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Tung based reactive diluents for alkyd systems: Film properties
Progress in Organic Coatings 73 (2012) 283–290
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Tung based reactive diluents for alkyd systems: Film properties Kosin Wutticharoenwong, Jamie Dziczkowski, Mark D. Soucek ∗ University of Akron, Polymer Engineering Department, USA
a r t i c l e
i n f o
Article history: Received 4 January 2011 Received in revised form 21 March 2011 Accepted 23 March 2011 Available online 7 May 2011 Keywords: Alkyd Reactive diluent Diels–Alder Coatings Tung oil
a b s t r a c t Reactive diluents were prepared from tung oil via a Diels–Alder reaction with three different dienophiles: methacryloxypropyl trimethoxysilane (TOMAS), 2,2,2-trifluoroethyl methacrylate (TOF) and triallyl ether acrylate (TOTAE). The reactive diluents were mixed with a long oil soya alkyd, a metal drier package, and a wetting agent; and then cured. Formulations were prepared as a function of reactive diluent type and wt%. Tensile, thermomechanical, and coatings properties were evaluated after curing the films. The addition of two reactive diluents, TOMAS and TOTAE, improved the tensile strength and tensile modulus of the alkyd. The addition of the diluents, however, did not significantly change the elongation at break compared to the alkyd in any of the systems. Also, the addition of TOMAS and TOTAE increased both the crosslink density and glass transition temperature of the alkyd. Basic film properties including hardness, solvent resistance, impact resistance, adhesion, and gloss were not adversely affected by the introduction of any of the tung oil based diluents. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Researchers are continually attempting to develop “greener” coatings systems [1–3]. Seed-oil based materials are an attractive choice. Seed oils are classified into different categories (non-drying, semi-drying, and drying) based on the number of unsaturated sites located in the fatty acid side chains. The higher the number of unsaturated sites, the more readily a film is formed when exposed to the atmosphere. The process by which a seed oil based film is formed is commonly referred to as autoxidative curing [4–7]. Oxidation of the drying oil begins when molecular oxygen attacks an active center on a fatty acid chain, followed by the homolytic cleavage of the peroxide to produce free radicals. For non-conjugated seed oils, hydrogen is then abstracted from the doubly allylic methylene groups, afterwhich isomerization can occur to form a conjugated structure. Termination then occurs resulting in mostly ether and peroxy crosslinks. The push to develop new materials using renewable resources makes alkyds an attractive binder. Alkyds are composed of seed oils, such as linseed oil, soybean oil, or safflower oil reacted with an aromatic dibasic acid to and polyols to form a polymer backbone with pendent fatty acids. Alkyd based coatings have several advantages including high gloss, good color/gloss retention, good heat and solvent resistance, and an oxygen driven autoxidative crosslinking mechanism [1]. However, alkyds can require the use of organic solvents to reach the desired application viscosity, which
∗ Corresponding author. Tel.: +1 3309722583. E-mail address: [email protected] (M.D. Soucek). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.03.017
poses a problem in a time where environmental restrictions are more severe. An alternative to solventborne alkyds are waterborne alkyds which pose hydrolytic difficulties with respect to potlife stability. One possible solution to satisfy environmental concerns is to develop new reactive diluents. These materials function as an organic solvent in the coatings formulation, but are integrated into the film during the curing process. Tung oil based materials have been reported as reactive diluents for radiocurable formulations [8], and thermally induced cationic copolymerization [9]. In addition, tung has been used extensively as an autoxidative reactive diluent for alkyds. Since tung oil is a conjugated drying oil derived from the nuts of Aleurites fordiiz., the drying mechanism is slightly different than the nonconjugated seed oils [1,10]. Unlike the other seed oils (nonconjugated) the autoxidative process for tung oil [11,12] includes crosslinking reactions via through direct addition of free radicals to the conjugated double bonds. In this paper, the film properties of alkyds with three tung oil based reactive diluents [13] were investigated. The three reactive diluents were all Diels–Alder Adducts of tung oil which possessed a variety of different functionality: (1) tung oil modified with Alkyoxylsilane (TOMAS), tung oil modified with tetraallyl ether (TOTAE), tung oil modified with a flurorinated side chain (TOF). The three reactive diluents were formulated with a long oil soya alkyd to evaluate the effect of the reactive diluents. Film properties were investigated as a function of reactive diluent percent and reactive diluent type. The coatings were evaluated via tensile properties, Dynamic Thermal Analysis (DMA), and rudimentary coatings tests.
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Table 1 Alkyd formulations with reactive diluents. Samples
Soy Bean LOA (9)
Diluent (9)
Drier (2%) (9)
Wetting (1%) (9)
Tung Si-0 Tung Si-10 Tung Si-20 Tung Si-30 Tung F-0 Tung F-10 Tung F-20 Tung F-30 Tung AE-0 Tung AE-10 Tung AE-20 Tung AE-30
9.7000 8.7300 7.7600 6.7900 9.7000 8.7300 7.7600 6.7900 9.7000 8.7300 7.7600 6.7900 Total weight
0.0000 0.9700 1.9400 2.9100 0.0000 0.9700 1.9400 2.9100 0.0000 0.9700 1.9400 2.9100 10
0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 g
0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101
2. Experimental
2.2. Synthesis of long-oil alkyd resin
2.1. Materials
The alkyd resin was prepared by the monoglyceride process. The reaction was conducted in a 500 ml four-neck round bottom flask equipped with an inert gas inlet, thermometer, reflux condenser, and a mechanical agitator. The transesterification step involved soybean oil (200 g) and an excess of glycerol (44.75 g). Before heating the vessel was purged with argon gas for ∼15 min. The mixture was then heated to 120 ◦ C and lithium hydroxide catalyst was introduced into the reactor (0.10–0.15 wt% of polyol). The temperature was gradually increased to 240 ◦ C. After approximately 1 h, a small aliquot was removed and mixed in three parts 95 wt% ethanol. This was repeated until the resulting solution was clear (2 h). The reaction mixture from step 1 was cooled to around 100 ◦ C and a Dean-Stark trap was introduced to the reaction setup. The reactor was then charged with PA (71.65 g) and 100 mL of xylene for use as a reflux solvent. The mixture was then slowly heated to
Refined soybean oil used for alkyd synthesis was acquired from Cargill Inc., while lithium hydroxide, phthalic anhydride (PA), glycerol, phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA), p-toluenesulfonic acid and 2,2,2-trifluoroethyl methacrylate were all purchased from Aldrich. Tung oil was obtained from Waterlox Coatings Inc. and Methacryloxypropyl trimethoxysilane was purchased from Gelest Inc. Driers used in coatings formulations, 5 wt% cobalt Hydro-Cure II, 12 wt% Zirconium Hydro-Cure, and 5 wt% calcium Hydro-Cure were obtained from OMG Group. Tego wet 270 (Evonik, Inc.). All materials were used as received. The reactive diluents were prepared as previously described [13]. The structures are shown in Fig. 1.
