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Aphotopolymer orlight-activated resin is apolymer that changes its properties when exposed to light, often in theultraviolet orvisible region of theelectromagnetic spectrum.^[1] These changes are often manifested structurally, for example hardening of the material occurs as a result ofcross-linking when exposed to light. An example is shown below depicting a mixture ofmonomers,oligomers, andphotoinitiators that conform into a hardened polymeric material through a process calledcuring.^[2][3]

A wide variety of technologically useful applications rely on photopolymers; for example, someenamels andvarnishes depend on photopolymer formulation for proper hardening upon exposure to light. In some instances, an enamel can cure in a fraction of a second when exposed to light, as opposed to thermally cured enamels which can require half an hour or longer.^[4] Curable materials are widely used for medical, printing, andphotoresist technologies.

Changes in structural and chemical properties can be induced internally bychromophores that thepolymer subunit already possesses, or externally by addition ofphotosensitive molecules. Typically a photopolymer consists of a mixture of multifunctional monomers and oligomers in order to achieve the desired physical properties, and therefore a wide variety of monomers and oligomers have been developed that canpolymerize in the presence of light either through internal or externalinitiation. Photopolymers undergo a process called curing, where oligomers arecross-linked upon exposure to light, forming what is known as anetwork polymer. The result of photo-curing is the formation of athermoset network of polymers. One of the advantages ofphoto-curing is that it can be done selectively using high energy light sources, for examplelasers, however, most systems are not readily activated by light, and in this case a photoinitiator is required. Photoinitiators are compounds that upon radiation of light decompose into reactive species that activatepolymerization of specificfunctional groups on the oligomers.^[5] An example of a mixture that undergoes cross-linking when exposed to light is shown below. The mixture consists of monomericstyrene and oligomericacrylates.^[6]

Most commonly, photopolymerized systems are typically cured through UV radiation, sinceultraviolet light is more energetic. However, the development of dye-based photoinitiator systems have allowed for the use ofvisible light, having the potential advantages of being simpler and safer to handle.^[7]UV curing in industrial processes has greatly expanded over the past several decades. Many traditional thermally cured andsolvent-based technologies can be replaced by photopolymerization technologies. The advantages ofphotopolymerization over thermally curedpolymerization include higher rates of polymerization and environmental benefits from elimination of volatileorganic solvents.^[1]

There are two general routes for photoinitiation:free radical andionic.^[1][4] The general process involves doping a batch of neat polymer with small amounts of photoinitiator, followed by selective radiation of light, resulting in a highlycross-linked product. Many of these reactions do not require solvent which eliminatestermination path via reaction of initiators with solvent and impurities, in addition to decreasing the overall cost.^[8]

In ionic curing processes, an ionicphotoinitiator is used to activate thefunctional group of theoligomers that are going to participate incross-linking. Typicallyphotopolymerization is a very selective process and it is crucial that thepolymerization takes place only where it is desired to do so. In order to satisfy this, liquid neat oligomer can be doped with eitheranionic orcationic photoinitiators that willinitiate polymerization only when radiated withlight.Monomers, or functional groups, employed in cationic photopolymerization include:styrenic compounds,vinyl ethers, N-vinylcarbazoles,lactones, lactams, cyclicethers, cyclicacetals, and cyclicsiloxanes. The majority of ionic photoinitiators fall under the cationic class; anionic photoinitiators are considerably less investigated.^[5] There are several classes of cationic initiators, includingonium salts,organometallic compounds andpyridinium salts.^[5] As mentioned earlier, one of the drawbacks of the photoinitiators used for photopolymerization is that they tend to absorb in the shortUV region.^[7] Photosensitizers, orchromophores, 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 arefree 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 containepoxides, 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 tooxygen and does not require aninert atmosphere to perform well.^[1]

Photolysis

${\displaystyle \begin{array}{c}\\ [{\text{R}}^{\prime }-{\text{R}}^{+}{\text{X}}^{-}]{\underset{R^{\prime }-R+X-}{\overset{hv}{\to }}}^{\ast }⟶{\text{R}}^{\cdot ++}{\text{R}}^{\prime \cdot }+{\text{X}}^{-}\stackrel{\text{MH}}{\to }{\text{R}}^{+}-{\text{H}}^{+}\text{M}-{\text{R}}^{\prime }+{\text{X}}^{-}⟶{\text{R}}^{+}{\text{H}}^{+}{\text{X}}^{-}\\ \end{array}}$

M =Monomer

The proposed mechanism forcationicphotopolymerization begins with thephotoexcitation of the initiator. Once excited, bothhomolytic cleavage and dissociation of a counteranion takes place, generating acationic radical (R), an arylradical (R') and an unaltered counter anion (X). The abstraction of alewis acid by the cationic radical produces a very weakly bound hydrogen and afree radical. The acid is furtherdeprotonated by the anion (X) in solution, generating a lewis acid with the starting anion (X) as a counter ion. It is thought that the acidicproton generated is what ultimately initiates thepolymerization.^[9]

Since their discovery in the 1970s arylonium salts, more specificallyiodonium andsulfonium salts, have received much attention and have found many industrial applications. Other less common onium salts includeammonium andphosphonium salts.^[1]

A typicalonium compound used as aphotoinitiator contains two or threearene groups for iodonium and sulfonium respectively. Onium salts generally absorb short wavelength light in theUV region spanning from 225–300 nm.^[5]: 293 One characteristic that is crucial to the performance of the onium photoinitiators is that the counteranion is non-nucleophilic. Since theBrønsted acid generated during theinitiation step is considered the active initiator forpolymerization, there is atermination route where the counter ion of the acid could act as the nucleophile instead of a functional groups on the oligomer. Common counter anions includeBF−4,PF−6,AsF−6 andSbF−6. There is an indirect relationship between the size of the counter ion and percent conversion.

