Photopolymer

In ionic curing processes, an ionic photoinitiator is used to activate the functional group of the oligomers that are going to participate in cross-linking. Typically photopolymerization is a very selective process and it is crucial that the polymerization takes place only where it is desired to do so. In order to satisfy this, liquid neat oligomer can be doped with either anionic or cationic photoinitiators that will initiate polymerization only when radiated with light. Monomers, or functional groups, employed in cationic photopolymerization include: styrenic compounds, vinyl ethers, N-vinyl carbazoles, lactones, lactams, cyclic ethers, cyclic acetals, and cyclic siloxanes. The majority of ionic photoinitiators fall under the cationic class; anionic photoinitiators are considerably less investigated.[5] There are several classes of cationic initiators, including onium salts, organometallic compounds and pyridinium salts.[5] As mentioned earlier, one of the drawbacks of the photoinitiators used for photopolymerization is that they tend to absorb in the short UV region.[7] Photosensitizers, or chromophores, that absorb in a much longer wavelength region can be employed to excite the photoinitiators through an energy transfer.[5] Other modifications to these types of systems are free radical assisted cationic polymerization. In this case, a free radical is formed from another species in solution that reacts with the photoinitiator in order to start polymerization. Although there are a diverse group of compounds activated by cationic photoinitiators, the compounds that find most industrial uses contain epoxides, oxetanes, and vinyl ethers.[1] One of the advantages to using cationic photopolymerization is that once the polymerization has begun it is no longer sensitive to oxygen and does not require an inert atmosphere to perform well.[1]

Photolysis

${\displaystyle {\begin{matrix}{}\\{\ce {[R'-R+X^{-}]->[hv]{[R'-R+X-]^{\ast }}->{R^{.+}}+{R'^{.}}+X^{-}->[{\ce {MH}}]{{R+}-H}+{M-R'}+X^{-}->{R}+H+X^{-}}}\\{}\end{matrix}}}$

M = Monomer

Onium salts

Since their discovery in the 1970s aryl onium salts, more specifically iodonium and sulfonium salts, have received much attention and have found many industrial applications. Other less common onium salts include ammonium and phosphonium salts.[1]

Onium Salts

A typical onium compound used as a photoinitiator contains two or three arene groups for iodonium and sulfonium respectively. Onium salts generally absorb short wavelength light in the UV region spanning from 225–300 nm.[5]^: 293 One characteristic that is crucial to the performance of the onium photoinitiators is that the counter anion is non-nucleophilic. Since the Brønsted acid generated during the initiation step is considered the active initiator for polymerization, there is a termination route where the counter ion of the acid could act as the nucleophile instead of a functional groups on the oligomer. Common counter anions include BF−4, PF−6, AsF−6 and SbF−6. There is an indirect relationship between the size of the counter ion and percent conversion.

Nowadays, most radical photopolymerization pathways are based on addition reactions of carbon double bonds in acrylates or methacrylates, and these pathways are widely employed in photolithography and stereolithography.[12]

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.[1]

In the free radical mechanism of radiation curable systems, light absorbed by a photoinitiator generates free-radicals which induce cross-linking reactions of a mixture of functionalized oligomers and monomers to generate the cured film [13]

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.[4] 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, or through disproportionation, which occurs when an atom (typically hydrogen) is transferred from one radical chain to another resulting in two polymeric chains.

Initiation

${\displaystyle {\begin{aligned}Initiator+h_{\nu }&{\ce {->R^{\bullet }}}\\{\ce {{R^{\bullet }}+M}}\ &{\ce {->RM^{\bullet }}}\end{aligned}}}$

Propagation

${\displaystyle {\ce {{RM^{\bullet }}+M_{\mathit {n}}->RM_{{\mathit {n}}+1}^{\bullet }}}}$

Termination

combination

${\displaystyle {\ce {{RM_{\mathit {n}}^{\bullet }}+{^{\bullet }M_{\mathit {m}}R}->RM_{\mathit {n}}M_{\mathit {m}}R}}}$

disproportionation

${\displaystyle {\ce {{RM_{\mathit {n}}^{\bullet }}+{^{\bullet }M_{\mathit {m}}R}->{RM_{\mathit {n}}}+M_{\mathit {m}}R}}}$

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 to 3000. In general, monomers with higher functionality result in a tighter crosslinking density of the finished material.[5] Typically these oligomers and monomers alone do not absorb sufficient energy for the commercial light sources used, therefore photoinitiators are included.[4][13]

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.[13]

Free-rad types of photoinitiators1

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, giving 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.

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 functionalized 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.[4]

Acrylated epoxy oligomer

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 and 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 desired properties of the final material.[4]

Photopolymerization has wide-ranging applications, from imaging to biomedical uses.

Photoresists

Photoresists are coatings, or oligomers, that are deposited on a surface and are designed to change properties upon irradiation of light. These changes either polymerize the liquid oligomers into insoluble cross-linked network polymers or decompose the already solid polymers into liquid products. Polymers that form networks during photopolymerization are referred to as negative resist. Conversely, polymers that decompose during photopolymerization are referred to as positive resists. Both positive and negative resists have found many applications including the design and production of micro-fabricated chips. The ability to pattern the resist using a focused light source has driven the field of photolithography.

Differences between negative and positive photoresist

Negative resists

As mentioned, negative resists are photopolymers that become insoluble upon exposure to radiation. They have found a variety of commercial applications, especially in the area of designing and printing small chips for electronics. A characteristic found in most negative tone resists is the presence of multifunctional branches on the polymers used. Radiation of the polymers in the presence of an initiator results in the formation of a chemically resistant network polymer. A common functional group used in negative resists is epoxy functional groups. An example of a widely used polymer of this class is SU-8. SU-8 was one of the first polymers used in this field, and found applications in wire board printing.[17] In the presence of a cationic photoinitiator photopolymer, SU-8 forms networks with other polymers in solution. Basic scheme shown below.

SU-8 photopolyermization

SU-8 is an example of an intramolecular photopolymerization forming a matrix of cross-linked material. Negative resists can also be made using co-polymerization. In the event that two different monomers, or oligomers, are in solution with multiple functionalities, it is possible for the two to polymerize and form a less soluble polymer.

Manufacturers also use light curing systems in OEM assembly applications such as specialty electronics or medical device applications.[18]

Fine printing

A printing plate of a city map created in photopolymer.

Photopolymers can be used to generate printing plates, which are then pressed onto paper-like metal type.[21] This is often used in modern fine printing to achieve the effect of embossing (or the more subtly three-dimensional effect of letterpress printing) from designs created on a computer without needing to engrave designs into metal or cast metal type. It is often used for business cards.[22][23]

Washing the polymer plates after they have been exposed to ultra-violet light may result in monomers entering the sewer system, eventually adding to the plastic content of the oceans. Current water purification installations are not able to remove monomer molecules from sewer water. Some monomers, such as styrene, are toxic or carcinogenic.