Fig. 1. Structures of reactive diluents synthesized via the Diels–Alder reaction: (a) methacryloxypropyl trimethoxysilane (TOMAS) modified tung oil (b) 2,2,2-trifluoroethyl methacrylate (TOF) modified tung oil (c) triallyl ether acrylate (TOTAE) modified tung oil.
K. Wutticharoenwong et al. / Progress in Organic Coatings 73 (2012) 283–290
285
O O
H 2C CH
O
Linoleic (~51%)=R3
C C
Stearic (~15%)=R2
O
CH2 O
C Oleic (~25%)=R1
O
Triglyceride of Soybean Oil O O
H2C HC
O
R1
C
R2
H2C +
C
R3
HC CH2
OH
O
O C
O
OH
O
+
R1,2,3
C
O
CH2
OH
R3 most probable
O
O *
R1,2,3
O
OH
HC
(excess)
O
H2C
H2C
OH
HC OH CH2
O
CH2 O
C
O
O
O O
O O
O
O
O
OH
R3 most probable
O *
O
PHTHALIC ANHYDRIDE Alkyd Resin
Scheme 1. Reaction path of a soya-based alkyd synthesized via the monoglyceride process.
Fig. 2. Tensile strength (Pa) as a function of reactive diluent: TOMAS-Silicone, TOF-Fluorine, TOTAE-Allyl Ether.
O
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Fig. 3. Elongation at break (%) as a function of reactive diluent: TOMAS-Silicone, TOF-Fluorine, TOTAE-Allyl Ether.
220 ◦ C. After every hour of reaction, a sample was removed and the acid number was determined. The reaction was stopped once an acid number below 10 was achieved (4 h). The xylene was removed, and the acid number determination was based on ASTM D4462-98 with 1 M KOH and phenolphthalein indicator. The product was then cooled to room temperature and stored under argon atmosphere. Scheme 1 shows the reaction path for a soybean oil based alkyd synthesized by the monoglyceride process. 2.3. Coating formulation and film preparation Each diluent was formulated with the alkyd resin at three different levels (10 wt%, 20 wt% and 30 wt% based on total formulation), 2 wt% metal drier package (10 wt% Cobalt Hydro-Cure II, 80 wt% Zirconium Hydro-Cem, 10 wt% Calcium Hydro-Cem), and 1 wt% wetting agent. Formulations of each mixture can be found in Table 1. The contents were introduced into a sealed vial and mixed
on a roller mill for 2 h. A draw down bar was used to cast films on clean glass panels (6 mils wet film) and on aluminum panels (3 mils wet film). The wet films were cured in the oven at 120 ◦ C for 2 h, followed by a second cure at 160 ◦ C for 3 h. The films were kept at room temperature for 7 days before any tests were performed to ensure a through cure was achieved. 2.4. Instruments and characterization All experiments were conducted according to ASTM standards. The viscosity was measured at ambient temperature. The viscoelastic properties of the films were investigated using a Perkin-Elmer Rheometric Scientific dynamic mechanical thermal analyzer (DMTA) at a frequency of 1 Hz and a heating rate of 4 ◦ C/min over a range of −50 to 250 ◦ C. The gap distance was set at 3 mm for rectangular specimens (10 mm wide, 20 mm long, and 0.10 mm thick). The maximum of the tan delta was used to
Fig. 4. Tensile modulus (Pa) as a function of reactive diluent: TOMAS-Silicone, TOF-Fluorine, TOTAE-Allyl Ether.
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Table 2 Viscoelastic properties of the alkyd and alkyd/diluent cured films.
Alkyd Tung Si-10 Tung Si-20 Tung Si-30 Tung F-10 Tung F-20 Tung F-30 Tung AE-10 Tung AE-20 Tung AE-30
E (min) (N/m2 )
e (mol/cm3 )
Tg (◦ C)
1.86E + 06 2.37E + 06 3.42E + 06 4.19E + 06 2.50E + 06 3.79E + 06 1.77E + 06 3.23E + 06 1.68E + 06 3.83E + 06
202 244 354 450 263 406 190 344 246 384
37 50 52 50 49 48 31 51 48 51
3. Results and discussion The overall objective was to evaluate three reactive diluents with a model alkyd system. The synthesis of TOTAE, TOMAS, and TOF have been previously reported [13], and this is paper is an extension of the alkyd properties. Two of the reactive diluents, TOF and TOTAE (see Fig. 1b and c) cure via an autoxidative mechanism, the third, TOMAS (see Fig. 1a) cures via a moisture and autoxidative curing mechanisms. Of the two autoxidative reactive diluents, the TOTAE is anticipated to be more active as a consequence of the allyl ether groups, whereas the TOF is expected to have less autoxidative curing on account of less functionality. However, the fluorinated reactive diluent (TOF) was designed with strictly surface active properties in mind. 3.1. Tensile properties
Fig. 5. Modulus (E ) of reactive diluent type and wt% loading: (a) TOMAS-Silicone, (b) TOF-Fluorine, (c) TOTAE-Allyl Ether.
determine the glass transition temperature, while the crosslink density was determined by utilizing the lowest E point value at a minimum of 50 ◦ C beyond the point at which the Tg was found. The tensile properties of the films were evaluated using an Instron Universal Tester. The dimensions of the films for tensile testing were 0.05 mm in thickness, 13–17 mm wide, with an initial length of 10 mm. A crosshead speed of 10 mm/min with a load cell of 100 N was applied to determine elongation-at-break, tensile modulus, and tensile strength of each system. For each film, between five and eight samples were tested. The tensile data are reported as the mean with a standard deviation. All other film properties were found according to the corresponding ASTM standard, and the appropriate equipment was utilized when specified.
Tensile strength with increasing reactive diluent content for each of the different systems is shown in Fig. 2. Both the TOMAS and TOTAE increase in the tensile strength of the alkyd film when added to the formulation. There is an optimum loading in tensile strength close to 10 wt% for both the TOMAS and TOTAE. At 10 and 20 wt%, the TOMAS had the highest tensile strength until 30 wt% loading where the TOTAE had the highest value. The fluorine-modified tung oil containing films show no effect on the tensile strength of the films at any of the loading levels tested. The maximum value tensile strength value can be explained by balancing the intrinsic crosslink density of the soya-based alkyd with crosslinking provided by the tung-oil based diluent. The alkyd has both polyester and fatty acid character meaning that the polyester backbone of the alkyd is the only component that provides rigidity to the entire system, and in this case a baseline in tensile properties. A soya based alkyd will not achieve a high tensile strength alone as compared to a linseed based alkyd. Unfortunately, a lower reactive functionality for the soya-based alkyd is available for crosslinking. However, the tung oil has many reactive sites per chain and has a smaller molecular weight (larger mole fraction). It is assumed that the autoxidative crosslinking will begin with the tung oil derived diluents, since conjugated drying oils are more reactive than the non-conjugated soya-oil, and the higher mobility that the triglyceride possesses in comparison with the alkyd. However, there will reach a point in the curing mechanism that the tung oil materials will build molecular weight similar to that of the alkyd. At this point, the curing rate will then model that of a traditional soya-based alkyd resin more closely than that of tung oil due to the similarity in the diffusion rates of the two different materials. As the amount of reactive diluent increases to 30 wt% and above, the properties of the matrix are dominated by the triglyceride and not polyester backbone, resulting in a diminution of tensile strength. When comparing the results from the different reactive diluents, the alkoxysilane modified tung oil enhances the tensile strength
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Fig. 6. Tan ı for reactive diluent type and wt% loading: (a) TOMAS-Silicone, (b) TOF-Fluorine, (c) TOTAE-Allyl Ether.
