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Photopolymerized systems are typically cured through UV radiation, since ultraviolet light is more energetic; however, the development of dye-based photoinitiator systems have allowed for the use of visible light, having potential advantages of processes that are more simple and safe to handle. UV curing in industrial processes has greatly expanded over the past several decades. Many traditional thermally cured and solvent-based technologies can be replaced by photopolymerization technologies. The advantages of photopolymerization over thermally cured polymerization include high rates of polymerization and environmental benefits from elimination of volatile organic solvents and low energy inputs.

Free Radical Mechanism
Before the free radical nature of certain polymerizations was determined, certain monomers were observed to polymerize when exposed to light. The first to demonstrate the photoinduced free radical chain reaction of vinyl bromide was Ivan Ostromislensky, a Russian chemist who also studied the polymerization of synthetic rubber. Subsequently many compounds were found to become dissociated by light and found immediate use as photoinitiators in the polymerization industry. In the free radical mechanism of radiation curable systems 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 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. 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. ,

Free-radical photoinitiators
There are two types of free-radical photoinitators: A two component system where the radical is generated through abstraction of a hydrogen atom from a donor compound (also called co-initiator), and a one component system where two radicals are generated by cleavage. 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
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
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.

Applications
Photopolymerization is a widely used technology, used in applications ranging from imaging to biomedical uses. Below is a description of just some photopolymerization applications.

Medical Uses
Dentistry is one market where free radical photopolymers have found wide usage as adhesives, sealant composites, and protective coatings. These dental composites are based on a camphorquinone photoinitiator and a matrix containing methacrylate oligomers with inorganic fillers such as silicon dioxide. Photocurable adhesives are also used in the production of catheters, hearing aids, surgical masks, medical filters, and blood analysis sensors. Photopolymers have also been explored for uses in drug delivery, tissue engineering and cell encapsulation systems. Photopolymerization processes for these applications are being developed to be carried out in vivo or ex vivo. In vivo photopolymerization would provide the advantages of production and implantation with minimal invasive surgery.Ex vivo photopolymerization would allow for fabrication of complex matrices, and versatility of formulation. Although photopolymers show promise for a wide range of new biomedical applications, biocompatibility with photopolymeric materials must still be addressed and developed.

3D-Imaging
Stereolithography, digital imaging, and 3D inkjet printing are just a few 3D imaging technologies that make use of photopolymers. 3D imaging usually proceeds with CAD-CAM software, which creates a 3D image to be translated into a 3D plastic object. The image is cut in slices, where each slice is reconstructed through radiation curing of the liquid polymer,converting the image into a solid object. Photopolymers used in 3D imaging processes must be designed to have a low volume shrinkage upon polymerization in order to avoid distortion of the solid object. Common monomers utilized for 3D imaging include multifunctional acrylates and methacrylates combined with a non-polymeric component in order to reduce volume shrinkage. A competing composite mixture of epoxide resins with cationic photoinitiators is becoming increasingly used since their volume shrinkage upon ring-opening polymerization is significantly below those of acrylates and methacrylates. Free-radical and cationic composed of both epoxide and acrylate monomers have also been employed, gaining the high rate of polymerization from the acryilic monomer, and better mechanical properties from the epoxy matrix.