09.21.15
Abstract
Linear, soluble polymers were produced from unsaturated bio-based compounds using a carbocationic polymerization process. The unsaturated bio-based compounds that were first converted to vinyl ether monomers and subsequently polymerized were plant oil triglycerides, cardanol, and eugenol. As a result of the much higher reactivity of the vinyl ether group compared to the unsaturation derived from the bio-based compounds and the ability to tailor the cationic polymerization process, polymerization was exclusively limited to vinyl ether groups. By preserving the unsaturation derived from the bio-based compounds, the polymers could be crosslinked into insoluble coatings by autoxidation. In addition, the unsaturation can be converted to other functional groups, such as epoxy, which enable other crosslinking mechanisms. This document describes some of the polymers and coatings that have been produced with the technology.
Introduction
Prior to the ample supply of petrochemicals, coatings were largely derived from renewable resources such as plant oils, fats, plant proteins, polysaccharides, terpenes, and minerals.1 As a result of the low cost and tremendous diversity of petrochemicals, development of new coating components based on renewable/bio-based resources was largely abandoned. Due to concerns with the finite supply of fossil resources, geo-political events, the environment, and human health, the use of bio-based materials in the coatings industry is making a resurgence.
In general, technology innovation within the coatings industry has been largely driven by regulations aimed at protecting both the environment and human health. These regulations have historically been focused on the reduction of the VOC content of coatings. However, due to growing consumer demand for environmentally friendly products, the chemical and materials industries have been placing more emphasis on the complete environmental impact of products. The total environmental impact of a product or material is typically assessed by conducting a life cycle analysis.
Of the coating resin technologies utilized today, alkyd resins utilize a significant fraction of bio-based materials. Alkyd resins were developed in the mid-1920s primarily as a means to reduce the drying time of coatings based on drying oils such as linseed, tung, walnut, perilla, and poppy seed oil.2 Plant oil triglycerides are highly flexible molecules and, as a result, a significant degree of crosslinking is required for a drying oil-based coating to become dry-to-touch. With the availability of petrochemicals, aromatic monomers, such as phthalic anhydride and isophthalic acid, were used to produce polyesters modified with fatty acid ester chains derived from a plant oil. The higher glass transition temperature (Tg) of these polyesters, referred to as alkyds, enabled films to become dry-to-touch shortly after solvent evaporation from the film. Chemical resistance and film hardness were developed over time due to crosslinking by autoxidation.
The mechanism of the oxidative process, commonly referred to as autoxidation, is a free-radical process that possesses initiation, propagation, and termination steps.3-7 As shown in Figure 1, initiation occurs by abstraction of a bis-allylic hydrogen by singlet oxygen to produce the carbon-centered radical (I). This radical is delocalized over the pentadiene structure and reacts with oxygen to produce the peroxy radical and conjugation in the fatty acid ester chain (II). The peroxy radical can participate in a number of reactions including hydrogen abstraction to produce the hydroperoxide (III). The hydroperoxide is thermally unstable and can undergo hemolytic cleavage to produce an ether radical and a hydroxyl radical (IV). Crosslinks are formed primarily by radical coupling reactions that result in a variety of crosslinks including ether bonds, peroxide bonds, and carbon-carbon bonds.
The general classes of resins/polymers currently used in the coatings industry include epoxies, polyurethanes, alkyds, acrylics, polyesters, and amino resins. Of these, acrylics represent the highest volume of resins used in the coatings industry. The utility of acrylic resins can be largely attributed to the tremendous diversity in thermal and physiochemical properties that can be achieved through copolymerization. Most coating films derived from acrylic resins are thermoplastic and thus possess limited chemical and stain resistance. It has long been recognized that the incorporation of fatty acid ester chains into the pendent groups of acrylic resins would be a useful method for introducing crosslinks into coating films to provide enhanced properties. However, the incorporation of the linoleic and linolenic fatty acid esters needed for effective crosslinking into an acrylate or methacrylate monomer would be expected to be problematic due to the presence of the readily extractable bis-allylic hydrogen atoms. These bis-allylic hydrogen atoms would be expected to lead to extensive chain transfer and perhaps gelation during the polymerization. Further, radical addition to double bonds present in the fatty acid ester chains could also lead to gelation during polymerization.
In the last few decades, tremendous progress has been made in the carbocationic polymerization of vinyl monomers.8 Although carbocations are generally very reactive species, polymerization processes have been developed that enable very controlled polymerization. In fact, living carbocationic polymerization systems have been developed for a number of monomers including vinyl ethers, isobutylene, and styrene. The controlled reactivity of the propagation step with these living polymerization systems is generally believed to be the result of a propagation-step that involves an equilibrium between dormant and active species. In addition, the active carbocation is stabilized through interactions with the counter anion and/or interactions with Lewis base additives present in the polymerization system. A number of polymerization variables can be used to tailor the nature of a carbocationic polymerization including temperature, initiator composition, Lewis acid coinitiator composition and concentration, addition of a Lewis base, Lewis base composition and concentration, and solvent composition. This document provides an overview of a polymer technology that enables the production of novel poly(vinyl ether)s based on renewable materials and their application as coatings binders.