the most, while the fluorinated diluent shows no changes. The allyl ether modified tung oil falls somewhere in between the two. These results are consistent based on previous work done by Soucek et al. on inorganic/organic hybrid materials that contain alkoxysilanes, where the strength of the coatings increased with increasing alkoxysilane functionality [14]. The reason for the higher tensile strength in the films that contain the alkoxysilane modified diluents versus the allyl ether modified films is attributed to the additional crosslinking mechanism that is introduced by the alkoxysilane groups. As mentioned earlier, there is an in situ polycondensation of the silicon alkoxide within the organic polymer matrix, thus increasing the functionality available for crosslinking [15–17]. The fluorine containing diluents do not exhibit the same properties as
the other two reactive diluents, as a consequence of less functionality available for crosslinking [13]. The elongation at break and tensile modulus as a function of reactive diluent are shown in Figs. 3 and 4, respectively. Based on the results, there is no pronounced effect on the elongation at break by adding any of the diluents at any loading level. Since alkyd films are so readily crosslinked and form strong networks, the elongation at break of these films is fairly low, even though alkyds have some flexibility from the fatty acid groups. The tensile modulus of the allyl ether (TOTAE) and alkoxysilane (TOMAS) modified systems show the most difference from that of the alkyd. This trend is similar to the tensile strength where the additional crosslinkable sites of these reactive diluents resulted in an enhancement of
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Table 3 Coatings properties of neat alkyd resin and diluent/alkyd mixtures. Sample
Film thickness (micron)
MEK double rubs
Impact resistance (Ib/in)
Pencil hardness
Mandrel bend flexibility (% Elongation)
Cylindrical bend flexibility (mm)
Neat Alkyd AS-10 AS-20 AS-30
30 20 20 20
120 68 69 80
>40 >40 >40 >40
2B B HB HB
>32% >32% >32% >32%
2 2 2 2
F-10 F-20 F-30
20 20 20
117 175 125
>40 >40 >40
2B 2B 2B
>32% >32% >32%
2 2 2
AE-10 AE-20 AE-30
20 20 20
100 106 71
>40 >40 >40
HB HB HB
>32% >32% >32%
2 2 2
AS-x: Alkyd with x-wt% alkoxysilane modified tung oil. F-x: Alkyd with x-wt% fluorine modified tung oil AE-x: Alkyd with x-wt% allyl ether modified tung oil.
Table 4 Coatings properties of neat alkyd resin and diluent/alkyd mixtures (cont.). Sample
Film thickness (micron)
Viscosity (CP)
20◦ Gloss
60◦ Gloss
Crosshatch adhesion
Neat Alkyd AS-10 AS-20 AS-30 F-10 F-20 F-30 AE-10 AE-20 AE-30
30 20 20 20 20 20 20 20 20 20
1670 1334 1085 868 1147 883 681.5 1056 748 490
161.2 164.2 173.8 172.3 165.2 167.1 163.6 170.0 170.2 161.5
150.7 153.9 157.9 156.3 152.9 155.9 153.9 158.7 161.2 149.7
5B 5B 5B 5B 5B 5B 5B 5B 5B 5B
AS-x: Alkyd with x-wt% alkoxysilane modified tung oil. F-x: Alkyd with x-wt% fluorine modified tung oil AE-x: Alkyd with x-wt% allyl ether modified tung oil.
the tensile moduli. Again, the highest tensile moduli are the alkoxy silane TOMAS at 10 and 20 wt%. However, at 30 wt% the alkoxysilane TOMAS and the allyl ether TOTAE have the same value. 3.2. Viscoelastic properties The viscoelastic properties of all systems are shown in Figs. 5 and 6. All of the films had narrow ␣-transitions in tan ı data (see Fig. 6) and modulus transitions (see Fig. 5) indicating a uniformity of the crosslinked network. For the alkoxy silane based TOMAS, all the ␣-transitions were higher than the alkyd. This was in agreement with the increase in storage moduli in the rubbery plateau. The allyl ether TOTAE exhibits the same behavior for the ␣-transition, however at 30 wt%, a decrease in the storage modulus is observed which is consistent with the tensile data. The storage moduli of the fluorinated TOF with 10 and 20 wt% loading exhibits a similar behavior to the TOTAE. Unlike the TOTAE, the tan ı data of the TOF is in agreement with the lower modulus at 30 wt%. The expression used to calculate the crosslink density is given in Eq. (1), where e is the number of moles of elastically effective chains per cubic centimeter of the film, E min is the minimum storage modulus in the rubbery plateau in N/m2 , R is the gas constant (8.314 N m/g mol K), and T is the absolute temperature in Kelvin. E min = 3e RT
(1)
The values for E min , e , and Tg can be found for all samples in Table 2. As expected, the crosslink density of the film did increase with addition of all three diluents, except for the mixture that contains 30 wt% FTO. The glass transition temperatures of all cured films containing the diluents are all significantly higher except for the formulation that contains 30 wt% FTO. There appears to be a maximum level of incorporation of the FTO that has been crossed, above which the effects on film properties are not beneficial to
the system. The increase in crosslink density and glass transition temperature at 10 and 20 wt% is due to the residual unreacted double bonds of the ␣-eleostearate which can react with itself and crosslink the alkyd. The increase in crosslink density for the TOMAS and the TOTAE series reflects the additional functionality of the trimethoxysilane and triallyl ether, respectively. The apparent inconsistency of tensile data, Tg , and crosslink data at >20 wt% can be explained by a transition from an alkyd dominated system which is crosslinked via pendent fatty acid groups to a reactive diluent dominated system. The film properties of oxidizing alkyds emanating from the rigidity of the polyester backbone balance with the eventual crosslinking of the pendent fatty acid groups. The molecular weight of the polyester backbone by itself is insufficient to build-up properties, so some level of chain extension is needed for the polyester backbone to build its molecular weight and hence its optimal properties (i.e. tensile versus molecular weight). The size, ability to diffuse, and multiple functionality of the reactive diluents are idea for building up the crosslink density, and as a consequence, the mechanical properties as well. After 20 wt% loading, the matrix begins to be dominated by the reactive diluent and not the polyester backbone of the alkyd. 3.3. General film properties Film properties of the formulated alkyds are presented in Tables 3 and 4. In addition to reducing viscosity, the diluents are expected to enhance the film properties of the alkyd films. Several trends can be noted and discussed about the behavior of the films. The pencil hardness of the films improved as predicted for the reactive diluents and remained the same for systems containing the fluorinated material. Solvent resistance, on the other hand, gave unexpected results. The MEK double rub test showed that coatings with TOMAS reduced the solvent resistance, whereas incorporation of FTO and TAETO does not significantly improve the solvent resistance of the coatings. However, one should take MEK rubs
