Synthesis of UV-curable tung oil and UV-curable tung oil based alkyd
Introduction
Tung oil, or Chinese wood oil, has been used as a protective coating for hundred years, especially in Asian countries [1]. The primary usage of tung oil is as a wood varnish. Advantages of using tung oil as a coating include: faster drying times, higher water resistance, and higher hardness compared to unconjugated drying oils such as linseed or soybean oils [2]. Similar to unconjugated drying oils (linseed, sunflower, soybean, etc.), a heat treatment process can be used to increase the molecular weight build (oligomerization) of the tung oil for coating applications. These heat treatment processes are known as heat bodied or blown. Heat-bodied oil is the process of heating the drying oil up to 300 °C under inert atmosphere, whereas the process of making blown oils is achieved by passing air or oxygen gas through the drying oil at 140–150 °C. At very high temperature or long processing time, the risk of gelation is high. Therefore, the processing time of both methods needs to be closely monitored. Tung oil, being conjugated, has the advantages of faster drying time, higher water resistance, and higher hardness compared to other drying oils which are not conjugated [3]. In addition, since tung oil is conjugated, it can more easily be chemically modified to achieve desirable properties.
Phenolic-modified drying oil, such as tung oil, has been studied for many decades [4], [5], [6], [7], [8], [9]. Both types of phenolic resins, Novolac or Resole, have been claimed to react with tung oil to form a chroman ring [10], [11], [12], as shown in Scheme 1. With the Novolac reaction, base catalysts, such as hexamethylenetetramine, are necessary to accelerate the reaction rate, while Resole can react directly without using a catalyst. Another means of modifying drying oils is via Diels–Alder chemistry [13]. Since tung oil contains ∼80% α-eleosterate and thus has conjugated double bonds, tung oil functions as a moderately active diene. A number of modified-drying oils have been prepared by this method with various dienophiles [14], [15], [16], [17]. Of these, Trumbo and Mote studied a copolymer based on tung oil [15]. They employed the Diels–Alder reaction to prepare a copolymer by reacting tung oil with diacrylate monomers. Time and temperature of the reaction were varied to observe the conversion of the copolymers. They reported 80% conversion of copolymer when using 1,6-hexanediol diacrylate as the dienophile without a catalyst.
Drying oils can also be modified with phthalic anhydride (or other phthalic acids) to form an alkyd resin. Various types of seed oils, such as linseed, sunflower, safflower, soybean, can be used to prepare alkyd resins via the “monoglyceride process”. In the monoglyceride process, the seed oil is first reacted with a polyol and then a polybasic acid to form polyester-based polymer, as alkyd depicted in Fig. 1. A second method for the preparation of alkyds is called the fatty acid process where the drying oil is saponified and the three components, fatty acid, polyol, and polybasic acid, are reacted in one step. This method affords better process control, but the production cost is somewhat higher compared to the monoglyceride process [2].
In coating applications, volatile organic compounds (VOCs) are a major environmental and toxic issue worldwide [18], [19]. To alleviate problems associated with VOCs, reactive diluents have been studied with a particular focus on UV-curing [20], [21], [22], [23]. With increasing environmental regulations in recent years, many natural products have been modified to become UV-curable materials [17], [24], [25]. A study of Soucek and co-workers demonstrated the synthesis and subsequent photopolymerization of an epoxide-modified linseed oil [17]. In their study, norbornylized linseed oil was prepared by reacting linseed oil with cyclopentadiene via the Diels–Alder reaction followed by epoxidation of norbornylized linseed oil. The norbornyl epoxidized linseed oil was photopolymerized via a cationic super acid initiator.
Several natural materials such as natural rubber [25], linseed oil [17], and castor oil [26] have been studied and modified by different methods for UV-curing. Free radical based photocuring, in particular acrylate or methacrylate monomers [21], has advantage of fast cure along with industrial/commercial acceptance. Thus, the purpose of this study was to develop UV-curable tung oil based on acrylate photoinitiated free radical curing. The tung oil was bodied with a phenolic resin and then reacted with trimethylolpropane trimethacrylate (TMPTMA). A tung oil based alkyd was also reacted with TMPTMA ester was via the same reaction. The UV-curable tung oil (UVTO) and tung oil alkyd (UVTA) were characterized by 1H NMR, 13C NMR, MALDI-TOF mass spectroscopy, and gel permeation chromatography (GPC). Photo-curing kinetics of the UVTO and the UVTA were investigated by Photo-DSC.
Section snippets
Materials
Trimethylolpropane trimethacrylate (TMPTMA) and tripropylene glycol diacrylate (TPGDA) were received from Sartomer. Tung oil, phenothiazine, xylene isomer, phthalic anhydride (99%), lead (IV) oxide (97%), and glycerol (99%) were purchased from Aldrich. Phenolic resin was purchased from Georgia-Pacific Resins Incorporation. Irgacure 2100 (Phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide), structure shown in Fig. 2, was obtained from Ciba Company. All chemicals were used as received without
Results
In this study, a new class of UV-curable resin was synthesized by introducing a trifunctional methacrylate, which provides additional functionality for photo-initiated crosslinking, on the bodied tung oil and tung oil alkyd. Trimethylolpropane trimethacrylate (TMPTMA) was a target monomer, which was used in this study. The α-eleosterate of tung oil has an available s-cis diene conformation and the methacrylate was a sufficiently good dienophile for the Diels–Alder reaction to occur. On account
Discussion
The tung oil was bodied with a Novolac resin before the Diels–Alder reaction. What was also interesting was the lack of the formation of the chroman ring (see Fig. 6). For tung oil to afford acceptable coating properties, phenolic resins were typically heated with the tung oil during a bodying process. It appears that the phenolic resin does not necessarily undergo a cyclo-addition reaction with the tung oil. In this case, the phenolic resin appeared to be a spectator.
It is important to note
Conclusion
UV-curable tung oil (UVTO) and UV-curable tung-based alkyd (UVTA) were prepared by reacting trimethylolpropane trimethacrylate (TMPTMA) onto the α-eleosterate of a tung oil and tung oil alkyd molecule via a Diels–Alder reaction. The reactions were conducted at elevated temperature and atmospheric pressure. The structure of UVTO and UVTA was identified by 1H NMR and 13C NMR. MALDI-TOF spectroscopy was used to confirm the molecular weight of the UVTO. The curing kinetics of the UVTO and UVTA were
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