Polymers Based on Plant Oils
Using simple base-catalyzed transesterification of a vinyl ether alcohol with either a plant oil triglyceride or fatty alkyl ester, a novel vinyl ether monomer was produced.9 Figure 2 shows the synthetic process using 2-(vinyloxy)ethanol as the vinyl ether alcohol and methyl soyate as the fatty alkyl ester. As illustrated in Figure 2, this monomer, 2-(vinyloxy)ethyl soyate (2-VOES), is a mixture based on the fatty acid ester composition of methyl soyate. As illustrated in Figure 3, the polymerization system developed for these plant oil-based vinyl ether monomers involves the use of the addition product of isobutyl vinyl ether and acetic acid as the initiator, ethylaluminum sesquichloride as the coinitiator, and toluene as the solvent. Using this system, a living polymerization was achieved.10
For most carbocationic polymerizations of a vinyl ether produced using a Lewis Acid coinitiator, such as ethylaluminum sesquichloride, an appropriate concentration of Lewis base is needed to obtain a living polymerization. The mechanism of ‘Lewis-base assisted living cationic polymerization’ is believed to involve an equilibrium between dormant and active chain ends with the concentration of active chain ends being much lower than that of the dormant chain ends. The Lewis base is believed to reduce both the concentration and the reactivity of active chain ends. As described by Kanazawa et al.,11,12 the Lewis base: (1) complexes with the Lewis acid coinitiator resulting in the formation of monomeric Lewis acid species and an adjustment of acidity; (2) stabilizes active chains through direct interaction; and (3) stabilizes the counteranion generated upon initiation. For the polymerization of 2-VOES, it is believed that the ester group present in the monomer serves the role of a Lewis base additive typically utilized in a ‘Lewis base-assisted’ living carbocationic polymerization. Obtaining a living polymerization enabled control of polymer molecule weight, narrow molecular weight distribution polymers, and the production of block copolymers.13
To date, plant oil-based poly(vinyl ether) homopolymers have been produced using soybean oil (SBO), hydrogenated SBO (HSBO), corn oil (CO), and palm oil (PO) as the parent plant oil. As expected, the thermal properties of these novel polymers varied as a function of the oil or fatty methyl ester used to produce the monomer. For example, polymers based on SBO and CO were amorphous liquids at room temperature, while the polymer based on HSBO was a waxy solid. The solid nature of the latter can be attributed to the high chain packing efficiency of the saturated fatty acid ester pendent chains. While the polymers based on SBO and CO were liquids at room temperature, side chain crystallization was observed using differential scanning calorimetry (DSC). For example, a weak, broad endotherm with a peak maximum at -25 °C was observed for the SBO-based polymer. This polymer also displayed a Tg at -92 °C.9,13 Compared to the parent oil, i.e., SBO, the heat of fusion for the SBO-based polymer was much lower indicating that the higher viscosity and polymeric nature of the latter significantly inhibited fatty acid ester chain crystallization. The polymer based on PO showed a melting temperature just below room temperature, which is consistent with the relative content of saturated fatty acid ester chains compared to the polymers based on SBO and HSBO.14 The content of saturated fatty acid esters in PO is intermediate between that of SBO and HSBO.
As a result of the polymeric nature of the plant oil-based polymers, the number of bis-allylic protons, allylic protons, and double bonds per molecule is much higher than that of the parent oil. As a result, the degree of autoxidation needed to produce a crosslinked network is significantly reduced compared to the parent oil. In fact, it was demonstrated that a coating produced by simply blending titanium dioxide with an SBO-based polymer, i.e., poly(2-VOES), became tack-free in less than one tenth the time required for an analogous coating based on linseed oil to become tack-free.15 Thus, by converting a semi-drying oil, such as SBO, to a polymer, drying properties can be achieved that are far superior to that of a drying oil. This feature of the plant oil-based polymer technology also translates to other curing chemistries. For example, the double bonds in poly(2-VOES) were converted to epoxide groups using peracetic acid and crosslinked networks produced using an anhydride curing agent. To illustrate the relative difference in the time required to reach the gel-point for this network as compared to an analogous network based on epoxidized SBO (ESBO), rheological measurements were made as a function of time at 100 °C. For the epoxidized poly(2-VOES)/anhydride mixture, viscosity began to rise after just 20 min, while 2 h was required for the ESBO/anhydride mixture.13
A similar trend was also observed for a comparison between polyurethane networks based on a polyol derived from poly[(2-vinyloxy)ethyl palmitate] [poly(2-VOEP)] to analogous networks based on a polyol derived from PO. Hydroxy groups were incorporated into poly(2-VOEP) and PO by first epoxidizing the double bonds in the materials and then ring-opening the epoxide groups with methanol.14 In addition to providing a lower degree of functional group conversion to produce a crosslinked network, the molecular architecture of a plant oil-based polymer enables a significantly higher crosslink density compared to the triglyceride analog. This feature can be attributed to the methine carbon atoms present in the polymer backbone that function as additional crosslinks in the network when the material is cured.13 Further, cure shrinkage for crosslinked networks derived from a plant oil-based polymer would be expected to be lower than that for an analogous network based on the parent triglyceride simply because of the molecular weight difference between the two materials.