Although less common,transition metal complexes can act as cationicphotoinitiators as well. In general, the mechanism is more simplistic than theonium ions previously described. Most photoinitiators of this class consist of a metal salt with a non-nucleophilic counter anion. For example,ferrocinium salts have received much attention for commercial applications.^[10] The absorption band for ferrocinium salt derivatives are in a much longer, and sometimesvisible, region. Upon radiation the metal center loses one or moreligands and these are replaced byfunctional groups that begin thepolymerization. One of the drawbacks of this method is a greater sensitivity tooxygen. There are also severalorganometallic anionic photoinitiators which react through a similar mechanism. For theanionic case, excitation of a metal center is followed by eitherheterolytic bond cleavage orelectron transfer generating the active anionicinitiator.^[5]

Generallypyridiniumphotoinitiators are N-substitutedpyridine derivatives, with a positive charge placed on thenitrogen. The counter ion is in most cases a non-nucleophilic anion. Upon radiation,homolytic bond cleavage takes place generating a pyridiniumcationic radical and a neutralfree radical. In most cases, ahydrogen atom is abstracted from theoligomer by the pyridinium radical. The free radical generated from the hydrogen abstraction is then terminated by the free radical in solution. This results in a strong pyridinium acid that can initiatepolymerization.^[11]

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 thefree radical nature of certainpolymerizations was determined, certainmonomers were observed to polymerize when exposed to light. The first to demonstrate the photoinduced free radical chain reaction ofvinyl bromide wasIvan Ostromislensky, a Russian chemist who also studied the polymerization ofsynthetic rubber. Subsequently, many compounds were found to become dissociated by light and found immediate use asphotoinitiators 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 undergochain-growth polymerization, which includes three basic steps:initiation,chain propagation, andchain termination. The three steps are depicted in the scheme below, whereR• represents the radical that forms upon interaction with radiation during initiation, andM 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 throughcombination, in which two chain radicals are joined, or throughdisproportionation, which occurs when an atom (typically hydrogen) is transferred from one radical chain to another resulting in two polymeric chains.

Initiation

Propagation

${\displaystyle {\text{RM}}^{\bullet +}{\text{M}}_{\mathit{\text{n}}}^{}⟶{\text{RM}}_{\mathit{\text{n}}+1}^{\bullet }}$

Termination

combination

${\displaystyle {\text{RM}}_{\mathit{\text{n}}}^{\bullet +\bullet }{\text{M}}_{\mathit{\text{m}}}^{}\text{R}⟶{\text{RM}}_{\mathit{\text{n}}}^{}{\text{M}}_{\mathit{\text{m}}}^{}\text{R}}$

disproportionation

${\displaystyle {\text{RM}}_{\mathit{\text{n}}}^{\bullet +\bullet }{\text{M}}_{\mathit{\text{m}}}^{}\text{R}⟶{\text{RM}}_{\mathit{\text{n}}}^{+}{\text{M}}_{\mathit{\text{m}}}^{}\text{R}}$

Most composites that cure through radical chain growth contain a diverse mixture of oligomers and monomers withfunctionality 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]

There are two types of free-radical photoinitators: A two component system where the radical is generated throughabstraction of a hydrogen atom from a donor compound (also called co-initiator), and a one-component system where two radicals are generated bycleavage. Examples of each type of free-radical photoinitiator is shown below.^[13]

Benzophenone,xanthones, andquinones are examples of abstraction type photoinitiators, with common donor compounds being aliphatic amines. The resultingR• 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 effectivechain 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 typicallyepoxides,urethanes,polyethers, orpolyesters, each of which provide specific properties to the resulting material. Each of these oligomers are typically functionalized by anacrylate. An example shown below is an epoxy oligomer that has been functionalized byacrylic 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]

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 includestyrene,N-Vinylpyrrolidone, andacrylates. 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.

