About the Author
James H Clark is Professor of Chemistry and Director of the Green Chemistry Centre of Excellence, The University of York, UK. He has led the green chemistry movement in Europe for the last 15 years and was the first scientific editor of the journal Green Chemistry and is Editor-in-chief of the RSC Green Chemistry book series.
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Green Materials from Plant Oils
By Zengshe Liu, George Kraus
The Royal Society of ChemistryCopyright © 2015 The Royal Society of Chemistry
All rights reserved.
Photo-cured Materials from Vegetable Oils
YANCHANG GAN AND XUESONG JIANG
School of Chemistry & Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China
Photo-curing is one of the most effective processes for the rapid transformation of liquid multifunctional monomer resins to cross-linked polymer networks. Photo-curing technology has found an increasing number of industrial applications over the past decade due to its unique benefits, for example solvent-free formulations, and high-speed and room-temperature processing. Photo-curing technology has become attractive, especially in the paint, ink, adhesive, optical disk, photo-lithography and coating industries due to its very low consumption of energy and its low emissions of volatile organic compounds.
The raw materials for photo-curing technology such as monomer resins, photo-initiators and functional additives, are usually produced from fossil oil. However, due to the growing demand for petroleum-based products and the resulting negative impact on the environment, there has been a growing interest in the utilization of renewable resources as an alternative to petroleum-based polymers. The replacement of petroleum-based raw materials with renewable resources constitutes a major contemporary challenge in terms of both economical and environmental aspects.
Vegetable oils are inexpensive, environmentally friendly, renewable, naturally raw materials with low toxicity and functional groups such as hydroxy, epoxy, carboxyl and C=C. Vegetable oils are extracted primarily from the seeds of oilseed plants. Their competitive cost, worldwide availability, and built-in functionality make them attractive. In recent years, there has been a growing trend in using vegetable oils as renewable resources, especially in oleochemical products. Several derivatives of vegetable oils are used as polymerizable monomers in radiation-curable systems due to their environmentally friendly character and low cost. For example, the long fatty acid chains of vegetable oils provide some brittle resin systems such as epoxy, urethane and polyester resins, which have good flexibility and toughness. Vegetable oils exhibit many excellent properties which can be utilized in preparing valuable polymeric materials such as polyester amide, epoxy, polyurethane, alkyd polymers and have many applications in other areas. Epoxidized vegetable oils, such as soybean oil and epoxidized palm oil have been used in UV-curable coating systems. Vernonia oil contains epoxide groups, which means it can be utilized as a polymerizable monomer directly in cationic UV-cured coatings. In this chapter, we summarize the photocured materials that can be obtained from vegetable oils including photocurable monomers, photo-initiator systems and the photo-curing approach.
1.2 Photo-curable Monomers Derived from Vegetable Oils
Vegetable oils consist of mainly triglycerides formed between glycerol and various fatty acids, which have a three-armed star structure (Figure 1.1). Triglycerides are comprised of three fatty acids joined at a glycerol junction. Most of the common oils contain fatty acids that vary from 14 to 22 carbons in length, with 0 to 3 double bonds per fatty acid. Table 1.1 summarizes the most common fatty acids present in vegetable oils. As shown in Table 1.1, most fatty acids are long straight-chain compounds with an even number of carbons, and the double bonds in most of these unsaturated fatty acids possess a cis configuration.
Because the internal double bonds in the triglyceride structure are not sufficiently reactive for various polymerization processes, the vegetable oils must be modified with efficient photo-polymerizable groups such as acrylate or epoxy when being used as photo-curing monomers.
The triglycerides contain several reactive positions, as shown in Figure 1.1: ester groups (a); C=C double bonds (b); acrylic positions (c); and the a-position of ester groups (d) can act as the starting points in different reactions. The C=C double bond reactive positions are usually used as the starting points for introducing highly efficient reactive groups. The general modification pathway is shown in Figure 1.2.