Probably the most useful aspect of the plant oil-based polymer technology is the ability to widely tailor properties through copolymerization. This was demonstrated using a number of comonomers including cyclohexyl vinyl ether (CHVE), menthol vinyl ether (MVE), and pentaethylene glycol ethyl vinyl ether (PEGEVE). Since homopolymers of the plant oil-based vinyl ethers possess a very low Tg, it was of interest to increase polymer Tg by copolymerization. As illustrated in Figure 4, both CHVE and MVE possess the cyclohexyl ring attached to the vinyl ether oxygen atom with MVE possessing a substituted ring. CHVE is commercially available, while MVE was synthesized in-house. MVE represents a potentially bio-based monomer since menthol is a naturally occurring terpene that can be obtained from the peppermint plant, Mentha x piperita (Lamiaceae).16 As expected, copolymerization of 2-VOES with these cycloaliphatic vinyl ether monomers enabled the formation of crosslinked films with increased Tgs compared to the control film based on the homopolymer of 2-VOES. Figure 5 shows the variation in Tg as a function of comonomer content for films cured at room temperature by autoxidation.17,18
Copolymerization of 2-VOES with PEGEVE was utilized as a means to provide dispersability of the polymer in water without the need for surfactant. The amphiphilic copolymers produced were shown to be surface active as determined by measuring critical micelle concentration.19 Three different copolymers were produced that varied with respect to PEGEVE repeat unit content. From these copolymers, aqueous dispersions were produced that also contained a water-based drier package. The solids content of the dispersions was 30 wt.% and all three copolymers gave stable dispersions. Figure 6a shows the variation in drying time with copolymer composition, while Figure 6b provides an image of a coating cast and cured at ambient conditions on a glass panel. As shown in Figure 6a, coatings cured relatively fast with the tack-free time decreasing with increasing 2-VOES repeat unit content. From Figure 6b, it can be seen that cured films had excellent optical clarity, which can be attributed to the lack of surfactant in the films.
Polymers Based on Cardanol
Cardanol is derived from cashew nut liquid, which is a byproduct of cashew nut processing.20 The primary component of cashew nut liquid is anacardic acid, which can be converted to cardanol by thermal decarboxylation. Cardanol is a mixture of four different meta-alkyl phenols that differ with respect to the degree of unsaturation in the alkyl side chain, as shown in Figure 7.21 As a result of the success obtained with the plant oil-based poly(vinyl ether)s described above, it was of interest to produce and characterize poly(vinyl ether)s containing cardanol units in the pendent chains of the repeat units. A novel vinyl ether monomer of cardanol, i.e., cardanol ethyl vinyl ether, was produced using the Williamson ether synthesis reaction shown in Figure 7. This monomer was successfully polymerized using the same polymerization system used for the production of the plant oil-based vinyl ethers. With this polymerization system, a soluble, tacky polymer was produced.
A solution of the homopolymer was combined with a drier package and films were cast on steel panels. Three different curing conditions were investigated. Curing at room temperature was done over a period of two weeks, while curing at 120 °C and 150 °C was done for one hour. For elevated temperature curing, coated panels were placed into the oven shortly after the solvent had evaporated from the film. In addition to coated steel panels, free film specimens were prepared by casting films over Teflon™-laminated glass panels. Table 1 displays the properties of the coatings and free films. In general, the coatings produced were relatively flexible and possessed sub-ambient Tgs. Although the coatings possessed sub-ambient Tgs, they exhibited good solvent resistance as expressed by the number of methyl ethyl ketone (MEK) double rubs.
Polymers Based on Eugenol
Eugenol is a major component of Ocimum, Cinnamon, and Clove oils.22 Eugenol is also a potential product from the breakdown of lignin, and approximately 50 million tons of lignin is produced annually from the pulp and paper industries worldwide.23
Analogous to the monomer based on cardanol, eugenol ethyl vinyl ether (EEVE) was produced from eugenol and 2-chloroethyl vinyl ether. This monomer was readily polymerized by cationic polymerization to produce a soluble polymer that was a viscous, tacky liquid at room temperature with a Tg of approximately 2 °C. Using proton nuclear magnetic resonance spectroscopy, preservation of the allyl group derived from eugenol was confirmed.
As illustrated in Figure 8, the methylene hydrogen atoms between the vinyl group and the phenyl group should be very labile to abstraction by singlet oxygen since the radical can be resonance stabilized by both the adjacent double bond and the phenyl ring. As a result, it was of interest to determine if poly(EEVE) could be cured into a crosslinked film by autoxidation. A poly(EEVE) sample with a number-average molecular weight of 17,900 g/mol was dissolved in toluene at 35 wt.%. To this solution, a drier package containing cobalt 2-ethylhexanoate, zirconium 2-ethylhexanoate, and zinc carboxylate was added. With this system, a cast film became dry-to-touch in 10 min, Coating specimens were produced using three different curing conditions, i.e., curing at room temperature for 3 weeks, 120 °C for 1 h, and 150 °C for 1 h. In addition to films cast on steel substrates, free films were produced and used to determine film mechanical and viscoelastic properties. Table 2 shows the properties obtained for the coatings and films produced.
