An Alternative to Alkyds Novel Biobased Polymers for Coatings Applications

Technical Paper Biopolymers An alternative to alkyds Novel biobased polymers for coatings applications

Corresponding author: Harjoyti Kalita Mukund Sibi Metal-based catalysts have been developed to increase the Dr Bret J. Chisholm Deep Kalita Bret J. Chisholm overall rate of autoxidation. Examples of these catalysts, NDSU Center for referred to as driers, include carboxylates based on metals Nanoscale Science and Engineering High molecular weight linear polymers have been such as cobalt, manganese, iron, vanadium, lead and zirco- [email protected] produced based on soybean oil and possessing a nium. Even with an optimised drier package, the curing rate fatty acid ester side chain in the repeat unit. The of drying oils is often undesirably long for many coating ap- unsaturation of the parent plant oil can be pre- plications. served, allowing coatings to cure by autoxidation, This problem can be at least partly attributed to the high or be converted to other functional groups to permit molecular mobility of plant oil triglycerides. This means that the use of other curing mechanisms. a substantial level of autoxidation is required to produce a film that can be touched and handled without affecting the rior to the ample supply of low cost petrochemicals, appearance or damaging the coating. To overcome this is- european plant oils were used extensively as binders for paints sue, alkyd resins were developed. coatings and coatings. The utility of plant oil triglycerides as coat- handbook P ing binders stems from the unsaturated fatty acid esters that

Book tip Alkyds and their modifications outlined Brock, provide a means to crosslink the liquid plant oil into an in- Groteklaes, soluble film by simply exposing the thin film of oil to the Alkyd resins are polyester oligomers containing fatty acid es- Mischke atmosphere. ter residues that enable crosslinking via autoxidation as well www.european- The crosslinking mechanism is a complex free-radical pro- as aromatic moieties that function to increase glass transi- coatings.com/ cess, referred to as autoxidation, and involves abstraction tion temperature (Tg) [5]. Increasing the Tg with aromatic books of bis-allylic hydrogen atoms by singlet oxygen present in monomers allows the coating film to become dry-to-touch the atmosphere [1-4]. Since the concentration of bis-allylic with little to no crosslinking required. With alkyd coatings, hydrogens is critical to the autoxidation process, only plant especially those based on soybean oil, chemical resistance oils with relatively high levels of linoleic and linolenic fatty is generally limited and develops slowly with time. Besides acid esters are useful, such as linseed, tung, poppy seed and enabling curing via autoxidation, the unsaturation in plant walnut oil. oils has also been used to incorporate other functional Plant oils such as these are referred to as ‘drying oils.’ Soy- groups into the triglyceride. For example, epoxy groups can bean oil (SBO), which is the plant oil currently produced in be easily introduced into unsaturated plant oils by oxidising the highest volume in North America, is referred to as ‘semi- the unsaturation to epoxy groups using, for example, per- drying’ since it is capable of autoxidising to a cured film, but acetic acid as the oxidant [6, 7]. From the epoxy-functional the time required to do so is too long to be useful for paints plant oil, other functional groups can be incorporated using and coatings. epoxy ring-opening reactions, such as ring-opening with acrylic acid to produce an acrylate-functional material or ring-opening with methanol to produce a hydroxy-functional material. Acrylate-functional plant oils are useful for the production of radiation-curable coatings, and hydroxy-functional ones for polyurethane coatings [8-11]. base Novel technology yields R = fatty alkyl group derived highly unsaturated polymers from soybean oil The authors have developed a polymer technology Figure 1: Schematic showing the production of a vinyl ether monomer from SBO based on plant oils that allows the production of linear using methyl soyate as the plant oil-based component polymers with fatty acid ester side chains [12]. The technology involves the production of vinyl ether monomers from either the plant oil triglyceride or alkyl esters derived from a plant oil using simple base-catalysed trans- esterification. Figure 1 provides a schematic describing the production Figure 2: An of a vinyl ether monomer from SBO using methyl soyate illustration of the Soybean oil Poly [(2-vinyloxy)ethyl soyate] as the plant oil-based component. This particular mono- difference in mo- mer is 2-(vinyloxy) ethylsoyate (2-VOES). By using the ap- lecular architecture = fatty acid ester chain propriate polymerisation system, a living polymerisation

between SBO and = double bond was produced for this monomer using a cationic poly- poly(2-VOES) merisation mechanism.