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with a grain of salt. It is important to note that the thickness of the coating does affect the result of the solvent resistance test, in which the neat alkyd films were thicker than those films that contain the diluents. FTO generally exhibited relatively higher solvent resistance than SFTO and TAETO. This result may be due to the excellent chemical resistance of fluorinated groups in FTO or just a more frictionless surface. The remaining properties, flexibility and impact resistance, are not affected by the incorporation of the diluents. Additional coatings properties such as the gloss and crosshatch adhesion appear to not be significantly effected by the diluent content (see Table 4). The primary goal of this study was to replace a percentage of the organic solvent used in the alkyd-based coatings in order to reduce the volatile organics needed for application viscosity. Overall, modified tung oil diluents generally provided good film and coatings properties. Thus, the reduction of the viscosity without having any adverse effects on the coatings properties indicated the attainment of this goal. A previously published paper outlines the synthesis of the reactive diluents [13]. Drying time and viscosity data was also included. All the reactive diluents were shown to reduce the viscosity of the alkyd, and the TOTAE had a faster drying time than the others. The TOTAE however may have some difficult with the evolution of acrolein. It was presumed that the allyl ether groups increased the speed of the drying process. The Tg and crosslink density data support this supposition. The TOMAS was slower with respect to drying, but was expected to participate in the drying process. Again, the Tg and crosslink density data are supported of the alkoxysilane groups reacting and thus adding to the crosslink density. As a result, the tensile modulus, tensile strength, and hardness were also improved. The fluorine TOF generally showed relatively higher solvent resistance than the other diluents, which was moderately surprising. The TOF also participated in the film formation process as evidenced by the Tg and crosslink density data. By incorporating alkoxysilanes into these new diluents, hybrid inorganic–organic coatings can be formed through the sol–gel process. This process involves in situ polycondensation of metal or silicon alkoxide with an organic polymer matrix [15–17]. The inorganic microdomain is formed by hydrolysis, condensation, and gelation of the metal or silicon alkoxide in solution; while the organic polymer matrix of the drying oil forms cross-linking structure by an autoxidative process. Fluorinated polymers exhibit low inter- and intramolecular interactions. These characteristics lead to low cohesive energy, which in turn provides low surface energy. Moreover, fluorinated materials provide high thermal stability, chemical inertness, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, valuable electrical properties, and low dielectric constants and dissipation factors [18]. 4. Conclusions Incorporation of the diluents into alkyd-based coatings formulations has shown that the new functionalized tung oil derivatives act effectively as diluents. The viscosity of the alkyd
was significantly decreased with increased loading of the diluents, replacing the need for organic solvent in the formulation. All the reactive diluents participated in the film formation process. The allyl ether TOTAE and alkoxyl silane TOMAS provided additional crosslinking sites which were reflected in the tensile data as well as the overall film properties. However, addition of diluents shows no significant change in the elongation at break and impact resistance compared to the alkyd. The balance of the polyester backbone to the pendent fatty acid of the alkyd can be altered after >20 wt% of a triglyceride reactive diluent is added into the system. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.porgcoat.2011.03.017. References [1] Z.W. Wicks, F.N. Jones, P.S. Pappas, Organic Coatings: Science and Technology, 2nd ed., Wiley-Interscience, New York, 1992. [2] P. Swaraj, Surface Coatings Science and Technology, 2nd ed., John Wiley and Sons, New York, 1996. [3] F.N. Jones, Toward solventless liquid coatings, Journal of Coatings Technology 68 (852) (1996) 25–36. [4] W.J. Muizebelt, et al., Oxidative crosslinking of alkyd resins studied with mass spectrometry and NMR using model compounds, Journal of Coatings Technology 70 (876) (1998) p83. [5] G. Teng, et al., Spectroscopic investigation of the blowing process of soyabean oil, Surface Coatings International Part B: Coatings Transactions 86 (B3) (2003) 221–229. [6] N.M. Martyak, D. Alford, R. Picker, C. Dowling, Controlled oxidative curing of tung oil, Journal of Coatings Technology: Coatings Technology (2005 May) 36. [7] J. Mallegol, et al., Thermal (DSC) and chemical (iodometric titration) methods for peroxides measurements in order to monitor drying extent of alkyd resins, Progress in Organic Coatings 41 (1–3) (2001) 171–176. [8] D. Poortere, Radiocurable Composition, 1978, U C B. [9] F. Li, R.C. Larock, Synthesis, structure and properties of new tung oil–styrene–divinylbenzene copolymers prepared by thermal polymerization, Biomacromolecules 4 (4) (2003) 1018–1025. [10] H.W. Chatfield, Varnish Constituents, Interscience, New York, 1944. [11] W.J. Muizebelt, et al., Crosslink mechanisms of high-solids alkyd resins in the presence of reactive diluents, Progress in Organic Coatings 40 (1–4) (2000) 121–130. [12] W.J. Muizebelt, M.W.F. Nielen, Oxidative crosslinking of unsaturated fatty acids studied with mass spectrometry, Journal of Mass Spectrometry 31 (5) (1996) 545–554. [13] (a) K. Wutticharoenwong, M.D. Soucek, Synthesis of tung oil based reactive diluents, Macromolecular Materials and Engineering 295(12) (2011) 1097.; (b) K. Wutticharoenwong, Bio-based Reactive Diluents and Thiol-ene Photopolymerization for Environmentally Benign Coatings, Ph.D. Thesis, 2007. [14] H. Ni, et al., Alkoxysilane-modified polyurea coatings, Polymeric Materials Science and Engineering 81 (1999) 405–406. [15] C.R. Wold, M.D. Soucek, Mixed metal oxide inorganic/organic coatings, Journal of Coatings Technology 70 (882) (1998) 43–51. [16] S.J. Tuman, M.D. Soucek, Novel inorganic/organic coatings based on linseed oil and sunflower oil with sol–gel precursors, Journal of Coatings Technology 68 (854) (1996) 73–81. [17] R.L. Ballard, et al., Effects of an acid catalyst on the inorganic domain of inorganic–organic hybrid materials, Chemistry of Materials 11 (3) (1999) 726–735. [18] B. Ameduri, B. Boutevin, Well-architectured Fluoropolymers: Synthesis Properties and Applications, 1st ed., Elsevier, Amsterdam, 2004.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Tung based reactive diluents for alkyd systems: Film properties Kosin Wutticharoenwong, Jamie Dziczkowski, Mark D. Soucek ∗ University of Akron, Polymer Engineering Department, USA
a r t i c l e
i n f o
Article history: Received 4 January 2011 Received in revised form 21 March 2011 Accepted 23 March 2011 Available online 7 May 2011 Keywords: Alkyd Reactive diluent Diels–Alder Coatings Tung oil
