Photo-curing is one of the most effective processes for the rapid transformation of liquid multifunctional monomer resins to cross-linked polymer networks.^1–3  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.^4,5  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.^6,7 

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.⁸  The replacement of petroleum-based raw materials with renewable resources constitutes a major contemporary challenge in terms of both economical and environmental aspects.⁹ 

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.^10,11  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.¹²  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.^13–15  Epoxidized vegetable oils, such as soybean oil and epoxidized palm oil have been used in UV-curable coating systems.^16,17  Vernonia oil contains epoxide groups, which means it can be utilized as a polymerizable monomer directly in cationic UV-cured coatings.¹⁸  In this chapter, we summarize the photo-cured materials that can be obtained from vegetable oils including photo-curable monomers, photo-initiator systems and the photo-curing approach.

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.¹⁹  Table 1.1 summarizes the most common fatty acids present in vegetable oils.^9,19,20  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 α-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.

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.^21,22  Some raw vegetable oil such as vernonia oil already contain large numbers of epoxide groups.²³  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.^24,25  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.²³ 

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.²⁶  The chemistry of the Prileshajev epoxidation of unsaturated fatty compounds is well known.²⁷  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.²⁸  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.²⁹  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.³⁰ 

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,³¹  and an 87.4% conversion rate for the epoxidation of jatropha oil.³²  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 side-reactions did not occur on a large scale.^8,33 

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 methyltrioxorhenium (4 mol%) and pyridine.³⁴  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.³⁵  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.^36–41 

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).^42,43  Vegetable oils like soybean, castor, lesquerella, palm and vernonia have been successfully converted to acrylates and methacrylates respectively.^42,44–46  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.¹⁰  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.^8,47 

In the report of Wuzella et al.,⁴⁸  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 vegetable 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 Applewhite⁴⁹  and Pelletier⁵⁰  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).⁵¹ 

Epoxidized vegetable oils can be reacted with polyhydric alcohols to prepare vegetable-oil-based polyols. Cheong's group used a vegetable-oil-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).⁵² 

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).^19,53 

The Patel group⁵⁴  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.⁵⁵  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.⁵⁶  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.⁵⁷ 

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 1.11.⁷  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.^58–61 

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.^8,62–64 

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.⁶⁵  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.⁶³ 

The cross-linking mechanism (Figure 1.12)^9,62,69  was investigated in detail, and the formation of the lipidic network was attributed to the successive formation of radical species, isomerization, hydroperoxidation and cross-linking. The oxidation process, accelerated by UV irradiation using metal-based catalysts has also been studied.^63,66–69 

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.⁴⁸  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-phenyl-propan-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.^55,70,71  For example, the Patel group has prepared a novel binder system for UV-curing coatings based on tobacco seed (Nicotiana rustica) oil derivatives.⁷¹  The UV-curing films of tobacco seed oil show good thermal stability at 100 °C, 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,⁷¹  or acrylated oligomers such as acrylated-PEG (polyethylene glycol) or acrylated-poly(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.^72,73 

The Lecamp group⁷⁴  described a new synthesis process for vegetable-oil-based materials. It may provide potential opportunities to synthesize polymer materials from renewable resources by a clean and simple process that is totally transposable to lesser drying oils. First, linseed oil was thermally polymerized in bulk at 300 °C under an inert atmosphere. Then, the obtained stand oil was functionalized in a two-step one-spot process without solvent in order to graft onto it some photo-polymerizable groups. Finally, the materials were prepared by UV-curing of the modified linseed oil. The obtained materials were globally flexible, hydrophobic and non-bio-degradable. Compared to a naturally oxidized linseed-oil-based materials, the thermal stability and hydrophobicity remained unchanged.

Multifunctional epoxide monomers can be converted into highly cross-linked polymer networks by UV irradiation in the presence of cationic photo-initiators. The cationic photo-polymerization of epoxidized oils is insensitive to oxygen, thus is highly attractive for many applications such as inks and adhesives.⁷  The mechanism of photo-initiated cationic polymerization is illustrated in Figure 1.15.