As shown in Figure 9, the presence of the vinyl groups in poly(EEVE) enables the production of polyepoxide resins by simple oxidation using, for example, m-chloroperoxybenzoic acid. An epoxidized version of poly(EEVE) was produced and crosslinked films generated using diethylenetriamine (DETA) as the crosslinking agent and a 1/1 epoxy/NH ratio. For comparison purposes, crosslinked films of the diglycidyl ether of bisphenol-A (DGEBPA) were also produced using DETA. Figure 10 displays the viscoelastic properties of the network derived from epoxidized poly(EEVE) and DETA. From the tangent delta data, a Tg of 130 °C was determined. It was very interesting to observe this high of a Tg considering the relatively high molecular mobility of the poly(vinyl ether) polymer backbone. Obviously the very high crosslink density derived from the high number of epoxy groups per polymer molecule enables such a high Tg.
Table 3 provides a comparison of the properties of coatings cast and cured on steel substrates. As shown in Table 3, the hardness, flexibility, and adhesion of the coating based on epoxidized poly(EEVE) was similar to that of the analogous coating based on DGEBPA. The primary difference between these two coatings involved the chemical and impact resistance. The chemical resistance, as expressed using the MEK double rub test, was dramatically better for the coatings based on epoxidized poly(EEVE). After 1,000 MEK double rubs, no visible damage to the coating was observed. In contrast, the coating based on DGEBPA failed after 310 double rubs. With regard to impact resistance, the coating based on the epoxidized poly(EEVE) showed a lower impact resistance than the coating based on DGEBPA. The higher MEK resistance and lower impact resistance associated with the epoxidized poly(EEVE) is consistent with a higher crosslink density for this coating.
Conclusion
The results presented in this document demonstrate the utility of a carbocationic polymerization system for producing novel bio-based polymers that retain the unsaturation derived from the bio-based component. The high reactivity of the vinyl ether functional group and the appropriate choice of the polymerization system enabled linear polymers to be produced with relatively narrow molecular weight distributions. The ability to retain unsaturation from the bio-based component enabled the production of crosslinked coatings using autoxidation. The high number of allylic hydrogens and double bonds per molecule associated with these unsaturated poly(vinyl ether)s results in relatively fast curing by autoxidation due to the gel-point being reached at relatively low extents of reaction. Another important aspect of this polymer technology is the ability to utilize copolymerization to tailor polymer and coating properties. The unsaturation present in the polymer produced can also be easily converted to other functional groups, such as the epoxy group, to enable other crosslinking mechanisms. CW
Acknowledgement
The authors thank the Department of Energy (grant DE-FG36-08GO088160), United States Department of Agriculture/National Institute of Food and Agriculture (grant 2012-38202-19283), United Soybean Board, National Science Foundation (grants IIA-1330840, IIA-1355466, and IIP-1401801), and North Dakota Soybean Council for financial support.
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
References
1. Derksen, J.T.P.; Cuperus, F.P.; Kolster, P. Renewable resources in coatings technology: A review. Prog. Org. Coat. 1996, 27, 45-53.
2. Hofland, A. Alkyd resins: From down and out to alive and kicking. Prog. Org. Coat. 2012, 73, 274-282.
3. Mallegol, J.; Gardette, J.L.; Lemaire. J. Long-term behavior of oil-based varnishes and paints I. Spectroscopic analysis of curing drying oils. J. Am. Oil Chem. Soc. 1999, 76, 967-976.
4. Mallegol, J.; Lemaire, J.; Gardette, J.L. Drier influence on the curing of linseed oil. Prog.Org.Coat. 2000, 39, 107-113.
5. Mallegol, J.; Gardette, J.L.; Lemaire, J. Long-term behavior of oil-based varnishes and paints. Fate of hydroperoxides in drying oils. J. Am. Oil Chem. Soc. 2000, 77, 249-255.
6. Mallegol, J.; Gardette, J.L.; Lemaire, J. Long-term behavior of oil-based varnishes and paints. Photo- and thermooxidation of cured linseed oil. J. Am. Oil Chem. Soc. 2000, 77, 257-263.
7. Soucek, M.D.; Khattab, T.; Wu, J. Review of autoxidation and driers. Prog. Org. Coat. 2012, 73, 435-454.
8. Aoshima, S.; Kanaoka, S. A renaissance in living cationic polymerization. Chem. Rev. 2009, 109, 5245-5287.
9. Alam, S.; Chisholm, B.J. Coatings derived from novel, soybean oil-based polymers produced using carbocationic polymerization. J. Coat. Technol. Res. 2011, 8, 671-683.
10. Chernykh, A.; Alam, S.; Jayasooriya, A.; Bahr, J.; Chisholm, B.J. Living carbocationic polymerization of a vinyl ether monomer derived from soybean oil, 2-(vinyloxy)ethyl soyate. Green Chem. 2013, 15, 1834-1838.
11. Kanazawa, A.; Kanaoka, S.; Aoshima, S. Major progress in catalysts for living cationic polymerization of isobutyl vinyl ether: Effectiveness of a variety of conventional metal halides. Macromolecules 2009, 42, 3965-3972.
12. Aoshima, S.; Yoshida, T.; Kanazawa, A.; Kanaoka, S. New stage in living cationic polymerization: An array of effective Lewis Acid catalysts and fast living polymerization in seconds. J. Polym. Sci. Part A: Polym. Chem. Ed. 2007, 45, 1801-1813.
13. Alam, S.; Kalita, H.; Jayasooriya, A.; Samanta, S.; Bahr, J.; Chernykh, A.; Weiz, M.; Chisholm, B.J.; 2-(Vinyloxy)ethyl soyate as a versatile platform chemical for coatings: An overview. Eur. J. Lipid Sci. Technol. 2014, 116, 2-15.