a b s t r a c t Reactive diluents were prepared from tung oil via a Diels–Alder reaction with three different dienophiles: methacryloxypropyl trimethoxysilane (TOMAS), 2,2,2-trifluoroethyl methacrylate (TOF) and triallyl ether acrylate (TOTAE). The reactive diluents were mixed with a long oil soya alkyd, a metal drier package, and a wetting agent; and then cured. Formulations were prepared as a function of reactive diluent type and wt%. Tensile, thermomechanical, and coatings properties were evaluated after curing the films. The addition of two reactive diluents, TOMAS and TOTAE, improved the tensile strength and tensile modulus of the alkyd. The addition of the diluents, however, did not significantly change the elongation at break compared to the alkyd in any of the systems. Also, the addition of TOMAS and TOTAE increased both the crosslink density and glass transition temperature of the alkyd. Basic film properties including hardness, solvent resistance, impact resistance, adhesion, and gloss were not adversely affected by the introduction of any of the tung oil based diluents. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Researchers are continually attempting to develop “greener” coatings systems [1–3]. Seed-oil based materials are an attractive choice. Seed oils are classified into different categories (non-drying, semi-drying, and drying) based on the number of unsaturated sites located in the fatty acid side chains. The higher the number of unsaturated sites, the more readily a film is formed when exposed to the atmosphere. The process by which a seed oil based film is formed is commonly referred to as autoxidative curing [4–7]. Oxidation of the drying oil begins when molecular oxygen attacks an active center on a fatty acid chain, followed by the homolytic cleavage of the peroxide to produce free radicals. For non-conjugated seed oils, hydrogen is then abstracted from the doubly allylic methylene groups, afterwhich isomerization can occur to form a conjugated structure. Termination then occurs resulting in mostly ether and peroxy crosslinks. The push to develop new materials using renewable resources makes alkyds an attractive binder. Alkyds are composed of seed oils, such as linseed oil, soybean oil, or safflower oil reacted with an aromatic dibasic acid to and polyols to form a polymer backbone with pendent fatty acids. Alkyd based coatings have several advantages including high gloss, good color/gloss retention, good heat and solvent resistance, and an oxygen driven autoxidative crosslinking mechanism [1]. However, alkyds can require the use of organic solvents to reach the desired application viscosity, which
∗ Corresponding author. Tel.: +1 3309722583. E-mail address: [email protected] (M.D. Soucek). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.03.017
poses a problem in a time where environmental restrictions are more severe. An alternative to solventborne alkyds are waterborne alkyds which pose hydrolytic difficulties with respect to potlife stability. One possible solution to satisfy environmental concerns is to develop new reactive diluents. These materials function as an organic solvent in the coatings formulation, but are integrated into the film during the curing process. Tung oil based materials have been reported as reactive diluents for radiocurable formulations [8], and thermally induced cationic copolymerization [9]. In addition, tung has been used extensively as an autoxidative reactive diluent for alkyds. Since tung oil is a conjugated drying oil derived from the nuts of Aleurites fordiiz., the drying mechanism is slightly different than the nonconjugated seed oils [1,10]. Unlike the other seed oils (nonconjugated) the autoxidative process for tung oil [11,12] includes crosslinking reactions via through direct addition of free radicals to the conjugated double bonds. In this paper, the film properties of alkyds with three tung oil based reactive diluents [13] were investigated. The three reactive diluents were all Diels–Alder Adducts of tung oil which possessed a variety of different functionality: (1) tung oil modified with Alkyoxylsilane (TOMAS), tung oil modified with tetraallyl ether (TOTAE), tung oil modified with a flurorinated side chain (TOF). The three reactive diluents were formulated with a long oil soya alkyd to evaluate the effect of the reactive diluents. Film properties were investigated as a function of reactive diluent percent and reactive diluent type. The coatings were evaluated via tensile properties, Dynamic Thermal Analysis (DMA), and rudimentary coatings tests.
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Table 1 Alkyd formulations with reactive diluents. Samples
Soy Bean LOA (9)
Diluent (9)
Drier (2%) (9)
Wetting (1%) (9)
Tung Si-0 Tung Si-10 Tung Si-20 Tung Si-30 Tung F-0 Tung F-10 Tung F-20 Tung F-30 Tung AE-0 Tung AE-10 Tung AE-20 Tung AE-30
9.7000 8.7300 7.7600 6.7900 9.7000 8.7300 7.7600 6.7900 9.7000 8.7300 7.7600 6.7900 Total weight
0.0000 0.9700 1.9400 2.9100 0.0000 0.9700 1.9400 2.9100 0.0000 0.9700 1.9400 2.9100 10
0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 0.204 g
0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101 0.101
2. Experimental
2.2. Synthesis of long-oil alkyd resin
2.1. Materials
The alkyd resin was prepared by the monoglyceride process. The reaction was conducted in a 500 ml four-neck round bottom flask equipped with an inert gas inlet, thermometer, reflux condenser, and a mechanical agitator. The transesterification step involved soybean oil (200 g) and an excess of glycerol (44.75 g). Before heating the vessel was purged with argon gas for ∼15 min. The mixture was then heated to 120 ◦ C and lithium hydroxide catalyst was introduced into the reactor (0.10–0.15 wt% of polyol). The temperature was gradually increased to 240 ◦ C. After approximately 1 h, a small aliquot was removed and mixed in three parts 95 wt% ethanol. This was repeated until the resulting solution was clear (2 h). The reaction mixture from step 1 was cooled to around 100 ◦ C and a Dean-Stark trap was introduced to the reaction setup. The reactor was then charged with PA (71.65 g) and 100 mL of xylene for use as a reflux solvent. The mixture was then slowly heated to
Refined soybean oil used for alkyd synthesis was acquired from Cargill Inc., while lithium hydroxide, phthalic anhydride (PA), glycerol, phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA), p-toluenesulfonic acid and 2,2,2-trifluoroethyl methacrylate were all purchased from Aldrich. Tung oil was obtained from Waterlox Coatings Inc. and Methacryloxypropyl trimethoxysilane was purchased from Gelest Inc. Driers used in coatings formulations, 5 wt% cobalt Hydro-Cure II, 12 wt% Zirconium Hydro-Cure, and 5 wt% calcium Hydro-Cure were obtained from OMG Group. Tego wet 270 (Evonik, Inc.). All materials were used as received. The reactive diluents were prepared as previously described [13]. The structures are shown in Fig. 1.