The Soucek group⁷⁵  have reported the preparation and photo-polymerization of cyclohexene-derivatized linseed oil (CLO) and epoxycyclohexene-derivatized linseed oil (ECLO). The structures of CLO and ECLO and the photo-polymerization kinetics are shown in Figure 1.16. It was found that the addition of reactive or non-reactive diluents could reduce the viscosity of the formulations and increase the final epoxy conversion and the polymerization rate (Figure 1.17).⁷⁶  The properties of UV-curing hybrid films derived from epoxynorbornane-functionalized linseed oil and tetraethylorthosilane (TEOS) were also studied. The results indicated that the incorporation of TEOS can improve the performance of the films and enhance the tensile strength, thermal stability, fracture toughness, and general coating properties of epoxynorbornane-modified linseed oil.⁷⁷ 

Ortiz's group studied the acceleration effect of substituted benzyl alcohols on the cationic photo-polymerization rate of epoxidized natural oils.⁷⁸  The Crivello group described the effect of the structure of both the cation and anion of diaryliodinium and triarylsulfonium photo-initiators on the polymerization rate of epoxidized triglycerides as renewable monomers. The obtained cured raw vernonia oil films based on this system exhibited a high degree of flexibility and impact strength.⁷⁹  Johansson's group has prepared UV-curable resins from a hydroxy functionalized hyperbranched polyether onto which an epoxy functional fatty acid, vernolic acid, had been grafted (Figure 1.18).⁸⁰  The Lalevée group⁸¹  explored the cationic polymerization process of epoxidized soybean oil (ESO) using various photo-initiators under air and solar irradiation. A fundamental research study on photo-initiators was described for the case of the cationic photo-polymerization of triglycerides. Their newly developed photo-initiating systems are highly efficient under air upon solar irradiation, leading to 60% conversion, and form a completely tack-free and uncolored coating after 1 h.

The thiol-ene photo-reaction is a unique class of photo-polymerization that is advantageous with regard to the polymerization process and also the performance of the resulting polymers.^82–84  As shown in Figure 1.19, the thiol-ene reaction proceeds in a free-radical step-growth polymerization manner (Propagation A), and the excess acrylate continues to polymerize through homopolymerization (Propagation B).^51,84  In thiol-ene polymerizations a broad range of physical properties are achieved because a wide variety of enes such as acrylate, vinyl ether, allyl ether, vinyl acetate, and alkene are used. Thiol-ene photo-polymerization occurs through a step-growth reaction mechanism that offers many advantages such as delayed gel points, uniform structures, low polymerization shrinkage and reduced stress,^85–87  which make thiol-ene polymers high-performance materials,^88–90  especially for photo-curing coatings.^91,92 

The unsaturated nature of triglycerides obtained from vegetable oils make them good candidates for thiol-ene coupling. The fatty compounds can be transformed into polymeric networks by adding multifunctional thiols under UV or thermal radical conditions; the low thermal stability of lipidic compounds justifies the use of UV irradiation instead of thermal activation in the thiol-ene coupling reaction.^62,93–95  The thiol-ene photo-induced coupling reaction is a powerful method for the chemical modification of triglycerides or lipidic compounds to prepare vegetable oil derivates.^96,97  The Auvergne group⁹⁸  demonstrated an efficient thiol addition onto vegetable oils, leading to bio-based polyols (Figure 1.20). The most important feature of the reaction was the influence of the number of double bonds per chain in the vegetable oil on the thiol grafting yield. Indeed, disulfide formation and intermolecular recombination occurred. In the case of oleic acid functionalization, transesterification was also detected. Despite these side-reactions, byproducts were found to exhibit alcohol functional groups. Interestingly, the photo-reaction can therefore be performed under mild conditions, requiring neither solvent nor photo-initiator. The crude product could also be purified by an easy procedure. This one-step route to produce fatty polyols represents a significant advance compared to the traditional epoxidation approach (that occurs in two steps) and further evidence of the synthetic usefulness of thiol addition in materials chemistry. Indeed, this synthetic method can readily incorporate different reactive functionalities onto vegetable oils and their derivatives, thus leading to functional precursors suitable for polymer synthesis. Through a similarly efficient coupling procedure, Meier⁹⁹  described the syntheses of materials derived from plant oils. By coupling both transesterification and thiol-ene addition, the Caillol group¹⁰⁰  synthesized a pseudo-telechelic diol from vegetable oil.