14. Kalita, H.; Jayasooriya, A.; Fernando, S.; Chisholm, B.J. Novel high molecular weight polymers based on palm oil. J. Oil Palm Res. in press (2015).
15. Kalita, H.; Alam,S.; Kalita, D.; Chernykh, A.; Tarnavchyk, I.; Bahr, J.; Samanta, S.; Jayasooriya, A.; Fernando, S.; Selvakumar, S.; Popadyuk, A.; Wickramaratne, D.S.; Sibi, M.; Voronov, A.; Bezbaruah, A.; Chisholm, B.J. Synthesis and characterization of novel soybean oil-based polymers and their application in coatings cured by autoxidation. In Soy-based Chemicals and Materials, in press (2015).
16. Farco, J. A.; Grundmann, O. Menthol--pharmacology of an important naturally medicinal “cool.” Mini Rev. Med. Chem. 2013, 13, 124-131.
17. Kalita, H.; Selvakumar, S.; Jayasooriya, A.; Fernando, S.; Samanta, S.; Bahr, J.; Alam, S.; Sibi, M.; Vold, J.; Ulven, C.; Chisholm, B.J. Novel Bio-Based Poly(vinyl ether)s and Their Application as Alkyd-Type Surface Coatings. Green Chem. 2014, 16, 1974-1986.
18. Kalita, H.; Alam, S.; Jayasooriya, A.; Fernando, S.; Samanta, S.; Bahr, J.; Selvakumar, S.; Sibi, M.; Vold, J.; Ulven, C.; Chisholm, B. J. Novel alkyd-type coating resins produced using cationic polymerization. J. Coat. Technol. Res., submitted for publication (2015).
19. Alam, S.; Kalita, H.; Kudina, O.; Popadyuk, A.; Chisholm, B.J.; Voronov, A. Soy-based surface active copolymers as a safer replacement for low molecular weight surfactants. ACS Sustainable Chem. Eng. 2013, 1, 19-22.
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[1] For a more detailed discussion and additional material see Biobased and Environmentally Benign Coatings, edited by Atul Tiwari, Anthony Galanis and Mark D. Soucek, Wiley-Scrivener, 2016.
This paper was a keynote presentation at the 42nd Annual International Waterborne Symposium, which was held February 8-13 in New Orleans, LA. The Sympoisum is hosted by the The School of Polymers and High Performance Materials at The University of Southern Mississippi.
Linear, soluble polymers were produced from unsaturated bio-based compounds using a carbocationic polymerization process. The unsaturated bio-based compounds that were first converted to vinyl ether monomers and subsequently polymerized were plant oil triglycerides, cardanol, and eugenol. As a result of the much higher reactivity of the vinyl ether group compared to the unsaturation derived from the bio-based compounds and the ability to tailor the cationic polymerization process, polymerization was exclusively limited to vinyl ether groups. By preserving the unsaturation derived from the bio-based compounds, the polymers could be crosslinked into insoluble coatings by autoxidation. In addition, the unsaturation can be converted to other functional groups, such as epoxy, which enable other crosslinking mechanisms. This document describes some of the polymers and coatings that have been produced with the technology.
Introduction
Prior to the ample supply of petrochemicals, coatings were largely derived from renewable resources such as plant oils, fats, plant proteins, polysaccharides, terpenes, and minerals.1 As a result of the low cost and tremendous diversity of petrochemicals, development of new coating components based on renewable/bio-based resources was largely abandoned. Due to concerns with the finite supply of fossil resources, geo-political events, the environment, and human health, the use of bio-based materials in the coatings industry is making a resurgence.
In general, technology innovation within the coatings industry has been largely driven by regulations aimed at protecting both the environment and human health. These regulations have historically been focused on the reduction of the VOC content of coatings. However, due to growing consumer demand for environmentally friendly products, the chemical and materials industries have been placing more emphasis on the complete environmental impact of products. The total environmental impact of a product or material is typically assessed by conducting a life cycle analysis.
Of the coating resin technologies utilized today, alkyd resins utilize a significant fraction of bio-based materials. Alkyd resins were developed in the mid-1920s primarily as a means to reduce the drying time of coatings based on drying oils such as linseed, tung, walnut, perilla, and poppy seed oil.2 Plant oil triglycerides are highly flexible molecules and, as a result, a significant degree of crosslinking is required for a drying oil-based coating to become dry-to-touch. With the availability of petrochemicals, aromatic monomers, such as phthalic anhydride and isophthalic acid, were used to produce polyesters modified with fatty acid ester chains derived from a plant oil. The higher glass transition temperature (Tg) of these polyesters, referred to as alkyds, enabled films to become dry-to-touch shortly after solvent evaporation from the film. Chemical resistance and film hardness were developed over time due to crosslinking by autoxidation.
The mechanism of the oxidative process, commonly referred to as autoxidation, is a free-radical process that possesses initiation, propagation, and termination steps.3-7 As shown in Figure 1, initiation occurs by abstraction of a bis-allylic hydrogen by singlet oxygen to produce the carbon-centered radical (I). This radical is delocalized over the pentadiene structure and reacts with oxygen to produce the peroxy radical and conjugation in the fatty acid ester chain (II). The peroxy radical can participate in a number of reactions including hydrogen abstraction to produce the hydroperoxide (III). The hydroperoxide is thermally unstable and can undergo hemolytic cleavage to produce an ether radical and a hydroxyl radical (IV). Crosslinks are formed primarily by radical coupling reactions that result in a variety of crosslinks including ether bonds, peroxide bonds, and carbon-carbon bonds.
