Epoxy photoinitiated polymerization

See also Epoxy coatings Epoxy chalcone, 10 450 12,13-Epoxy-cis-9-octadecenoic (vernolic) acid, physical properties, 5 35t Epoxy coatings, 10 436 450 17 845. See also Epoxy can coatings for corrosion protection, 7 199 markets for, 10 442-449 performance of, 10 423 waterborne, 10 439 Epoxy composites, 10 450, 451 Epoxy compounds, photoinitiated polymerization of, 23 716 Epoxy content analysis, 10 385 Epoxy cresol novolac (ECN) resins, 10 367, 369... 

Crivello, J.V. and Lee, J.L., The synthesis, characterization and photoinitiated cationic polymerization of silicon-containing epoxy resins. J. Polym. Sci. Polym. Chem. Ed., 28, 479-503 (1990). 

Structure and Reactivity Relationships in the Photoinitiated Cationic Polymerization of 3,4-Epoxy cyclohexylmethyl-3, 4 -epoxy cyclohexane... 

To evaluate the reactivity of model compounds III-VIII in photoinitiated cationic polymerization, we have employed real-time infrared spectroscopy (RTIR). Thin film samples of the model compounds containing 0.5 mol% of (4-n-octyloxyphenyl)phenyliodonium SbF - as a photoinitiator were irradiated in a FTIR spectrometer at a UV intensity of 20 mW/cm2. During irradiation, the decrease in the absorbance of the epoxy ether band at 860 cm-1 was monitored. 

Cationic photoinitiators are used in coatings, printing inks, adhesives, sealants, and photoresist applications. Most of the applications involve vinyl ether polymerizations or ringopening polymerizations of epoxy monomers (Sec. 7-2b). 

All of these species (XIV, XV) have been for the most part applied towards function in the olefin polymerization arena use of these novel anions for the stabilization of other electrophilic species remains to be explored. Recently, the imidazolide anion XVI, as well as the perfluorinated tetraaryl borate derived from the diborane IX of Chart 2, have been used to stabilize iodonium cations.222 These cations are used as photoinitiators for cationic polymerization of epoxy resins in photolithography applications. While use of the [B(C6F5)4] led to a breakthrough in this area of research,223 higher activities are observed for more WCAs. 

The generation of a protonic acid (HX) is responsible for the initiation of cationic polymerization. Various monomers are polymerized by sulfonium photoinitiators. Especially this system has been shown to be potentially useful for UV curable coatings of metal and plastics with epoxy resins. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

A considerable number of detailed descriptions on synthesis, production, and applications of epoxy resins exists. Because the aim of this chapter is the application of cationic initiators, and more particularly photoinitiators, to the polymerization of epoxies leading to cross-linked products (curing reaction), only litterature dealing with these aspects will be cited. For the general aspects of epoxy resins the scientific and patent literature may be found in detailed reviews [119,120] and classical books [121-123]. 

The main advantages of cationic photoinitiators is that they have high reaction rates and require a low energy. They can operate at a low temperature, they are not inhibited by oxygen, they do not promote the polymerization of epoxy groups in the dark, and they are often stable at elevated temperatures. Some disadvantages exist that is, inhibition by bases, chain-transfer reaction by water, and the presence of acids in cured products. 

Photoinitiated epoxy crosslinking is generally based (with one exception on cationic ring-opening polymerization of the oxirane group, yielding polyether structures. 

Many different photoinitiators based on onium -type compounds with anions of low nucleophilicity also have been described in the literature as effective catalysts for the polymerization of epoxides Thus, diaryliodonium salts diaryliodosyl salts triarylsulfonium salts and related compoundstri-phenylsulfoxonium saltsdialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenylsulfonium salts seem to be most suitable as photoinitiators for epoxy curing. Some of the principles of the reaction mechanism involving these initiators are discussed in detail in the following Sections. Various other onium photoinitiators such as diarylchloronium and diarylbromonium salts , thiopyrylium salts 3), triarylselenonium salts and onium salts of group Va elements >... 

Thermodynamic data of the epoxy polymerization with triphenylsulfonium salt photoinitiators can be obtained by differential scanning calorimetric measurements (DSC). Figure 6 shows the DSC diagram of the polymerization of Bisphenol-A di-glycidylether with triphenylsulfonium hexafluorophosphate, irradiation being carried... 

In our laboratory, we have found that triarylsulfonium salts (III) in which the anions are of the type BF , AsFg , PFg , SbFg , etc., are excellent photoinitiators for the polymerization of epoxy resins as well as a variety of other monomers (12, 13). Similar results have also been reported by another group of investigators (14). 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

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