![]() The trace of the AlkydITX is unimodal and narrowed, which is similar to the behavior of autoacceleration in the UV-curing of acrylate 9. The first is the plot of the AlkydITX resembles a diagonal-like line from the upper left to lower right i.e., the reaction rate decreases dramatically with the conversion. In part b, the R p is plotted as a function of α. This is also called photoassisted autoxidation 6. Thus, this approach was expected to result in the reduction of the amount of Co salts used because it can lower the barrier to the reaction. The utilization of sunlight is an important factor for outdoor jobs to accelerate the curing process. After that, it may convert to a more stable but less energetic triplet state (marked by 3) by intersystem crossing (marked by ISC). ITX in the ground state can be initially excited to a high-energy singlet state (marked by 1) after being irradiated, as illustrated in Fig. 2-isopropylthioxanthone (ITX) is an efficient and common Type II photoinitiator and is recommended for use in formulations because it can absorb light energy in the visible region. Taking a cue from photopolymerization, we suggested that hydrogen abstractor photoinitiators (Type II) can help to shorten the hydrogen abstraction process. So the alkyd monomer is active as a hydrogen donor. The bond dissociation energy (BDE) of bis-allylic C-H in lipids was reported to be approximately 50 kcal/mol lower than that of alkyl C-H (100 kcal/mol) and mono-allylic C-H (80 kcal/mol) 5. Thus, it seems worthwhile to innovate a formulation for increased efficiency of hydrogen abstraction. However, there is a general consensus that hydrogen abstraction (i.e., dehydrogenation) is the rate-determining step in this autoxidation. The details of the mechanism have not been clearly understood thus far for specifics, the reader is referred to the original literature 2, 3, 4. ), leading to polymerization (Haber and Weiss reaction). ![]() In the autoxidation, the Co salts mainly catalyze the decomposition of hydroperoxides to generate alkoxy radicals (RO The radical readily reacts with oxygen in air, leading to the intermediate alkyl hydroperoxide –C(OOH)–CH=CH–CH=CH–. The autoxidation reaction involves dehydrogenation from a methylene group between two double bonds in a lipid chain, yielding the pentadienyl radical –CH=CH– ♼H–CH=CH–. For this purpose, a detailed understanding of the mechanism of autoxidation is necessary. Because an ideal solution is not yet available, the alternative approach is to minimize the usage of Co salts. The problem is that some cobalt alternatives are either inadequate in terms of activity or are strongly colored. However, although several substitutes, such as Mn or Fe-based complexes, are applied in industry, Co-based complexes remain widely used. Therefore, much effort has been focused on the search of substitutes for cobalt complexes. Compared with other through and auxiliary driers, such as Ca, Zr, Al, and Ba-based catalysts, Co salts are still commonly used in commercial formulations. ![]() They are often used as a drier in the curing of alkyd monomers. Recently, cobalt organometallic complexes (Co salts, e.g., cobalt carboxylate) have received considerable attention due to their latent carcinogenic properties 1. More and more engineers use environmentally friendly additives to replace hazardous compounds in various formulations. On the basis of these observed results, an accelerated mechanism of the photoassisted autoxidation of alkyd resins was proposed. These theoretical results fit well with the experimental result of linear free energy relationship. The vertical excitation energy obtained from the time-dependent density functional theory was used to explain the anomalous behavior of the photoinitiators. The reaction rate constant and the Wigner tunneling factor were predicted from the result of quantum chemistry. ![]() The thermodynamic data of transition state theory, such as the activation energy, were calculated by using quantum chemistry program. Nanosecond laser flash photolysis was used to obtain the quenching rate constants between alkyd monomers and isopropylthioxanthone according to the Stern-Volmer equation. A linear free energy relationship was used to theoretically predict the hydrogen abstraction ability of photoinitiators. The results show that isopropylthioxanthone can increase the efficiency of hydrogen abstraction at the beginning of the curing reaction. The combined influence of the two additives on autoxidation was analyzed using real-time infrared spectroscopy. The system was polymerized using a Cobalt-salt complex as an air-sensitive drier and an isopropylthioxanthone photoinitiator as a light-sensitive accelerator, as well as using irradiation in the form of visible light. A novel bicomponent alkyd system was designed to decrease the usage of Cobalt-based drier due to its latent carcinogensis.
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