„Photopolymer“ – Versionsunterschied

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== Process ==


[[File:Photolysis General.jpg|center|Photolysis of cationic photoinitiator]]
# exposing imagewise a photopolymerizable element to actinic radiation emitting a wavelength in the range of 365 nm.
# removing the unexposed or unpolymerized areas of the plate, generally through the use of a solvent.
# drying the resulting plate.
The photopolymerized element can then be detackified by exposing the element to ultraviolet radiation emitting a wavelength in the range of 254 nm. To ensure final plate hardening and photopolymerization, the printing element can be further post-exposed to radiation emitting at wavelengths in the range of 365 nm.


===Cationic Photoinitiators===
Current platemaking processes utilize various sources of radiation for developing relief images and maximizing plate hardening. For example, actinic radiation from a variety of sources can be used, including commercial ultraviolet fluorescent tubes, medium, high, and low pressure mercury vapor lamps, argon glow lamps, photographic flood lamps, pulsed xenon lamps, carbon arc lamps, light-emitting diodes, etc.
The proposed mechanism for [[cationic]] [[polymerization|photopolymerization]] begins with the [[photoexcitation]] of the initiator. Once excited, both [[homolysis|homolytic]] cleavage and dissociation of a counter [[anion]] takes place, generating [[radical ion|cationic radical]] (R), an aryl [[radical]](R') and unaltered counter [[anion]] (X). The abstraction of a [[lewis acid]], in figure above a [[hydrogen]], by the [[radical ion|cationic radical]] produces a very weakly bound [[hydrogen]] and a [[free radical]]. The [[lewis acid|acid]] is further [[deprotonation|deprotonated]] by the [[anion]](X) in solution generating a [[lewis acid]] with the starting [[anion]] (X) as a counter ion. It is thought that the acidic [[proton]] generated is what ultimitely initiates the [[polymerization]].<ref>{{cite book|last=Zhdankin|first=Viktor|title=Hypervalent Iodine Chemistry: Preparation, Structure, and Synthetic Applications of Polyvalent Iodine Compounds|date=2013|publisher=John Wiley & Sons Ltd|page=427}}</ref>
====Onium Salts====
Since there discovery in the 1970's aryl [[onium compounds|onium]] salts, more specifically [[halonium ion|iodonium]] and [[sulfonium]] salts, have received much attention and have found many industrial applications.Other, less common, onium salts not mentioned here include [[ammonium]] and [[phosphonium]] salts.<ref>{{cite journal|last=Crivello|first=J.|coauthors=E. Reichmanis|title=Photopolymer Materials and Processes for Advanced Technologies|journal=Chemistry of Materials|date=2014|volume=26|pages=533-548}}</ref>
[[File:Onium salts 2.jpg|center|Onium Salts]]
The typical [[onium compound]] used as a [[photoinitiator]] contains two or three [[arene]] groups for [[halonium ion|iodonium]] and [[sulfonium]] respectively. [[onium compounds|Onium salts]] generally absorb short wavelength light in the [[ultraviolet|UV region]] spanning from 225-300nm.<ref>{{cite book|last=Fouassier|first=Jean|title=Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency|date=2012|publisher=John Wiley & Sons Ltd|page=293}}</ref> One characteristic that is crucial to the performance of the [[onium compounds|onium]] [[photoinitiator]]s is that the counter [[anion]] is non-[[nucleophile|nucleophilic]], . Since the [[Brønsted acid]] generated during the [[initiation (chemistry)|initiation]] step is considered the active initiator for [[polymerization]], there is a [[termination (chemistry)|termination]] route where the counter ion of the acid could act as the [[nucleophile]] instead of a functional groups on the oligomer. Common counter [[anion]]s include: {{chem|B|F|4|-}}, {{chem|P|F|6|-}}, {{chem|As|F|6|-}}, {{chem|Sb|F|6|-}}. There is a indirect relationship between the size of the counter ion and percent conversion.
====Organometallic====
Although less common, [[transition metal]] complexes can act as cationic [[photoinitiator]]s as well. In the general the mechanism is more simplistic than the [[onium compounds|onium]] ions previously described. Most [[photoinitiator]]s of this class consist of a metal salt with a non-nucleophilic counter anion. For example, [[ferrocene|ferrocinium]] salts have received much attentions from commercial applications. <ref>{{cite book|last=Meier|first=K|title=Proceedings of the RadCure Europe|date=1985|publisher=Basle Technical Paper}}</ref> The absorbtion band for [[ferrocene|ferrocinium]] salt derivatives are in a much longer, and sometimes [[visible spectrum|visible]], region. Upon radiations the metal center loses a [[ligand]](s) and the [[ligand]](s) are replaced by [[functional group]]s that begin the [[polymerization]]. Some of the drawbacks of this method is a greater sensitivity to [[oxygen]]. There are also several [[organometallic]] anionic [[photoinitiator]]s which react through a similar mechanism. For the [[anionic]] case, excitation of a metal center followed by either by [[heterolytic]] bond cleavage or [[electron transfer]] generating the active [[anionic]] [[photoinitiator|initiator]].<ref name="Photoinitiators for Polymer Synthesis: Scope, Reactivity and Efficiency" />


