Cationic photoinitiator

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Janke, C.J., Lopata, V.J., Havens, S.J., Dorsey, G.F. and Moulton, R.J., High energy electron beam curing of epoxy resin systems incorporating cationic photoinitiators, US Patent 5,877,229, 1999. 

Materials. The radical-type photopolymerizable formulation consisted of a mixture of hexanediol diacrylate (HDDA from UCB) and a polyurethane-diacrylate (Actilane 20 from Arkros). A bis-acylphosphine oxide (BAPO from Ciba) was used as photoinitiator at a typical concentration of 1 wt %. The cationic type photopolymerizable resin consisted of a mixture of the divinylether of triethyleneglycol (RapiCure DVE-3 from ISP) and a divinylether derivative of bis-phenol A (DVE-BPA). The cationic photoinitiator (Cyracure UVI-6990 from Union Carbide) had a composition of 50 wt % of mixed triarylsulfonium hexafluorophosphate salts and 50 wt % of propylene carbonate. The BAPO initiator... 

Figure 1. IR absorption spectra of 4-allyloxystyrene (a) uncured monomer (b) after 10 s UV exposure with cationic photoinitiator (c) after UV exposure and heating at 200°C for 1 hour and (d) after an additional 16 hours heating at 200°C.

The development of new classes of cationic photoinitiators has played a critical role in the production of highly sensitive, acid-catalyzed deep-uv photoresists. Sulfonium salts have been widely used in this respect (4). These materials are relatively easy to prepare and structural modifications can be used to produce desired wavelength sensitivity. Triphenylsulfonium salts are particularly well suited for deep-uv application and in addition can be photosensitized for longer wavelength. These salts are quite stable thermally and certain ones such as the hexafluoroantimonate salt are soluble in casting solvents and thus easily incorporated within resist materials. 

Acid-catalyzed photoresist films acid diffusion, 35 acid generation, 303233/341 advantages, 28 catalytic chain length, 3435r development of classes of cationic photoinitiators, 28 experimental procedure, 35-36 generation mechanism from irradiation of triphenylsulfonium salts, 28-29 merocyanine dye method for acid analysis, 30,31/33/... 

Films of polyphthalaldehyde, sensitized by cationic photoinitiators, have been imaged at 2-5-mJ/cm in the deep ultraviolet (DUV) (see Section 3.10), at 1 pC/cm (20 kV) electron beam radiation and at an unspecified dose of Al-A x-ray radiation. The ultimate utility of this "self-developing" resist system will depend upon its efficacy as an etch barrier. It seems clear that such materials would not serve as adequate etch masks for... 

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). 

With the narrow range of wavelength of the excimer lamp, a specific initiator can be selected with the maximum efficiency in that range. Thus, it is possible to reduce the amount of photoinitiator from that used for conventional UV sources and reduce the cost of the material and amounts of unreacted initiator in the finished coated substrate. Excimer lamps can be used for both free radical and cationic photoinitiators. ... 

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

The cationic photoinitiator triarylsulfonium, diaryliodonium, and related aryl diazonium salts containing nonnucleophilic counterions, were used by Konarski et al. (2) to prepare adhesives. 

Carbosilane methacrylate oligomers were prepared in a two-step process. Initially a di-allyloxy intermediate was methacrylated using methacrylic anhydride to introduce a thermal crosslinker. Hydrosilation of this product was performed using chloroplatinic acid and sym-divinyltetramethyldisiloxane with 1,3- or 1,4-bis-dimethylsilylbenzene. The cationic photoinitiator diphenyl iodonium hexafluorophosphate was used to polymerize all blended compositions. 

Photochemical decomposition of cationic photoinitiators can be sensitized by energy or electron transfer from the excited state of a sensitizer. Certain sensitizers, such as isopropylthioxantone, anthracene or even certain dyes absorbing in the... 

Cationic photoinitiation is based on the ring opening of the oxirane group. The photoinitiators of practical importance belong to three main classes of compounds diazonium salts, onium (e.g. iodonium and sulfonium) salts, and organometallic complexes,55 which upon irradiation by UV light decompose and yield an acid catalyst. 

Many papers have been published on positive electron-beam resists. These resists are mostly polymers which are degraded upon electron-beam irradiation. The resulting lower molecular weight polymer in the exposed area can be selectively removed by a solvent under certain developing conditions. The development is accomplished by the difference in the rate of dissolution between the exposed and unexposed areas, which is a function of the molecular weight of the polymer. Recently, Willson and his co-workers reported the new type of positive resist, poly(phthalaldehyde), the exposure of which in the presence of certain cationic photoinitiators resulted in the spontaneous formation of a relief image without any development step (/). 

Table 2.23 Decomposition reactions of some cationic photoinitiators...

Polymeric cationic curable formulations containing an d -biphenylthioxanthenium salt initiator 638 have been reported to reduce the residual odor and benzene levels associated with other -phenyl cationic photoinitiator systems <2006W02006/060281 >. The mechanism of photo-acid generation from related rS -aryl thiopyranium salts... 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

Photoinitiators are perhaps the most important component in uv cured radiation coatings. The photoinitiator is an ingredient that absorbs light and is responsible for the production of free radicals in a free radical polymerized system or cations in a cationic photoinitiated system. The photoinitiators are usually added to the reactive coating formulations in concentration ranges from less than 1 to 20 percent by weight based on the total formulation. The absorption bands of the photoinitiators should overlap the emission spectra of the various commercial light sources. 

Visible light cured epoxy adhesives and coatings have been developed for architectural, industrial, and maintenance applications and for products difficult to heat or uv/EB cure because of their size. These are clear, one-part epoxy resins that cure by exposure to visible light for a few hours. They are formulated with cycloaliphatic epoxy compounds and a cationic photoinitiator that generates a strong acid when exposed to sunlight. 

The cationic photoinitiator has relatively limited absorbance spectra, and the cure is more sluggish than a uv cured epoxy-acrylate. As a result, the uv may be used to develop handling strength in the epoxy adhesive, and a room temperature or elevated-temperature postcure may be necessary after the initial uv exposure for the adhesive to achieve ultimate properties. 

Cationic photoinitiators are frequently found in classes of compounds such as the triaryl sulfonium, tetraaryl phosphonium, and diaryliodonium salts of large protected anions (hexafluorophosphates or antimonates). These compounds are soluble in most epoxy resins, do not activate epoxy cure until exposed to uv light, are insensitive to room lighting, and have long storage life at room temperature. Cationic photoinitiators form an acid catalyst when exposed to uv light and consequently start the cationic chemical reaction. 

The cationic photoinitiators are sensitive to moisture, and the acid species formed can promote corrosion. Frequently the formulation for these curing methods contains solvent, which evaporates during curing, so that an important environmental advantage is lost. For these reasons cationic uv cure is usually preferred over free radical cure only when the higher-performance properties are justifiable. 

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