UV-Curable, Seed Oil-Based Coatings by Free-Radical Paper Technical Photopolymerization: Part 1 By Mark D. Soucek, number of seed oil-based, linseed oil have traditionally been used Jinping Wu, UV-curable coatings have been as drying oils and raw materials for 6 Narin Thanamongkollit, synthesized and reacted via alkyd resins. Tung oil has been used P. Chittavanich, AUV-initiated, free-radical as a protective coating for hundreds of Zhengang Zong and polymerization. These UV-curable, years. The primary usage of tung oil is seed oil-based coatings include as a wood varnish. Advantages of using Jianxia Chen acrylated tung oil (UVTO), tung tung oil as a coating include faster This is the first of a two-part series. oil alkyd (UVTA) and norbornene drying time, higher water resistance linseed oil (NLO). Both the chemical and higher hardness compared to structure and coating properties were unconjugated drying oil (such as investigated. Not only UV-curable soybean oil).6,7 In addition, since tung clear coatings, but also pigmented and oil is conjugated, it can more easily inorganic/organic UV-curable hybrid be chemically modified to achieve coatings were developed. A schematic desirable properties. model for UV-curing hybrid coatings Free-radical polymerization of was proposed. acrylic monomers initiated by UV radiation has been drawing much Introduction attention in coating applications—and Environmentally benign coatings it has been increasingly important in have been the focus of attention recent years. This is mainly due to for many years.1,2 With increasing the extensive use of this process in environmental regulations, there has photoactive polymer-based systems.8 been a growing interest in radiation- This process has a large source of curable monomers and many natural available raw materials. It also has products have been modified to be extremely fast curing rates, well-known UV-curable materials. The natural chemical reactions, and high or 100% reactivity, inherent low viscosity3 solids content. In a free-radical, and ease of modification of seed oils photopolymerizable formulation, the make this type of binder a superior photoinitiator decomposes into free candidate for environmentally benign radicals when it is irradiated by UV coatings.4,5 Naturally occurring seed light and then the crosslinking reaction oils are mainly triglycerides consisting leads to the formation of a three- of a mixture of saturated oleic, linoleic dimensional network. The drawback and linolenic fatty acid. They represent of free-radical photomerization is a biomass alternative technology the oxygen inhibition. However, the that is in need of modification to be surface curing can be obtained by high competitive with modern polymeric rates of radical polymerization not binders. Seed oils such as tung oil and only to consume oxygen located at the ISSUE 2 2013 RADTECH REPORT 37 Figure 1 Chemical structures of tung oil and UV-curable tung oil (UVTO) Technical Paper Technical surface, but also to overcome the rate the α-eleosterate of a tung oil and could be used to functionalize both of oxygen diffusion in resins.9 tung oil alkyd molecule via a Diels- tung oil and tung oil-based alkyd. In the past few years, our group of Alder reaction. The reactions were The high content of α-eleosterate researchers has developed and studied conducted at elevated temperature (~80 wt%) of tung oil triglyceride a series of UV-curable, seed oil-based and atmospheric pressure. An inhibitor is considered a major source of coating materials, especially tung oil (phenothiazine) was added to avoid conjugated diene, while the acrylate and linseed oil-based, UV-curable homopolymerziaton of TMPTMA. The molecule is a decent dienophile. The coatings. In this paper, UV-curable UVTO and UVTA were formulated carbonyl group on acrylate serves coatings crosslinked by free-radical with a free-radical reactive diluent, as an electron-withdrawing group, photopolymerization are discussed. tripropylene glycol diacrylate (TPGDA), which plays an important role by UVTO and UVTA are crosslinkable and photoinitiator Irgacure 2100. lowering the activated energy of the through an acrylate group. Thiol-ene Photocuring kinetics of the UVTO reaction. The study by Trumbo and photopolymerization of NLO and a new and the UVTA were investigated Mote12 also reported the Diels-Alder thiol siloxane colloid was also developed by Photo-DSC. The change of heat cycloaddition between tung oil and and studied for the UV-curable organic/ flow as a function of time from DSC diacrylate monomers, 1,6-hexanediol inorganic hybrid coatings. thermogram proved the capability and, and 1,4-butanediol diacrylate, at thus, demonstration of the concept elevated temperature without catalyst. Tung Oil-Based, UV-Curable of UV-curing for both UVTO and However, the resultant products in Clear Coatings UVTA. The UVTO and UVTA were their study were copolymers of tung oil Free-radical-based photocuring— characterized by 1H NMR, 13C NMR, and diacrylate. in particular, acrylate or methacrylate MALDI-TOF mass and Gel Permeation Our group used a trifunctional monomers—has