Photoinitiators-Light Absorbing

Thickness of the laminar layer is deterrnined both by the need to reproduce fine detail in the object and by the penetration depth of the actinic laser light into the monomer bath (21,76). There is thus a trade-off between precision of detail in the model and time required for stereohthography, ie, the number of layers that have to be written, and an optimum Light-absorbing initiator concentration in the monomer bath corresponding to the chosen layer thickness. Titanocene-based initiators, eg, bis-perfluorophenyltitanocene has been recommended for this apphcation (77). Mechanistic aspects of the photochemistry of titanocenes and mechanisms of photoinitiation have been reviewed (76). 

The rate of formation of radical from the photoinitiator molecule (the only light-absorbing molecule in the system) Vr- is generally given by Eq. (4) ... 

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

BAPO. The photoinitiator, BAPO, absorbs throughout the UV but also in the long UV and short visible light regions. It has the following structure,... 

Pigments generally inhibit uv curing to some degree since the pigments absorb and/or scatter uv radiation. This interferes with the ability of the photoinitiator to absorb the light energy required to initiate the polymerization reactions. Thus, the majority of commercial radiation curable adhesives are clear or contain silica. 

Since photoinitiation occurs in specific monomeric mixtures, as Marcus theory predicts, the properties of the to-be-photopolymerized mixture (polarity, viscosity, electron-donating or electron-accepting properties) may play an important role in the overall efficiency of the process. Considering this, the monomer can participate in the photoinduced electron transfer process, either as a light-absorbing chromophore, as a hydrogen atom source, or as an electron donor/electron acceptor. 

This type of photoinitiating system has been studied carefully using a series of 6-alkoxy-2,4-diiodo-3-fluorones and 2-acyl- or 7-alkyl-2,4,5-triiodo-3-fluorones [117-120] as the primary light absorbers. The systems have been further evaluated as initiators for photopolymerization in the presence of a tertiary aromatic amine. Photoinitiated polymerization was triggered with an argon-ion (514.5 nm line) or He-Ne (632 nm line) laser. 

General scheme for photoinduced cationic polymerization is depicted in Scheme 11.1. A photosensitive compound, namely, photoinitiator (PI), absorbs incident light and undergoes decomposition leading to production of initiating species. Active species, namely, a radical cation (R+") in turn, react with cationic polymerizable monomers (M), and yield polymer (Scheme 11.1). 

It is also important to select photosensitizers and photoinitiators with absorption bands that overlap the emission spectra of the various commercial UV and visible light sources (only the light absorbed by a molecule evokes a photochemical reaction). Some of the most common photoinitiator systems in use today are as follows ... 

A composition of an electron transfer free radical photo initiating system was reported recently. This composition uses a light absorber, a dye, and an electron donor The donor is a sulfur-containing aromatic carboxylic acid. The structure of the donor is such that upon photocleaving, the leaving group forms free radicals. It is claimed that the experimental results show a transformation of the sulfur-containing aromatic carboxylic acid into an ammonium salt. This is accompanied by a substantial increase in photoinitiation ability ... 

A combination of benzophenone and 1,3-dioxane is a convenient hydrogen abstraction-type photoinitiator system for the free radical photopolymerization of methyl methacrylate, styrene and other monomers. As an advantage, this system does not require an additional hydrogen donor as other conventional initiators. In a similar way, mixtures of thioxanthone derivatives and fluorenes can be used as visible light absorbing oil- and water-soluble photoinitiators for free radical polymerization of methyl methacrylate, ethyl 2-(2-phosphonoethoxymethyl)acrylate and trimethylolpropane triacrylate. Photopolymerization and laser flash photolysis studies reveal that initiation occurs by intra- and intermolecular hydrogen abstraction by the thioxanthone-like triplet excited state. 

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