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Since the reactivity of the vinyl ether double bond is much higher towards relatively stabilised carbocations than that of the internal double bonds derived from SBO, all of the unsaturation derived from the SBO is preserved in the polymer. This unsaturation can then be utilised to produce coating films either directly or by derivatisation to other functional groups. The value of various plant oil- based polymers such as these for coating applications is outlined below. Figure 2 illustrates the difference in molecular architecture between SBO and poly(2-VOES). While SBO pos- Figure 3: Compari- sesses three fatty acid esters per molecule, poly(2-VOES) son of the colour possess tens to hundreds of fatty acid esters per mole- of poly(2-VOES) de- cule. Due to the dramatically higher number of allylic hy- Linseed oil Poly(2-VOES) from rived from vacuum drogens per molecule associated with poly(2-VOES), the distilled monomer distilled 2-VOES to extent of radical coupling reactions needed to reach the linseed oil gel point is much lower for poly(2-VOES) than for SBO. A marked difference in tack-free time was obtained Figure 4: Differ- when a drier package was added to coatings derived ence in colour from poly(2-VOES) and SBO. Even with an autoxidation between a paint catalyst, it took 40 hours – almost two days – for SBO to based on poly(2- convert from a liquid film to a tack-free film at ambient VOES) derived from conditions. In contrast, poly(2-VOES) became tack-free in vacuum distilled 6.1 hours [13, 14]. This result clearly shows the benefit 2-VOES and an of increasing the number of fatty acid ester groups per analogous paint molecule by converting the SBO to a polymer. Coating based on poly Coating based on linseed oil based on linseed (2-VOES) derived from oil (both contained distilled 2-VOES rutile TiO as the Purified monomer 2 yields colourless polymer pigment) As discussed above, prior to the petrochemical revolu- tion, most paints were mixtures of a drying oil and pig- ments. While these paints and coatings have been large- colour of the oil, which is problematic for the production ly replaced with petrochemical-derived coatings, plant oil of white or pale coloured paints [15]. based paints are still used by many artists. Linseed oil in It was demonstrated that essentially colourless poly(2- particular possesses a relatively high concentration of li- VOES) can be produced by simply purifying 2-VOES by nolenic acid esters that allow curing without the addition vacuum distillation prior to polymerisation. Figure 3 pro- of a drier. Its significant drawback is the strong yellow vides a comparison of the colour of poly(2-VOES) derived from vacuum distilled 2-VOES to linseed oil. White paints were prepared from both linseed oil and the colourless poly(2-VOES) and the appearance and drying time de- termined. Results at a glance Figure 4 shows the difference in colour between the two paints produced using rutile titanium dioxide as the white A plant oil-based polymer technology was de- pigment. The image displayed in Figure 4 clearly shows veloped that enables linear, high molecular weight that the coating derived from the colourless poly(2-VOES) polymers to be produced possessing a fatty acid was essentially a pure white, while that derived from ester side chain in the repeat unit. linseed oil was clearly an off-white that appears more a beige or cream colour than pure white. The unsaturation derived from the parent plant With regard to drying time, the linseed oil-based paint oil can be preserved, allowing air-drying coatings took 316 hours to become dry-to-touch, while the poly(2- crosslinking by autoxidation to be produced using VOES)-based paint only took 26 hours. It should be noted soybean oil as the starting material. Drying is much that neither the poly(2-VOES)-based paint nor the linseed faster than that of the oil itself. oil based paint contained a drier to catalyse the curing process. It is very interesting that poly(2-VOES) cures Alternatively, the unsaturation can be converted quite fast even without a drier package. to other functional groups such as epoxy, acrylate, methacrylate or hydroxyl groups that allow other Copolymerisation extends curing mechanisms to be used. range of properties Purification of the initial monomer allows essentially A major advantage of this plant oil polymer technol- colourless resins to be produced from soybean oil. ogy is that copolymerisation can be utilised to dramatically change polymer thermal, physical, and mechanical