Fig. 1. Structures of reactive diluents synthesized via the Diels–Alder reaction: (a) methacryloxypropyl trimethoxysilane (TOMAS) modified tung oil (b) 2,2,2-trifluoroethyl methacrylate (TOF) modified tung oil (c) triallyl ether acrylate (TOTAE) modified tung oil.
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285
O O
H 2C CH
O
Linoleic (~51%)=R3
C C
Stearic (~15%)=R2
O
CH2 O
C Oleic (~25%)=R1
O
Triglyceride of Soybean Oil O O
H2C HC
O
R1
C
R2
H2C +
C
R3
HC CH2
OH
O
O C
O
OH
O
+
R1,2,3
C
O
CH2
OH
R3 most probable
O
O *
R1,2,3
O
OH
HC
(excess)
O
H2C
H2C
OH
HC OH CH2
O
CH2 O
C
O
O
O O
O O
O
O
O
OH
R3 most probable
O *
O
PHTHALIC ANHYDRIDE Alkyd Resin
Scheme 1. Reaction path of a soya-based alkyd synthesized via the monoglyceride process.
Fig. 2. Tensile strength (Pa) as a function of reactive diluent: TOMAS-Silicone, TOF-Fluorine, TOTAE-Allyl Ether.
O
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Fig. 3. Elongation at break (%) as a function of reactive diluent: TOMAS-Silicone, TOF-Fluorine, TOTAE-Allyl Ether.
220 ◦ C. After every hour of reaction, a sample was removed and the acid number was determined. The reaction was stopped once an acid number below 10 was achieved (4 h). The xylene was removed, and the acid number determination was based on ASTM D4462-98 with 1 M KOH and phenolphthalein indicator. The product was then cooled to room temperature and stored under argon atmosphere. Scheme 1 shows the reaction path for a soybean oil based alkyd synthesized by the monoglyceride process. 2.3. Coating formulation and film preparation Each diluent was formulated with the alkyd resin at three different levels (10 wt%, 20 wt% and 30 wt% based on total formulation), 2 wt% metal drier package (10 wt% Cobalt Hydro-Cure II, 80 wt% Zirconium Hydro-Cem, 10 wt% Calcium Hydro-Cem), and 1 wt% wetting agent. Formulations of each mixture can be found in Table 1. The contents were introduced into a sealed vial and mixed
on a roller mill for 2 h. A draw down bar was used to cast films on clean glass panels (6 mils wet film) and on aluminum panels (3 mils wet film). The wet films were cured in the oven at 120 ◦ C for 2 h, followed by a second cure at 160 ◦ C for 3 h. The films were kept at room temperature for 7 days before any tests were performed to ensure a through cure was achieved. 2.4. Instruments and characterization All experiments were conducted according to ASTM standards. The viscosity was measured at ambient temperature. The viscoelastic properties of the films were investigated using a Perkin-Elmer Rheometric Scientific dynamic mechanical thermal analyzer (DMTA) at a frequency of 1 Hz and a heating rate of 4 ◦ C/min over a range of −50 to 250 ◦ C. The gap distance was set at 3 mm for rectangular specimens (10 mm wide, 20 mm long, and 0.10 mm thick). The maximum of the tan delta was used to
Fig. 4. Tensile modulus (Pa) as a function of reactive diluent: TOMAS-Silicone, TOF-Fluorine, TOTAE-Allyl Ether.
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Table 2 Viscoelastic properties of the alkyd and alkyd/diluent cured films.
Alkyd Tung Si-10 Tung Si-20 Tung Si-30 Tung F-10 Tung F-20 Tung F-30 Tung AE-10 Tung AE-20 Tung AE-30
E (min) (N/m2 )
e (mol/cm3 )
Tg (◦ C)
1.86E + 06 2.37E + 06 3.42E + 06 4.19E + 06 2.50E + 06 3.79E + 06 1.77E + 06 3.23E + 06 1.68E + 06 3.83E + 06
202 244 354 450 263 406 190 344 246 384
37 50 52 50 49 48 31 51 48 51
3. Results and discussion The overall objective was to evaluate three reactive diluents with a model alkyd system. The synthesis of TOTAE, TOMAS, and TOF have been previously reported [13], and this is paper is an extension of the alkyd properties. Two of the reactive diluents, TOF and TOTAE (see Fig. 1b and c) cure via an autoxidative mechanism, the third, TOMAS (see Fig. 1a) cures via a moisture and autoxidative curing mechanisms. Of the two autoxidative reactive diluents, the TOTAE is anticipated to be more active as a consequence of the allyl ether groups, whereas the TOF is expected to have less autoxidative curing on account of less functionality. However, the fluorinated reactive diluent (TOF) was designed with strictly surface active properties in mind. 3.1. Tensile properties
Fig. 5. Modulus (E ) of reactive diluent type and wt% loading: (a) TOMAS-Silicone, (b) TOF-Fluorine, (c) TOTAE-Allyl Ether.
determine the glass transition temperature, while the crosslink density was determined by utilizing the lowest E point value at a minimum of 50 ◦ C beyond the point at which the Tg was found. The tensile properties of the films were evaluated using an Instron Universal Tester. The dimensions of the films for tensile testing were 0.05 mm in thickness, 13–17 mm wide, with an initial length of 10 mm. A crosshead speed of 10 mm/min with a load cell of 100 N was applied to determine elongation-at-break, tensile modulus, and tensile strength of each system. For each film, between five and eight samples were tested. The tensile data are reported as the mean with a standard deviation. All other film properties were found according to the corresponding ASTM standard, and the appropriate equipment was utilized when specified.
Tensile strength with increasing reactive diluent content for each of the different systems is shown in Fig. 2. Both the TOMAS and TOTAE increase in the tensile strength of the alkyd film when added to the formulation. There is an optimum loading in tensile strength close to 10 wt% for both the TOMAS and TOTAE. At 10 and 20 wt%, the TOMAS had the highest tensile strength until 30 wt% loading where the TOTAE had the highest value. The fluorine-modified tung oil containing films show no effect on the tensile strength of the films at any of the loading levels tested. The maximum value tensile strength value can be explained by balancing the intrinsic crosslink density of the soya-based alkyd with crosslinking provided by the tung-oil based diluent. The alkyd has both polyester and fatty acid character meaning that the polyester backbone of the alkyd is the only component that provides rigidity to the entire system, and in this case a baseline in tensile properties. A soya based alkyd will not achieve a high tensile strength alone as compared to a linseed based alkyd. Unfortunately, a lower reactive functionality for the soya-based alkyd is available for crosslinking. However, the tung oil has many reactive sites per chain and has a smaller molecular weight (larger mole fraction). It is assumed that the autoxidative crosslinking will begin with the tung oil derived diluents, since conjugated drying oils are more reactive than the non-conjugated soya-oil, and the higher mobility that the triglyceride possesses in comparison with the alkyd. However, there will reach a point in the curing mechanism that the tung oil materials will build molecular weight similar to that of the alkyd. At this point, the curing rate will then model that of a traditional soya-based alkyd resin more closely than that of tung oil due to the similarity in the diffusion rates of the two different materials. As the amount of reactive diluent increases to 30 wt% and above, the properties of the matrix are dominated by the triglyceride and not polyester backbone, resulting in a diminution of tensile strength. When comparing the results from the different reactive diluents, the alkoxysilane modified tung oil enhances the tensile strength
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Fig. 6. Tan ı for reactive diluent type and wt% loading: (a) TOMAS-Silicone, (b) TOF-Fluorine, (c) TOTAE-Allyl Ether.
