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 pro- duction 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 tech- nology involves the production of vinyl ether monomers from either the plant oil triglyceride or alkyl esters de- rived 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. 26 European Coatings JOURNAL 06 l 2015 www.european-coatings.com Technical Paper Biopolymers 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 architec- ture 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 dramati- cally change polymer thermal, physical, and mechanical www.european-coatings.com 06 l 2015 European Coatings JOURNAL 27 Technical Paper Biopolymers Figure 5: Tangent delta response obtained from dynamic mechani- cal analysis for films derived from poly (2-VOES) co- polymers based on CHVE or MVE as the comonomer; films were cured at ambi- ent temperature properties.