Many works focusing on UV-curable systems based on monomers derived from vegetable oils have been reported,^93–95,101  but UV-curable systems based on maleates, fumarates and acrylates have not been widely studied. Rawlins’ group¹⁰¹  reported the synthesis of thiol-ene UV-curable coatings based on vinyl ether, allyl, acrylate and derivatives of castor oil together with a multifunctional thiol. The films exhibited high solvent resistance and hardness as well as excellent adhesion and flexibility. Samuelsson et al. showed that coatings could be made by photo-induced thiol-ene polymerization using the internal double bonds of fatty esters (methyl oleate and methyl linoleate) and multifunctional thiols. The structure of both the unsaturated fatty derivatives and the thiol reagents affects the reactivity.¹⁰² 

Cádiz et al.¹⁰³  described polymer networks based on a kind of maleated soybean oil glyceride which was photo-polymerized with multifunctional thiols under very mild conditions (Figure 1.21). The mechanical properties of the polymer networks produced, resemble those of elastomeric materials. The materials exhibited a flexural modulus in the 240–340 MPa range and glass-transition temperatures below room temperature. Increasing the thiol functionality does not increase the rigidity of the materials due to structural factors. Bexell et al. reported the efficiency of a photo-induced reaction for coating aluminum surfaces with vegetable-oil-based films. In this case, linseed oil reacted with the thiol groups of mercaptosilane-treated aluminum.^104,105 

Because of flexible thioether linkages and the inherent disadvantages of vegetable oil, it is difficult to obtain cross-linked materials with hardness, toughness and high glass-transition temperatures. Additionally, the typically unpleasant odors of low-molecular-weight thiols have also limited their commercial utilization. To overcome these disadvantages and obtain photo-curing materials based on vegetable oils with enhanced performance, we first introduced polyhedral oligomeric silasesquioxanes (POSS) containing thiol groups into acrylated castor oil to develop a novel photo-cured hybrid material (Figure 1.22).⁵¹  The obtained hybrid materials were transparent to visible light and no obvious phase separation was observed. The introduction of POSS can decrease the surface energy of the obtained hybrid materials. The thermal stability of the obtained hybrid materials increased with the increase of POSS content. The thermal decomposition temperature of the hybrid materials is between 250 and 300 °C, and the Young's modulus of the cured hybrid film is 230 MPa, which is higher than pure cured acrylated castor oil films. These characteristics give the obtained hybrid material potential applications in coating, and these studies provided a novel alternative approach to preparing hybrid materials from renewable sources.

Due to their special structure and bio-compatibility, some vegetable oil derivates can be used as functional additives in resin formulations. Acrylated and epoxidized vegetable oils can be used as plasticizers to make photo-curing epoxy or acrylate resin films with greater flexibility and toughness.¹²  Due to their high adhesive properties, some photo-curing coating formulations may contain small proportions of vegetable oil derivates to enhance adhesion.^106,107  The Webster group¹⁰⁸  described the preparation of soybean-oil-based multithiol oligomers, which can be used as the cross-linking agent for photo-curing coating (Figure 1.23).

Kahraman's group¹⁰⁹  have prepared a kind of alkoxysilane-modified castor oil (Figure 1.24). They added the alkoxysilane-modified castor oil to a photo-curable resin formulation and obtained a hybrid UV-cured coating which is highly water repellent. Finally, highly hydrophobic and highly roughened coatings were prepared via a novel modification of an inexpensive and environmentally friendly bio-based renewable source. The most roughened coating showed a contact angle of 143°.

We synthesized a series of photo-initiators based on vegetable oil by introducing TX and co-initiator dimethylaminobenzene moieties into the ESO backbone through the reaction between carboxyl and epoxy groups (Figure 1.25). The obtained photo-initiators based on ESO exhibited high photo-efficiency in the photo-polymerization of acrylate monomers and low migration (Figure 1.26).¹¹⁰ 

Modifying vegetable oils can lead to new materials through a photo-polymerization process. The raw vegetable oils can be easily modified to obtain UV-curing monomers. Otherwise, modified vegetable-oil-based monomers can be introduced into polymeric materials to obtain specific properties, or form the basis of the polymers themselves. Consequently, photo-cured materials based on modified vegetable oils have a large variety of applications such as coatings, inks, bio-materials and so on.

Radiation-curing technology has developed very rapidly in recent years due to some obvious advantages. However, this technology retains some problems yet to be solved, such as the oxygen inhibition of radical polymerization, migration of photo-initiators and the low mechanical performance of the resulting materials. The development of non-hazardous photo-initiators which can be used safely in medicinal applications and in food contact coatings is a new opportunity. For industrial applications, new trends in the future will concern the development of ultrafast reactive monomers with low toxicity.