the most, while the fluorinated diluent shows no changes. The allyl ether modified tung oil falls somewhere in between the two. These results are consistent based on previous work done by Soucek et al. on inorganic/organic hybrid materials that contain alkoxysilanes, where the strength of the coatings increased with increasing alkoxysilane functionality [14]. The reason for the higher tensile strength in the films that contain the alkoxysilane modified diluents versus the allyl ether modified films is attributed to the additional crosslinking mechanism that is introduced by the alkoxysilane groups. As mentioned earlier, there is an in situ polycondensation of the silicon alkoxide within the organic polymer matrix, thus increasing the functionality available for crosslinking [15–17]. The fluorine containing diluents do not exhibit the same properties as
the other two reactive diluents, as a consequence of less functionality available for crosslinking [13]. The elongation at break and tensile modulus as a function of reactive diluent are shown in Figs. 3 and 4, respectively. Based on the results, there is no pronounced effect on the elongation at break by adding any of the diluents at any loading level. Since alkyd films are so readily crosslinked and form strong networks, the elongation at break of these films is fairly low, even though alkyds have some flexibility from the fatty acid groups. The tensile modulus of the allyl ether (TOTAE) and alkoxysilane (TOMAS) modified systems show the most difference from that of the alkyd. This trend is similar to the tensile strength where the additional crosslinkable sites of these reactive diluents resulted in an enhancement of
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Table 3 Coatings properties of neat alkyd resin and diluent/alkyd mixtures. Sample
Film thickness (micron)
MEK double rubs
Impact resistance (Ib/in)
Pencil hardness
Mandrel bend flexibility (% Elongation)
Cylindrical bend flexibility (mm)
Neat Alkyd AS-10 AS-20 AS-30
30 20 20 20
120 68 69 80
>40 >40 >40 >40
2B B HB HB
>32% >32% >32% >32%
2 2 2 2
F-10 F-20 F-30
20 20 20
117 175 125
>40 >40 >40
2B 2B 2B
>32% >32% >32%
2 2 2
AE-10 AE-20 AE-30
20 20 20
100 106 71
>40 >40 >40
HB HB HB
>32% >32% >32%
2 2 2
AS-x: Alkyd with x-wt% alkoxysilane modified tung oil. F-x: Alkyd with x-wt% fluorine modified tung oil AE-x: Alkyd with x-wt% allyl ether modified tung oil.
Table 4 Coatings properties of neat alkyd resin and diluent/alkyd mixtures (cont.). Sample
Film thickness (micron)
Viscosity (CP)
20◦ Gloss
60◦ Gloss
Crosshatch adhesion
Neat Alkyd AS-10 AS-20 AS-30 F-10 F-20 F-30 AE-10 AE-20 AE-30
30 20 20 20 20 20 20 20 20 20
1670 1334 1085 868 1147 883 681.5 1056 748 490
161.2 164.2 173.8 172.3 165.2 167.1 163.6 170.0 170.2 161.5
150.7 153.9 157.9 156.3 152.9 155.9 153.9 158.7 161.2 149.7
5B 5B 5B 5B 5B 5B 5B 5B 5B 5B
AS-x: Alkyd with x-wt% alkoxysilane modified tung oil. F-x: Alkyd with x-wt% fluorine modified tung oil AE-x: Alkyd with x-wt% allyl ether modified tung oil.
the tensile moduli. Again, the highest tensile moduli are the alkoxy silane TOMAS at 10 and 20 wt%. However, at 30 wt% the alkoxysilane TOMAS and the allyl ether TOTAE have the same value. 3.2. Viscoelastic properties The viscoelastic properties of all systems are shown in Figs. 5 and 6. All of the films had narrow ␣-transitions in tan ı data (see Fig. 6) and modulus transitions (see Fig. 5) indicating a uniformity of the crosslinked network. For the alkoxy silane based TOMAS, all the ␣-transitions were higher than the alkyd. This was in agreement with the increase in storage moduli in the rubbery plateau. The allyl ether TOTAE exhibits the same behavior for the ␣-transition, however at 30 wt%, a decrease in the storage modulus is observed which is consistent with the tensile data. The storage moduli of the fluorinated TOF with 10 and 20 wt% loading exhibits a similar behavior to the TOTAE. Unlike the TOTAE, the tan ı data of the TOF is in agreement with the lower modulus at 30 wt%. The expression used to calculate the crosslink density is given in Eq. (1), where e is the number of moles of elastically effective chains per cubic centimeter of the film, E min is the minimum storage modulus in the rubbery plateau in N/m2 , R is the gas constant (8.314 N m/g mol K), and T is the absolute temperature in Kelvin. E min = 3e RT
(1)
The values for E min , e , and Tg can be found for all samples in Table 2. As expected, the crosslink density of the film did increase with addition of all three diluents, except for the mixture that contains 30 wt% FTO. The glass transition temperatures of all cured films containing the diluents are all significantly higher except for the formulation that contains 30 wt% FTO. There appears to be a maximum level of incorporation of the FTO that has been crossed, above which the effects on film properties are not beneficial to
the system. The increase in crosslink density and glass transition temperature at 10 and 20 wt% is due to the residual unreacted double bonds of the ␣-eleostearate which can react with itself and crosslink the alkyd. The increase in crosslink density for the TOMAS and the TOTAE series reflects the additional functionality of the trimethoxysilane and triallyl ether, respectively. The apparent inconsistency of tensile data, Tg , and crosslink data at >20 wt% can be explained by a transition from an alkyd dominated system which is crosslinked via pendent fatty acid groups to a reactive diluent dominated system. The film properties of oxidizing alkyds emanating from the rigidity of the polyester backbone balance with the eventual crosslinking of the pendent fatty acid groups. The molecular weight of the polyester backbone by itself is insufficient to build-up properties, so some level of chain extension is needed for the polyester backbone to build its molecular weight and hence its optimal properties (i.e. tensile versus molecular weight). The size, ability to diffuse, and multiple functionality of the reactive diluents are idea for building up the crosslink density, and as a consequence, the mechanical properties as well. After 20 wt% loading, the matrix begins to be dominated by the reactive diluent and not the polyester backbone of the alkyd. 3.3. General film properties Film properties of the formulated alkyds are presented in Tables 3 and 4. In addition to reducing viscosity, the diluents are expected to enhance the film properties of the alkyd films. Several trends can be noted and discussed about the behavior of the films. The pencil hardness of the films improved as predicted for the reactive diluents and remained the same for systems containing the fluorinated material. Solvent resistance, on the other hand, gave unexpected results. The MEK double rub test showed that coatings with TOMAS reduced the solvent resistance, whereas incorporation of FTO and TAETO does not significantly improve the solvent resistance of the coatings. However, one should take MEK rubs
