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Free Radical Mechanism
In the free radical mechanism of radiation curable systems a mixture of low molecular weight monomers and oligomers are converted into highly crosslinked, chemically-resistant films by absorption of UV radiation. In general, light absorbed by a photoinitiator generates free-radicals which induce crosslinking reactions of a mixture of functionalized oligomers and monomers to generate the cured film [1] Photocurable materials that form through the free-radical mechanism undergo chain-growth polymerization, which includes three basic steps: initiation, chain propagation, and chain termination. The three steps are depicted in the scheme below, where R• represents the radical that forms upon interaction with radiation during initiation, and M is a monomer.[2] The active monomer that is formed is then propagated to create growing polymeric chain radicals. In photocurable materials the propagation step involves reactions of the chain radicals with reactive double bonds of the prepolymers or oligomers. The termination reaction usually proceeds through combination, in which two chain radicals are joined together, or through disproportionation, which occurs when an atom (typically hydrogen) is transferred from one radical chain to another resulting in two polymeric chains.
Most composites that cure through radical chain growth contain a diverse mixture of oligomers and monomers with functionality that can range from 2-8 and molecular weights from 500-3000. In general, monomers with higher functionality result is a tighter crosslinking density of the finished material. Typically these oligomers and monomers alone do not absorb sufficient energy for the commerical light sources used, therefore photoinitiators are included.[1] , [2]
Free-radical photoinitiators[1]
There are two types of free-radical photoinitators: Those generated through abstraction of a hydrogen atom from a donor compound (also called co-initiator), and those generated by cleavage to give two radical species. Examples of each type of free-radical photoinitiator is shown below.
Benzophenone, Xanthones, and Quinones are examples of abstraction type photoinitiators, with common donor compounds being aliphatic amines. The resulting R• species from the donor compound becomes the initiator for the free radical polymerization process, while the radical resulting from the starting photoinitiator (benzophenone in the example shown above) is typically unreactive.
Benzoin ethers, Acetophenones, Benzoyl Oximes, and Acylphosphines are some examples of cleavage-type photoinitiators. Cleavage readily occurs for the species to give two radicals upon absorption of light, and both radicals generated can typically initiate polymerization. Cleavage type photoinitiators do not require a co-initiator, such as aliphatic amines. This can be beneficial since amines are also effective chain transfer species. Chain-transfer processes reduce the chain length and ultimately the crosslink density of the resulting film.
Oligomers used in free radical curing processes[1]
The properties of a photocured material, such as flexibility, adhesion, and chemical resistance are provided by the functionalized oligomers present in the photocurable composite. Oligomers are typically epoxides, urethanes, polyethers, or polyesters, each of which provide specific properties to the resulting material. Each of these oligomers are typically functionallized by an acrylate. An example shown below is an epoxy oligomer that has been functionalized by acrylic acid. Acrylated epoxies are useful as coatings on metallic substrates, and result in glossy hard coatings. Acrylated urethane oligomers are typically abrasion resistant, tough, and flexible making ideal coatings for floors, paper, printing plates, and packaging materials. Acrylated polyethers and polyesters result in very hard solvent resistant films, however, polyethers are prone to UV degradation and therefore are rarely used in UV curable material. Often formulations are composed of several types of oligomers to achieve the desirable properties for a material.
Monomers used in free-radical curing processes[1]
The monomers used in radiation curable systems help control the speed of cure, crosslink density, final surface properties of the film, and viscosity of the resin. Examples of monomers include styrene, N-Vinylpyrrolidone, and acrylates. Styrene is a low cost monomer and provides a fast cure, N-vinylpyrrolidone results in a material that is highly flexible when cured, has low toxicity, and acrylates are highly reactive, allowing for rapid cure rates, and are highly versatile with monomer functionality ranging from monofunctional to tetrafunctional. Like oligomers, several types of monomers can be employed to achieve the desirable properties of the final material.
References
- ^ a b c d e Hoyle, Charles (1990). Radiation Curing of Polymeric Materials. Washington, DC: Am. Chem. Soc. pp. 1–15.
- ^ a b Ravve, A. (2006). Light-Associated Reactions of Synthetic Polymers. Spring Street, New York, NY 10013, USA: Springer Science+Business Media, LLC. ISBN 0-387-31803-8.
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