The general classes of resins/polymers currently used in the coatings industry include epoxies, polyurethanes, alkyds, acrylics, polyesters, and amino resins. Of these, acrylics represent the highest volume of resins used in the coatings industry. The utility of acrylic resins can be largely attributed to the tremendous diversity in thermal and physiochemical properties that can be achieved through copolymerization. Most coating films derived from acrylic resins are thermoplastic and thus possess limited chemical and stain resistance. It has long been recognized that the incorporation of fatty acid ester chains into the pendent groups of acrylic resins would be a useful method for introducing crosslinks into coating films to provide enhanced properties. However, the incorporation of the linoleic and linolenic fatty acid esters needed for effective crosslinking into an acrylate or methacrylate monomer would be expected to be problematic due to the presence of the readily extractable bis-allylic hydrogen atoms. These bis-allylic hydrogen atoms would be expected to lead to extensive chain transfer and perhaps gelation during the polymerization. Further, radical addition to double bonds present in the fatty acid ester chains could also lead to gelation during polymerization.
In the last few decades, tremendous progress has been made in the carbocationic polymerization of vinyl monomers.8 Although carbocations are generally very reactive species, polymerization processes have been developed that enable very controlled polymerization. In fact, living carbocationic polymerization systems have been developed for a number of monomers including vinyl ethers, isobutylene, and styrene. The controlled reactivity of the propagation step with these living polymerization systems is generally believed to be the result of a propagation-step that involves an equilibrium between dormant and active species. In addition, the active carbocation is stabilized through interactions with the counter anion and/or interactions with Lewis base additives present in the polymerization system. A number of polymerization variables can be used to tailor the nature of a carbocationic polymerization including temperature, initiator composition, Lewis acid coinitiator composition and concentration, addition of a Lewis base, Lewis base composition and concentration, and solvent composition. This document provides an overview of a polymer technology that enables the production of novel poly(vinyl ether)s based on renewable materials and their application as coatings binders.
Polymers Based on Plant Oils
Using simple base-catalyzed transesterification of a vinyl ether alcohol with either a plant oil triglyceride or fatty alkyl ester, a novel vinyl ether monomer was produced.9 Figure 2 shows the synthetic process using 2-(vinyloxy)ethanol as the vinyl ether alcohol and methyl soyate as the fatty alkyl ester. As illustrated in Figure 2, this monomer, 2-(vinyloxy)ethyl soyate (2-VOES), is a mixture based on the fatty acid ester composition of methyl soyate. As illustrated in Figure 3, the polymerization system developed for these plant oil-based vinyl ether monomers involves the use of the addition product of isobutyl vinyl ether and acetic acid as the initiator, ethylaluminum sesquichloride as the coinitiator, and toluene as the solvent. Using this system, a living polymerization was achieved.10
For most carbocationic polymerizations of a vinyl ether produced using a Lewis Acid coinitiator, such as ethylaluminum sesquichloride, an appropriate concentration of Lewis base is needed to obtain a living polymerization. The mechanism of ‘Lewis-base assisted living cationic polymerization’ is believed to involve an equilibrium between dormant and active chain ends with the concentration of active chain ends being much lower than that of the dormant chain ends. The Lewis base is believed to reduce both the concentration and the reactivity of active chain ends. As described by Kanazawa et al.,11,12 the Lewis base: (1) complexes with the Lewis acid coinitiator resulting in the formation of monomeric Lewis acid species and an adjustment of acidity; (2) stabilizes active chains through direct interaction; and (3) stabilizes the counteranion generated upon initiation. For the polymerization of 2-VOES, it is believed that the ester group present in the monomer serves the role of a Lewis base additive typically utilized in a ‘Lewis base-assisted’ living carbocationic polymerization. Obtaining a living polymerization enabled control of polymer molecule weight, narrow molecular weight distribution polymers, and the production of block copolymers.13
To date, plant oil-based poly(vinyl ether) homopolymers have been produced using soybean oil (SBO), hydrogenated SBO (HSBO), corn oil (CO), and palm oil (PO) as the parent plant oil. As expected, the thermal properties of these novel polymers varied as a function of the oil or fatty methyl ester used to produce the monomer. For example, polymers based on SBO and CO were amorphous liquids at room temperature, while the polymer based on HSBO was a waxy solid. The solid nature of the latter can be attributed to the high chain packing efficiency of the saturated fatty acid ester pendent chains. While the polymers based on SBO and CO were liquids at room temperature, side chain crystallization was observed using differential scanning calorimetry (DSC). For example, a weak, broad endotherm with a peak maximum at -25 °C was observed for the SBO-based polymer. This polymer also displayed a Tg at -92 °C.9,13 Compared to the parent oil, i.e., SBO, the heat of fusion for the SBO-based polymer was much lower indicating that the higher viscosity and polymeric nature of the latter significantly inhibited fatty acid ester chain crystallization. The polymer based on PO showed a melting temperature just below room temperature, which is consistent with the relative content of saturated fatty acid ester chains compared to the polymers based on SBO and HSBO.14 The content of saturated fatty acid esters in PO is intermediate between that of SBO and HSBO.