1.2.1 Epoxy Monomers based on Vegetable Oils
Epoxidation is one of the most important functionalization reactions involving C=C double bonds, and epoxidized vegetable oils show excellent promise as inexpensive, renewable monomers for photo-curing industrial applications. Some raw vegetable oil such as vernonia oil already contain large numbers of epoxide groups. The vernonia oil is extracted from the seeds of Vernonia galamensis with petroleum ether or hexane after the seeds are lipase-deactivated and coarse ground, and yields of up to 42% have been reported. One unique characteristic of vernonia oil is that about 80% of the oil is the triglyceride of vernolic acid. The structure is shown in Figure 1.3. Vernonia oil can be utilized as a polymerizable monomer directly in cationic UV-curing coating due to its high content of epoxy groups.
Most of the multifunctional epoxy monomers for photo-curing based on vegetable oils are prepared from the epoxidation of unsaturated fatty acids or triglycerides. The epoxidation of triglycerides or unsaturated fatty acids can be achieved in a straightforward fashion by reaction with molecular oxygen, hydrogen peroxide, or by chemo-enzymatic reactions. The chemistry of the Prileshajev epoxidation of unsaturated fatty compounds is well known. A short-chain peroxy acid, usually peracetic acid, is prepared from hydrogen peroxide (H2O2) and the corresponding acid either in a separate step or in situ (Figure 1.4).
This process is performed industrially on large scale, and more research is currently focusing on how to improve the conversion rate. An epoxidation reaction of mahua oil using hydrogen peroxide was done by Goud et al. They used H2O2 as the oxygen donor and glacial acetic acid as the oxygen carrier in the presence of sulfuric acid (H2SO4) and nitric acid (HNO3), and found that H2SO4 is the best inorganic catalyst for this system, producing a high conversion of double bonds to epoxide groups. Dinda et al. worked on the epoxidation kinetics of cottonseed oil using H2O2 and liquid inorganic acids i.e. hydrochloric (HCl) and phosphoric (H3PO4) acids, and HNO3 and H2SO4 as catalysts. They used carboxylic acid i.e. CH3COOH and HCOOH as oxygen carriers, but they found that acetic acid is a more effective oxygen carrier than formic acid. Among all the liquid inorganic acid catalysts, H2SO4 was found to be most efficient and effective.
Cai et al. also studied the kinetics of the in situ epoxidation of soybean oil, sunflower oil and corn oil by peroxyacetic acid using H2SO4 as the catalyst. They found that soybean oil had the highest conversion rate and the lowest activation energy for epoxidation using peroxyacetic acid, and an 87.4% conversion rate for the epoxidation of jatropha oil. Moreover, the epoxidation of soybean oil and the extent of side-reactions were studied using an ion-exchange resin as the catalyst. The results revealed that the reactions were first-order with respect to the double bond concentration and that sidereactions did not occur on a large scale.
The catalytic epoxidation of methyl linoleate with different transition metal complexes as catalysts was studied by Woo's group. Complete epoxidation with aqueous H2O2 (30%) can be obtained within four hours for methyl trioxorhenium (4 mol%) and pyridine. Gerbase's group proved the same catalyst could be successfully applied for the direct epoxidation of soybean oil in a bi-phasic system showing complete double bond conversion within two hours. Moreover, enzymes have been widely studied for the epoxidation of plant oils and their derivatives. The reaction proceeds via the enzymatic in situ formation of the peracids required for the chemical epoxidation of the double bonds. The general advantage of this kind of epoxidation is that there are no undesired ring-opening reactions of the epoxides obtained.
1.2.2 Acrylated Monomers based on Vegetable Oils
The double bonds present in vegetable oils can be transformed into acrylate groups through two steps, then the acrylated vegetable oils can be used as a binder in fast UV-curable coating mixtures. In a first modification step, the double bonds are converted to epoxide groups, then the epoxide groups are further converted to acrylate groups (Figure 1.5). Vegetable oils like soybean, castor, lesquerella, palm and vernonia have been successfully converted to acrylates and methacrylates respectively. As reported by Bajpai and co-workers, acrylated epoxidized soybean oil (AESO) has been prepared, using triethylamine and hydroquinone as a catalyst and gelling inhibitor, respectively. Similar strategies have been applied for the synthesis of AESO, which, along with maleinized soy oil monoglyceride and maleinized hydroxylated oil, were used to prepare composite materials with glass fibers and natural flax and hemp fibers.