Dentistry is one field in whichfree radical photopolymers have found wide usage as adhesives, sealant composites, and protective coatings. Thesedental composites are based on a camphorquinonephotoinitiator and a matrix containingmethacrylateoligomers with inorganic fillers such assilicon dioxide. Resin cements are utilized inluting castceramic, fullporcelain, andveneer restorations that are thin or translucent, which permits visible light penetration in order to polymerize the cement. Light-activated cements may be radiolucent and are usually provided in various shades since they are utilized in esthetically demanding situations.^[14]

Conventionalhalogen bulbs,argon lasers andxenonarc lights are currently used in clinical practice. A new technological approach for curing light-activated oralbiomaterials using a light curing unit (LCU) is based on bluelight-emitting diodes (LED). The main benefits of LED LCU technology are the long lifetime of LED LCUs (several thousand hours), no need for filters or a cooling fan, and virtually no decrease of light output over the lifetime of the unit, resulting in consistent and high quality curing. Simple depth of cure experiments ondental composites cured with LED technology show promising results.^[15]

Photocurable adhesives are also used in the production ofcatheters,hearing aids,surgical masks, medical filters, and blood analysis sensors.^[1] Photopolymers have also been explored for uses in drug delivery, tissue engineering and cell encapsulation systems.^[16] Photopolymerization processes for these applications are being developed to be carried outin vivo orex 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.

Stereolithography,digital imaging, and 3D inkjet printing are just a few3D printing technologies that make use of photopolymerization pathways. 3D printing usually utilizesCAD-CAM software, which creates a 3D computer model to be translated into a 3D plastic object. The image is cut in slices; each slice is then reconstructed through radiation curing of the liquidpolymer, converting the image into a solid object. Photopolymers used in 3D imaging processes require sufficient cross-linking and should ideally be designed to have minimal volume shrinkage uponpolymerization in order to avoid distortion of the solid object. Common monomers utilized for 3D imaging include multifunctionalacrylates andmethacrylates, often combined with a non-polymeric component in order to reduce volume shrinkage.^[12] A competing composite mixture of epoxide resins with cationic photoinitiators is becoming increasingly used since their volume shrinkage uponring-opening polymerization is significantly below those of acrylates and methacrylates.Free-radical andcationic polymerizations composed of both epoxide and acrylate monomers have also been employed, gaining the high rate of polymerization from the acrylic monomer, and better mechanical properties from the epoxy matrix.^[1]

Photoresists are coatings, oroligomers, that are deposited on a surface and are designed to change properties upon irradiation oflight. These changes eitherpolymerize the liquid oligomers into insolublecross-linked network polymers or decompose the already solid polymers into liquid products. Polymers that formnetworks duringphotopolymerization are referred to asnegative resist. Conversely,polymers that decompose during photopolymerization are referred to aspositive 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 ofphotolithography.

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 ofmultifunctional branches on thepolymers used. Radiation of the polymers in the presence of aninitiator results in the formation of a chemically resistantnetwork polymer. A commonfunctional group used in negative resists isepoxy functional groups. An example of a widely usedpolymer of this class isSU-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 acationic photoinitiator photopolymer,SU-8 formsnetworks with other polymers in solution. Basic scheme shown below.

SU-8 is an example of anintramolecularphotopolymerization forming a matrix ofcross-linked material. Negative resists can also be made using co-polymerization. In the event that two differentmonomers, oroligomers, are in solution with multiplefunctionalities, 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]

Exposure of apositive resist to radiation changes the chemical structure such that it becomes a liquid or more soluble. These changes in chemical structure are often rooted in the cleavage of specificlinkers in thepolymer. Once irradiated, the "decomposed" polymers can be washed away using a developersolvent leaving behind the polymer that was not exposed to light. This type of technology allows the production of very fine stencils for applications such asmicroelectronics.^[19] In order to have these types of qualities, positive resists utilize polymers withlabile linkers in their back bone that can be cleaved upon irradiation, or use aphoto-generated acid tohydrolyze bonds in the polymer. A polymer that decomposes upon irradiation to a liquid or more soluble product is referred to as apositive tone resist. Commonfunctional groups that can be hydrolyzed by a photo-generated acid catalyst includepolycarbonates andpolyesters.^[20]

Photopolymers can be used to generate printing plates, which are then pressed onto paper-likemetal type.^[21] This is often used in modern fine printing to achieve the effect ofembossing (or the more subtly three-dimensional effect ofletterpress 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]

Industrial facilities are utilizing light-activated resin as a sealant for leaks and cracks. Some light-activated resins have unique properties that make them ideal as a pipe repair product. These resins cure rapidly on any wet or dry surface.^[24]

Light-activated resins recently gained a foothold with fly tiers as a way to create custom flies in a short period of time, with very little clean up involved.^[25]

Light-activated resins have found a place in floor refinishing applications, offering an instant return to service not available with any other chemical due to the need to cure at ambient temperatures. Because of application constraints, these coatings are exclusively UV cured with portable equipment containing high intensity discharge lamps. Such UV coatings are now commercially available for a variety of substrates, such as wood, vinyl composition tile and concrete, replacing traditional polyurethanes for wood refinishing and low durability acrylics forVCT.

Washing the polymer plates after they have been exposed to ultra-violet light may result in^{[citation needed]} monomers entering the sewer system,^{[citation needed]} eventually adding to the plastic content of the oceans.^{[citation needed]} Current water purification installations are not able to remove monomer molecules from sewer water.^{[citation needed]} Some monomers, such asstyrene, are toxic orcarcinogenic.

• Polymers
• Photochemistry
• Adhesives

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