advantages of fast cure Chromatography (GPC). Details acrylate instead of a diacrylate along with industrial acceptance.10 Since about the synthetic chemistry and monomer. The flexibility and lack of tung oil contains ~80% α-eleosterate, characterization can be found in the steric hindrance of the previously and, thus, has conjugated double bonds, previously published papers.13,14 reported diacrylate system were the tung oil functions as a moderately active It is important to note that drying primary factors in telechelic-acrylic diene. A number of modified drying oils and alkyds are complex mixtures end groups reacting with the diene. oils have been prepared with various that are exceedingly difficult to In contrast, after the first Diels- dienophiles.11,12 Our group developed characterize with the degree of Alder reaction of the triacrylate with UV-curable, tung oil-based coatings by certainty that a single reactant can the α-eleosterate, the subsequent free-radical photopolymerization. A be characterized. The fact that the Diels-Alder reactions were sterically synthetic scheme for UVTO and UVTA triacrylate group can react with one or hindered. This left the remaining are shown in Figures 1 and 2. more of the eleosterates was observed two acrylic groups available for UVTO and UVTA were prepared in the propensity to gel. However, UV-polymerization. Maleic anhydride by reacting trimethylolpropane when the reaction conditions were was introduced onto the tung oil trimethacrylate (TMPTMA) onto controlled, the Diels-Alder reaction molecule via a Diels-Alder reaction 38 RADTECH REPORT ISSUE 2 2013 Figure 2 Chemical structures of tung oil alkyd and UV-curable tung oil alkyd (UVTA) Technical Paper Technical and then esterified with ethylene energy consumption and no volatile optimized for both through-cure and glycol to form tung oil-modified organic compounds. Formulation of surface cure. After curing, pencil polyol. This polyol was formulated pigmented UV-curing coatings has hardness, conical mandrel, impact with a cycloaliphatic epoxide and been a major challenge since the resistance, cross-cut adhesion and cured. In contrast, the modification earliest stage of their development, gloss were measured. In addition, the of tung oil for UV curing reported by mainly due to the reflection and viscoelastic properties of the optimized our group was based on free-radical absorption characteristics of pigment formulations were investigated. polymerization of the acrylate group. particles. Pigments prevent sufficient An important aspect of pigmented Although there have been UV-curable penetration of UV energy to efficiently coating—and especially UV-curable derivatives of drying oils, this study activate the photoinitiated crosslinking coating—is the degree of pigment was the first UV-curable tung oil alkyd reaction, resulting in the amplification dispersion. The dispersion was checked in the literature. It is hoped that the of the cure gradient in the film, which for each coating formulation via concept of UV-curable alkyd will is responsible for the creation of personal observation and a rub test. have great potential for widespread wrinkles on the film surface. There are also numerous methods that use in the field of coatings. Both A complete pigmented formulation have been employed to quantify degree the UV-curable drying oil and alkyd was developed based on UVTA in our of dispersion,15 but most of them are represent a considerable step forward previously published paper.14 A variety comparative rather than absolute in the replacement of solvents for the of reactive diluents were selected to and some of them depend on factors compliance of both North America and be formulated with the UVTA, such other than dispersion (e.g., gloss).16 To European environmental regulations. as TMPTMA, TPGDA, propoxylated validate the observation, the degree trimethylolpropane triacrylate of dispersion was determined by optical Tung Oil-Based, UV-Curable (POTMPTA), propoxylated neopentyl microscope and scanning electron Pigmented Coatings glycol diacrylate (PONPGDA), microscope (SEM) as shown in Figure 3. Besides the UVTO and UVTA neopentyl glycol diacrylate (NPGDA), It is observed in both micrographs that clear coating, our group also 1,6 hexanediol dimethacrylate the pigment is well dispersed. developed and investigated the (HDMA), dicyclopentadienyl According to Burrel,17 Basu,18 UVTA-based pigmented coatings.14 methacrylate (DCPDMA), lauryl Payne,19 Wicks and Chen,20 wrinkles Alkyd resins today are still one of the acrylate (LA), isodecyl methacrylate usually originate from the curing most important classes of coating (IDMA) and isobornyl acrylate (IBOA). gradient of the film, high cure rate at resins. They are used in both clear The color of yellow was chosen. the surface and low cure rate at the and pigmented coating. UV-curing The light source, photoinitiator and bottom. Therefore, an ideal solution to technology would result in low reactive diluent were evaluated and eliminate wrinkles is to decrease the ISSUE 2 2013 RADTECH REPORT 39 cure rate at the surface and increase substrate.