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Figure 5: Tangent delta response obtained from dynamic mechanical analysis for films derived from poly (2-VOES) copolymers based on CHVE or MVE as the comonomer; films were cured at ambient temperature

properties. Since cured films of poly(2-VOES) possess Tgs below 0 °C, it was of interest to investigate the use of copolymerisation to increase the Tg of cured films. Two comonomers were initially investigated, namely, cyclohexyl vinyl ether (CHVE) and menthol vinyl ether (MVE) [16, 17]. The cycloaliphatic pendent groups associated with these monomers were expected to substan- Figure 6: Varia- tially increase both polymer Tg and coating film Tg. tion in solvent Figure 5 displays the tangent delta response obtained resistance for from dynamic mechanical analysis for films cured at am- films derived bient temperature. As can be seen, the Tg of the cured from poly(2-VOES) networks increased with increasing comonomer content. copolymers based Figure 6 shows the variation in solvent resistance for the on CHVE or MVE as same films. The coatings derived from the copolymers the comonomer; were found to have somewhat lower solvent resistance films were cured at than the coating based on poly(2-VOES), which can be at- ambient tempera- tributed to the lower crosslink density of the copolymer- ture based films stemming from the lower concentration of fatty acid ester pendent groups.

Polyols for PU coatings give high crosslink density As discussed initially, the unsaturation in plant oils can be utilised to incorporate other functional groups that provide Figure 7: Com- alternative curing chemistries. For example, polyurethane parison of storage coatings were produced by converting a poly(2-VOES) sam- moduli for poly- ple to a polyol [HO-poly(2-VOES)] by first epoxidising the urethane networks double bonds of poly(2-VOES) with peracetic acid and then derived from ring-opening the epoxy groups with methanol. HO-poly(2-VOES) Polyurethane clear coatings were produced from the HO- and HO-SBO (data poly(2-VOES) by simply solution blending the polyol with reproduced from an isocyanate (isophorone diisocyanate trimer) and casting Alam and co- films from the solution. For comparison purposes, a polyol workers [13]) from SBO (HO-SBO) was also produced and an analogous coating produced. As shown in Figure 7, the crosslinked network from the HO- poly(2-VOES) provided a higher Tg and a substantially higher Figure 8: Illustra- storage modulus in the rubbery plateau region. The higher tion of the tertiary storage modulus in the rubbery plateau region is indicative carbon atoms of a higher crosslink density. present in poly The higher crosslink density associated with the use of the Teritiary carbon (2-VOES) and SBO atoms that serve as HO-poly(2-VOES) can be attributed to the higher number of that serve as addi- crosslinks in a fully tertiary carbon atoms per gram of material, as illustrated in tional crosslinks in cured network Figure 8. In addition to the urethane bonds formed during a fully crosslinked curing, these tertiary carbon atoms also serve as crosslinks network in the fully cured network.

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Versatile chemistry for high quality coatings

The plant oil-based polymer technology described here possesses several attributes with respect to coating applications. Compared to drying oils and alkyd resins, the higher functionality (i.e. double bonds and allylic hydrogens) per molecule results in faster curing by autoxidation. Also, due to the tertiary carbon atoms present in its back- bone, relatively high crosslink densities can be achieved. Since these plant oil vinyl ether monomers can be vacuum distilled, colourless monomer can be produced. It was demonstrated that polymerisation of the colourless monomer produces colourless polymer due to the mild conditions of the polymerisation process. The production of a colourless polymer is very important for the production of white and other pale-coloured coatings. Probably the most important attribute of this technology is the ability to utilise copolymerisation to tailor polymer properties. Copolymerisation has been used to increase the Tg of the plant oil-based polymers and to impart water-dis- persibility or solubility to the polymer. Due to the living nature of the polymerisation, block copolymers can also be produced with other vinyl ether monomers. While the unsaturation present in the plant oil-based polymers can be directly utilised to produce crosslinked films through autoxidation, the unsaturation can also be de- rivatised to other functional groups to provide other curing/ crosslinking mechanisms.

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.

REFERENCES

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