In the report of Wuzella et al., an acrylated epoxidized linseed oil (AELO) was synthesized from epoxidized linseed oil (ELO) through the ring opening of the oxirane group using acrylic acid as the ring-opening agent (Figure 1.6).
Esterification of hydroxylated vegetable oils using acrylic acid or acryloyl chloride is another efficient method for the preparation of acrylated vege table oils for UV-curing. The reaction is active and usually happens at low temperatures, which may minimize side-reactions such as the homopolymerization of acrylate monomers. The naturally occurring hydroxyl groups in castor oil are usually used to attach polymerizable acrylic moieties, by reacting castor oil with acryloyl chloride. The Applewhite and Pelletier groups reacted the hydroxyl groups of castor oil with acryloyl chloride to prepared acrylated castor oil (ACO). We also obtained ACO from castor oil using the same strategy (Figure 1.7).
Epoxidized vegetable oils can be reacted with polyhydric alcohols to prepare vegetable-oil-based polyols. Cheong's group used a vegetableoil-based polyol to prepare an acrylated polyol ester pre-polymer via the polycondensation esterification between polyol and acrylic acid, and thereafter produced a radiation-curable formulation from the pre-polymer (Figure 1.8).
Acrylate moieties have also been attached to triglyceride structures by the one-step addition of bromide and acrylate groups to a C=C bond. Soybean and sunflower oils have been modified in the presence of acrylic acid and N-bromosuccinimide (NBS, Figure 1.9).
The Patel group has synthesized a series of UV-curable polyurethane acrylate pre-polymer monomers by reacting polyols from sesame oil (edible) and using different ratio of polyols, aromatic isocyanate, and aliphatic isocyanate, 2-hydroxy ethyl methacrylate (HEMA) and dibutyltin dilaurate (DBTDL) as the catalyst. Polyols were prepared via the alcoholysis of triglyceride oil using a proprietary method, which was further reacted with toluene diisocyanate and isophorone diisocyanate in different ratios to develop a series of polyurethanes (Figure 1.10).
Homan et al. also employed an acrylate-bearing isocyanate group to produce acrylate castor oil. Patel et al. modified monoacylglycerol (MAG) with the diisocyanate reagents methylene bis(4-phenylisocyanate) (MDI) and toluene diisocyanate (TDI). The free terminal isocyanate groups of MAG reacted with the acrylate monomer, which bears a free –OH group. A final route for urethane-acrylated vegetable oils is the reaction of an acrylate bearing an isocyanate group and fatty chains, with a hyperbranched hydroxyl-terminated polyester.
1.3 Photo-cured Materials from Vegetable Oil Monomers
Photo-curing formulations are usually comprised of multifunctional monomers and oligomers, with small amounts of a photo-initiator which generates reactive species (free radicals or ions) upon UV exposure. The overall process can be represented schematically as shown in Figure 1.11. There are two major classes of UV-curable resins, and they differ in their polymerization mechanism of monomers i.e. acrylates or unsaturated polyesters (free radical polymerization) vs. photo-initiated cationic polymerization of multifunctional epoxides and vinyl ethers (cationic polymerization).
Many researchers pay attention to the chemistry behind the photo-curing process, and especially the photo-curing kinetics of ultrafast reactions for both cationic-type and radical-type polymerization of the multifunctional monomers from vegetable oils. Polymer networks based on vegetable oils with different structures and tailor-made properties have been obtained by photo-curing formulations containing one or more type(s) of monomer.