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with a grain of salt. It is important to note that the thickness of the coating does affect the result of the solvent resistance test, in which the neat alkyd films were thicker than those films that contain the diluents. FTO generally exhibited relatively higher solvent resistance than SFTO and TAETO. This result may be due to the excellent chemical resistance of fluorinated groups in FTO or just a more frictionless surface. The remaining properties, flexibility and impact resistance, are not affected by the incorporation of the diluents. Additional coatings properties such as the gloss and crosshatch adhesion appear to not be significantly effected by the diluent content (see Table 4). The primary goal of this study was to replace a percentage of the organic solvent used in the alkyd-based coatings in order to reduce the volatile organics needed for application viscosity. Overall, modified tung oil diluents generally provided good film and coatings properties. Thus, the reduction of the viscosity without having any adverse effects on the coatings properties indicated the attainment of this goal. A previously published paper outlines the synthesis of the reactive diluents [13]. Drying time and viscosity data was also included. All the reactive diluents were shown to reduce the viscosity of the alkyd, and the TOTAE had a faster drying time than the others. The TOTAE however may have some difficult with the evolution of acrolein. It was presumed that the allyl ether groups increased the speed of the drying process. The Tg and crosslink density data support this supposition. The TOMAS was slower with respect to drying, but was expected to participate in the drying process. Again, the Tg and crosslink density data are supported of the alkoxysilane groups reacting and thus adding to the crosslink density. As a result, the tensile modulus, tensile strength, and hardness were also improved. The fluorine TOF generally showed relatively higher solvent resistance than the other diluents, which was moderately surprising. The TOF also participated in the film formation process as evidenced by the Tg and crosslink density data. By incorporating alkoxysilanes into these new diluents, hybrid inorganic–organic coatings can be formed through the sol–gel process. This process involves in situ polycondensation of metal or silicon alkoxide with an organic polymer matrix [15–17]. The inorganic microdomain is formed by hydrolysis, condensation, and gelation of the metal or silicon alkoxide in solution; while the organic polymer matrix of the drying oil forms cross-linking structure by an autoxidative process. Fluorinated polymers exhibit low inter- and intramolecular interactions. These characteristics lead to low cohesive energy, which in turn provides low surface energy. Moreover, fluorinated materials provide high thermal stability, chemical inertness, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, valuable electrical properties, and low dielectric constants and dissipation factors [18]. 4. Conclusions Incorporation of the diluents into alkyd-based coatings formulations has shown that the new functionalized tung oil derivatives act effectively as diluents. The viscosity of the alkyd
was significantly decreased with increased loading of the diluents, replacing the need for organic solvent in the formulation. All the reactive diluents participated in the film formation process. The allyl ether TOTAE and alkoxyl silane TOMAS provided additional crosslinking sites which were reflected in the tensile data as well as the overall film properties. However, addition of diluents shows no significant change in the elongation at break and impact resistance compared to the alkyd. The balance of the polyester backbone to the pendent fatty acid of the alkyd can be altered after >20 wt% of a triglyceride reactive diluent is added into the system. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.porgcoat.2011.03.017. References [1] Z.W. Wicks, F.N. Jones, P.S. Pappas, Organic Coatings: Science and Technology, 2nd ed., Wiley-Interscience, New York, 1992. [2] P. Swaraj, Surface Coatings Science and Technology, 2nd ed., John Wiley and Sons, New York, 1996. [3] F.N. Jones, Toward solventless liquid coatings, Journal of Coatings Technology 68 (852) (1996) 25–36. [4] W.J. Muizebelt, et al., Oxidative crosslinking of alkyd resins studied with mass spectrometry and NMR using model compounds, Journal of Coatings Technology 70 (876) (1998) p83. [5] G. Teng, et al., Spectroscopic investigation of the blowing process of soyabean oil, Surface Coatings International Part B: Coatings Transactions 86 (B3) (2003) 221–229. [6] N.M. Martyak, D. Alford, R. Picker, C. Dowling, Controlled oxidative curing of tung oil, Journal of Coatings Technology: Coatings Technology (2005 May) 36. [7] J. Mallegol, et al., Thermal (DSC) and chemical (iodometric titration) methods for peroxides measurements in order to monitor drying extent of alkyd resins, Progress in Organic Coatings 41 (1–3) (2001) 171–176. [8] D. Poortere, Radiocurable Composition, 1978, U C B. [9] F. Li, R.C. Larock, Synthesis, structure and properties of new tung oil–styrene–divinylbenzene copolymers prepared by thermal polymerization, Biomacromolecules 4 (4) (2003) 1018–1025. [10] H.W. Chatfield, Varnish Constituents, Interscience, New York, 1944. [11] W.J. Muizebelt, et al., Crosslink mechanisms of high-solids alkyd resins in the presence of reactive diluents, Progress in Organic Coatings 40 (1–4) (2000) 121–130. [12] W.J. Muizebelt, M.W.F. Nielen, Oxidative crosslinking of unsaturated fatty acids studied with mass spectrometry, Journal of Mass Spectrometry 31 (5) (1996) 545–554. [13] (a) K. Wutticharoenwong, M.D. Soucek, Synthesis of tung oil based reactive diluents, Macromolecular Materials and Engineering 295(12) (2011) 1097.; (b) K. Wutticharoenwong, Bio-based Reactive Diluents and Thiol-ene Photopolymerization for Environmentally Benign Coatings, Ph.D. Thesis, 2007. [14] H. Ni, et al., Alkoxysilane-modified polyurea coatings, Polymeric Materials Science and Engineering 81 (1999) 405–406. [15] C.R. Wold, M.D. Soucek, Mixed metal oxide inorganic/organic coatings, Journal of Coatings Technology 70 (882) (1998) 43–51. [16] S.J. Tuman, M.D. Soucek, Novel inorganic/organic coatings based on linseed oil and sunflower oil with sol–gel precursors, Journal of Coatings Technology 68 (854) (1996) 73–81. [17] R.L. Ballard, et al., Effects of an acid catalyst on the inorganic domain of inorganic–organic hybrid materials, Chemistry of Materials 11 (3) (1999) 726–735. [18] B. Ameduri, B. Boutevin, Well-architectured Fluoropolymers: Synthesis Properties and Applications, 1st ed., Elsevier, Amsterdam, 2004.