Eschig / Progress in Organic Coatings 74 (2012) 705–711 [2] [3] [4] [5]
Frost and Sullivan, Wood Coatings-Technology Developments (D16C), 2008. J.T.P. Derksen, F.P. Cuperus, P. Kolster, Prog. Org. Coat. 27 (1996) 45–53. S. Friebel, O. Deppe, Chem. Ing. Tech. 81 (2009) 11. R. Tennebroek, J. Geurts, A. Overbeek, A. Harmsen, Surf. Coat. Int. Part B: Coat. Transact. 83 (2000) 13–19. [6] Y. Lu, R.C. Larock, Biomacromolecules 9 (2008) 3332–3340. [7] V.D. Athawale, J. Am. Oil Chem. Soc. (2010), doi:10.1007/s11746-010-1668-9.
711
[8] S. Friebel, C. Philipp, S. Eschig, U. Baumann ,White Biotechnology for Wood Coatings Industrié at PRA’s 6th Woodcoatings Congress in Amsterdam, The Netherlands, Paper 2, 2008. [9] S. Friebel, C. Philipp, Farbe & Lack (2010) 10. [10] A.K. Nanda, D.A. Wicks, Polymer 47 (2006) 1805–1811. [11] A. Patel, C. Patel, M. Patel, A. Dighe, Prog. Org. Coat. 67 (2010) 255–263. [12] G.N. Manvi, R.N. Jagtap, J. Dispersion Sci. Technol. 31 (2010) 1376–1382.