1.3.1 Photo-oxidation of Vegetable Oils for Direct Cross-linking
Drying oils are vegetable oils that are composed of mixtures of triglycerides. The high rate of unsaturation of these vegetable oils makes them sensitive to auto-oxidation under air. Drying oils are wildly applied as binders and film formers in paint and coating formulations because they can form polymer networks by auto-oxidation, peroxide formation and subsequent radical polymerization.
Linseed oil is the most successful example of a drying oil, and the superior performance of linseed oil compared to other vegetable oils is mainly due to its faster drying. Linseed oil is extensively used as a medium for paintings and elaborate linoleum, owing to its capacity to form a continuous thin layer easily, with good optical and mechanical properties within a reasonable time.
The cross-linking mechanism (Figure 1.12) was investigated in de tail, and the formation of the lipidic network was attributed to the successive formation of radical species, isomerization, hydroperoxidation and crosslinking. The oxidation process, accelerated by UV irradiation using metal-based catalysts has also been studied.
1.3.2 Photo-cured Polymer Networks based on Acrylated Vegetable Oils
Acrylated resins are the most widely used photo-curing systems, because of their high reactivity and the variety of available monomers and telechelic oligomers. A typical photo-curing formulation contains three basic components: (1) a photo-initiator which can generate free radicals by photolysis; (2) the acrylated functionalized oligomer which constitutes the backbone of the polymer network; and (3) the acrylated monomer which acts as a reactive diluent.7 As shown in Figure 1.13, the photo-initiator plays a key role in the polymerization, and it governs both the rate of polymerization and the cure depth. The final degree of the polymerization and the physical and chemical properties of the photo-curing polymers are determined by the chemical structure and functionality of both the monomer and the oligomer.
Wuzella et al. have studied the kinetic properties of acrylated epoxidized linseed oil monomers by UV-curing. They found that the photo-initiator affects both the reaction rate and the final double-bond conversation. The structures of the monomer and photo-initiators are shown in Figure 1.14. The monomer with the photo-initiator HAC (2-hydroxy-2-methyl-1-phenylpropan-1-one) showed the highest conversion rate and reached the highest level of double-bond conversion, followed by the mixtures with BP (benzophenone) TX (2,4-diethyl-9H-thioxanthen-9-one). The high conversion rate for HAC might be due to the quick generation of radical pairs through the efficient α-cleavage process of Type I photo-initiators. This UV-curable resin can be used for wood coating, as it contains sufficient cross-link density to withstand the solvent stress. Moreover, the polymer chains are flexible enough against scratches and exhibit good adhesion to wood substrates. The influence of acrylate-reactive diluents on the photo-curing rate was investigated in detail, as well as the relationship between the number of acrylate functional groups on the oil backbone and the hardness of the resulting materials. For example, the Patel group has prepared a novel binder system for UV-curing coatings based on tobacco seed (Nicotiana rustica) oil derivatives. The UV-curing films of tobacco seed oil show good thermal stability at 100 ºC, and the results of flexibility and adhesion tests revealed excellent performance. Higher functionalities of polyols, aromatic-type isocyanates, and lower oil ratios lead to poor adhesion and flexibility performance. Also, the aromatic nature of the isocyanate moiety further enhances the film hardness and toughness. Thus, the experimental sets based on higher polyols, higher functionality acrylate reactive diluents, and a lower proportion of oil gave better scratch hardness. Higher cross-linking densities showed better solvent and chemical resistance in the cured films.
Bio-degradable photo-cross-linked thin polymer networks based on acrylated hydroxy fatty acids have been reported. Di- and trimethacrylates, or acrylated oligomers such as acrylated-PEG (polyethylene glycol) or acrylatedpoly(ε-caprolactone) were used in co-polymerization. The bio-degradability of the resulting co-polymers was examined, and faster bio-degradation was observed for high-density cross-linking as a result of the low molecular weight between entanglements, that might otherwise block lipase attack sites.
Excerpted from Green Materials from Plant Oils by Zengshe Liu, George Kraus. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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