As a result of the polymeric nature of the plant oil-based polymers, the number of bis-allylic protons, allylic protons, and double bonds per molecule is much higher than that of the parent oil. As a result, the degree of autoxidation needed to produce a crosslinked network is significantly reduced compared to the parent oil. In fact, it was demonstrated that a coating produced by simply blending titanium dioxide with an SBO-based polymer, i.e., poly(2-VOES), became tack-free in less than one tenth the time required for an analogous coating based on linseed oil to become tack-free.15 Thus, by converting a semi-drying oil, such as SBO, to a polymer, drying properties can be achieved that are far superior to that of a drying oil. This feature of the plant oil-based polymer technology also translates to other curing chemistries. For example, the double bonds in poly(2-VOES) were converted to epoxide groups using peracetic acid and crosslinked networks produced using an anhydride curing agent. To illustrate the relative difference in the time required to reach the gel-point for this network as compared to an analogous network based on epoxidized SBO (ESBO), rheological measurements were made as a function of time at 100 °C. For the epoxidized poly(2-VOES)/anhydride mixture, viscosity began to rise after just 20 min, while 2 h was required for the ESBO/anhydride mixture.13
A similar trend was also observed for a comparison between polyurethane networks based on a polyol derived from poly[(2-vinyloxy)ethyl palmitate] [poly(2-VOEP)] to analogous networks based on a polyol derived from PO. Hydroxy groups were incorporated into poly(2-VOEP) and PO by first epoxidizing the double bonds in the materials and then ring-opening the epoxide groups with methanol.14 In addition to providing a lower degree of functional group conversion to produce a crosslinked network, the molecular architecture of a plant oil-based polymer enables a significantly higher crosslink density compared to the triglyceride analog. This feature can be attributed to the methine carbon atoms present in the polymer backbone that function as additional crosslinks in the network when the material is cured.13 Further, cure shrinkage for crosslinked networks derived from a plant oil-based polymer would be expected to be lower than that for an analogous network based on the parent triglyceride simply because of the molecular weight difference between the two materials.
Probably the most useful aspect of the plant oil-based polymer technology is the ability to widely tailor properties through copolymerization. This was demonstrated using a number of comonomers including cyclohexyl vinyl ether (CHVE), menthol vinyl ether (MVE), and pentaethylene glycol ethyl vinyl ether (PEGEVE). Since homopolymers of the plant oil-based vinyl ethers possess a very low Tg, it was of interest to increase polymer Tg by copolymerization. As illustrated in Figure 4, both CHVE and MVE possess the cyclohexyl ring attached to the vinyl ether oxygen atom with MVE possessing a substituted ring. CHVE is commercially available, while MVE was synthesized in-house. MVE represents a potentially bio-based monomer since menthol is a naturally occurring terpene that can be obtained from the peppermint plant, Mentha x piperita (Lamiaceae).16 As expected, copolymerization of 2-VOES with these cycloaliphatic vinyl ether monomers enabled the formation of crosslinked films with increased Tgs compared to the control film based on the homopolymer of 2-VOES. Figure 5 shows the variation in Tg as a function of comonomer content for films cured at room temperature by autoxidation.17,18
Copolymerization of 2-VOES with PEGEVE was utilized as a means to provide dispersability of the polymer in water without the need for surfactant. The amphiphilic copolymers produced were shown to be surface active as determined by measuring critical micelle concentration.19 Three different copolymers were produced that varied with respect to PEGEVE repeat unit content. From these copolymers, aqueous dispersions were produced that also contained a water-based drier package. The solids content of the dispersions was 30 wt.% and all three copolymers gave stable dispersions. Figure 6a shows the variation in drying time with copolymer composition, while Figure 6b provides an image of a coating cast and cured at ambient conditions on a glass panel. As shown in Figure 6a, coatings cured relatively fast with the tack-free time decreasing with increasing 2-VOES repeat unit content. From Figure 6b, it can be seen that cured films had excellent optical clarity, which can be attributed to the lack of surfactant in the films.
Polymers Based on Cardanol
Cardanol is derived from cashew nut liquid, which is a byproduct of cashew nut processing.20 The primary component of cashew nut liquid is anacardic acid, which can be converted to cardanol by thermal decarboxylation. Cardanol is a mixture of four different meta-alkyl phenols that differ with respect to the degree of unsaturation in the alkyl side chain, as shown in Figure 7.21 As a result of the success obtained with the plant oil-based poly(vinyl ether)s described above, it was of interest to produce and characterize poly(vinyl ether)s containing cardanol units in the pendent chains of the repeat units. A novel vinyl ether monomer of cardanol, i.e., cardanol ethyl vinyl ether, was produced using the Williamson ether synthesis reaction shown in Figure 7. This monomer was successfully polymerized using the same polymerization system used for the production of the plant oil-based vinyl ethers. With this polymerization system, a soluble, tacky polymer was produced.
A solution of the homopolymer was combined with a drier package and films were cast on steel panels. Three different curing conditions were investigated. Curing at room temperature was done over a period of two weeks, while curing at 120 °C and 150 °C was done for one hour. For elevated temperature curing, coated panels were placed into the oven shortly after the solvent had evaporated from the film. In addition to coated steel panels, free film specimens were prepared by casting films over Teflon™-laminated glass panels. Table 1 displays the properties of the coatings and free films. In general, the coatings produced were relatively flexible and possessed sub-ambient Tgs. Although the coatings possessed sub-ambient Tgs, they exhibited good solvent resistance as expressed by the number of methyl ethyl ketone (MEK) double rubs.
Polymers Based on Eugenol
Eugenol is a major component of Ocimum, Cinnamon, and Clove oils.22 Eugenol is also a potential product from the breakdown of lignin, and approximately 50 million tons of lignin is produced annually from the pulp and paper industries worldwide.23
Analogous to the monomer based on cardanol, eugenol ethyl vinyl ether (EEVE) was produced from eugenol and 2-chloroethyl vinyl ether. This monomer was readily polymerized by cationic polymerization to produce a soluble polymer that was a viscous, tacky liquid at room temperature with a Tg of approximately 2 °C. Using proton nuclear magnetic resonance spectroscopy, preservation of the allyl group derived from eugenol was confirmed.
As illustrated in Figure 8, the methylene hydrogen atoms between the vinyl group and the phenyl group should be very labile to abstraction by singlet oxygen since the radical can be resonance stabilized by both the adjacent double bond and the phenyl ring. As a result, it was of interest to determine if poly(EEVE) could be cured into a crosslinked film by autoxidation. A poly(EEVE) sample with a number-average molecular weight of 17,900 g/mol was dissolved in toluene at 35 wt.%. To this solution, a drier package containing cobalt 2-ethylhexanoate, zirconium 2-ethylhexanoate, and zinc carboxylate was added. With this system, a cast film became dry-to-touch in 10 min, Coating specimens were produced using three different curing conditions, i.e., curing at room temperature for 3 weeks, 120 °C for 1 h, and 150 °C for 1 h. In addition to films cast on steel substrates, free films were produced and used to determine film mechanical and viscoelastic properties. Table 2 shows the properties obtained for the coatings and films produced.
As shown in Figure 9, the presence of the vinyl groups in poly(EEVE) enables the production of polyepoxide resins by simple oxidation using, for example, m-chloroperoxybenzoic acid. An epoxidized version of poly(EEVE) was produced and crosslinked films generated using diethylenetriamine (DETA) as the crosslinking agent and a 1/1 epoxy/NH ratio. For comparison purposes, crosslinked films of the diglycidyl ether of bisphenol-A (DGEBPA) were also produced using DETA. Figure 10 displays the viscoelastic properties of the network derived from epoxidized poly(EEVE) and DETA. From the tangent delta data, a Tg of 130 °C was determined. It was very interesting to observe this high of a Tg considering the relatively high molecular mobility of the poly(vinyl ether) polymer backbone. Obviously the very high crosslink density derived from the high number of epoxy groups per polymer molecule enables such a high Tg.
Table 3 provides a comparison of the properties of coatings cast and cured on steel substrates. As shown in Table 3, the hardness, flexibility, and adhesion of the coating based on epoxidized poly(EEVE) was similar to that of the analogous coating based on DGEBPA. The primary difference between these two coatings involved the chemical and impact resistance. The chemical resistance, as expressed using the MEK double rub test, was dramatically better for the coatings based on epoxidized poly(EEVE). After 1,000 MEK double rubs, no visible damage to the coating was observed. In contrast, the coating based on DGEBPA failed after 310 double rubs. With regard to impact resistance, the coating based on the epoxidized poly(EEVE) showed a lower impact resistance than the coating based on DGEBPA. The higher MEK resistance and lower impact resistance associated with the epoxidized poly(EEVE) is consistent with a higher crosslink density for this coating.
Conclusion
The results presented in this document demonstrate the utility of a carbocationic polymerization system for producing novel bio-based polymers that retain the unsaturation derived from the bio-based component. The high reactivity of the vinyl ether functional group and the appropriate choice of the polymerization system enabled linear polymers to be produced with relatively narrow molecular weight distributions. The ability to retain unsaturation from the bio-based component enabled the production of crosslinked coatings using autoxidation. The high number of allylic hydrogens and double bonds per molecule associated with these unsaturated poly(vinyl ether)s results in relatively fast curing by autoxidation due to the gel-point being reached at relatively low extents of reaction. Another important aspect of this polymer technology is the ability to utilize copolymerization to tailor polymer and coating properties. The unsaturation present in the polymer produced can also be easily converted to other functional groups, such as the epoxy group, to enable other crosslinking mechanisms. CW
Acknowledgement
The authors thank the Department of Energy (grant DE-FG36-08GO088160), United States Department of Agriculture/National Institute of Food and Agriculture (grant 2012-38202-19283), United Soybean Board, National Science Foundation (grants IIA-1330840, IIA-1355466, and IIP-1401801), and North Dakota Soybean Council for financial support.
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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[1] For a more detailed discussion and additional material see Biobased and Environmentally Benign Coatings, edited by Atul Tiwari, Anthony Galanis and Mark D. Soucek, Wiley-Scrivener, 2016.
This paper was a keynote presentation at the 42nd Annual International Waterborne Symposium, which was held February 8-13 in New Orleans, LA. The Sympoisum is hosted by the The School of Polymers and High Performance Materials at The University of Southern Mississippi.