====Pyridinium Salts====
[[Category:3D printing]]
Generally [[pyridinium]] [[photoinitiator]]s are N-substituted [[pyridine]] derivatives, with a positive charge placed on the [[nitrogen]]. The counter ion is most cases is a non-nucleophilic anion. Upon radiation, [[homolysis|homolytic]] bond cleavage takes place generating a [[pyridinium]] [[radical ion|cationic radical]] and a neutral [[free radical]]. A [[hydrogen]], most cases, is abstracted from the [[oligomer]] by the [[pyridinium]] radical. The [[free radical]] generated from the [[hydrogen]] abstraction is then terminated by the [[free radical]] is solution. This results in a strong [[pyridinium]] acid that can initiate [[polymerization]].<ref>{{cite journal|last=TAKAHASHI|first=EIJI|coauthors=FUMIO SANDA, TAKESHI ENDO|title=Novel pyridinium salts as cationic thermal and photoinitiators and their photosensitization properties|journal=Journal of Polymer Science Part A: Polymer Chemistry|date=2002|volume=40|issue=8|page=1037}}</ref>


==Free Radical Mechanism==
{{Engineering-stub}}
Before the [[Radical polymerization|free radical]] nature of certain [[polymerization]]s 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 [[photoinitiator]]s in the polymerization industry.<ref name="Advanced Technologies" />
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 <ref name="Photocurable Coatings">{{cite book|last=Hoyle|first=Charles|title=Radiation Curing of Polymeric Materials|date=1990|publisher=Am. Chem. Soc.|location=Washington, DC|pages=1-15}}</ref>
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.<ref name="Light-associated reactions of synthetic polymers">{{cite book|last=Ravve|first=A.|title=Light-Associated Reactions of Synthetic Polymers|date=2006|publisher=Springer Science+Business Media, LLC|location=Spring Street, New York, NY 10013, USA|isbn=0-387-31803-8}}</ref> 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 [[kinetic chain length|combination]], in which two chain radicals are joined together, or through [[kinetic chain length|disproportionation]], which occurs when an atom (typically hydrogen) is transferred from one radical chain to another resulting in two polymeric chains.

[[File:Free rad mech1.jpg|center|Free rad mech1]]
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.<ref name="Photoinitiators for Polymer Synthesis: Scope, Reactivity and Efficiency">{{cite book|last=Fouassier|first=Jean Pierre|title=Photoinitiators for Polymer Synthesis: Scope, Reactivity and Efficiency|date=2012|publisher=Wiley-VCH Verlag GmbH & Co. KGaA|location=Weinheim, Germany|isbn=9783527648245}}</ref> Typically these oligomers and monomers alone do not absorb sufficient energy for the commercial light sources used, therefore photoinitiators are included.<ref name="Photocurable Coatings" />,<ref name="Light-associated reactions of synthetic polymers" />

===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.<ref name="Photocurable Coatings" />

[[File:Free-rad types of photoinitiators1.jpg|center|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 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 and Monomers===
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]], [[urethane|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.<ref name="Light-associated reactions of synthetic polymers" />
[[File:Acrylated epoxy oligomer.jpg|center|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, 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.<ref name="Light-associated reactions of synthetic polymers" />

=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 [[radical polymerization|free radical]] photopolymers have found wide usage as adhesives, sealant composites, and protective coatings. These [[dental composite|dental composites]] are based on a camphorquinone [[photoinitiator]] and a matrix containing [[methacrylate]] [[oligomer]]s with inorganic fillers such as [[silicon dioxide]]. [[Photocure|Photocurable]] adhesives are also used in the production of [[catheters]], [[hearing aid]]s, [[surgical mask]]s, medical filters, and blood analysis sensors.<ref name="Advanced Technologies" />
Photopolymers have also been explored for uses in drug delivery, tissue engineering and cell encapsulation systems.<ref name="photopolymerization of biomaterials">{{cite journal|last=Baroli|first=Biancamaria|title=Photopolymerization of biomaterials|journal=J. Chem. Technol. Biotechnol.|date=2006|volume=81|pages=491-499|doi=10.1002/jctb.1468}}</ref> 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 printing| 3D inkjet printing]] are just a few [[3D imaging]] technologies that make use of photopolymers. [[3D imaging]] usually proceeds with [[Computer-aided technologies|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 [[acrylate]]s and [[methacrylate]]s 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. [[Radical polymerization|Free-radical]] and [[cationic polymerization|cationic]] polymerizations 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. <ref name="Advanced Technologies" />

==Photoresists==
[[Photoresist]]s are coatings, or [[oligomer]]s, that are deposited on a surface and are designed to change properties upon irradiation of [[light]]. These changes either [[polymerization|polymerize]] the liquid [[oligomer]]s into insoluble [[branching (polymer chemistry)|cross-linked]] network polymers or decompose the already solid polymers into liquid products. Polymers that form [[branching (polymer chemistry)|networks]] during [[polymerization|photopolymerization]] are referred to as [[photoresist|negative resist]]. Conversely, [[polymer]]s that decompose during [[polymerization|photopolymerization]] are referred to as [[photoresist|positive resists]]. Both [[photoresist|positive]] and [[photoresist|negative resists]] have found many applications including the design and production of micro fabicated chips. The ability to pattern the resist using a focused light source has driven the field of [[photolithography]].
[[File:Photoresist Image.png|center|450x450px|Differences between negative and positive photoresist]]
===Negative Resists===
As mentioned, [[photoresist|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 characterisitc found in most [[photoresist|negative tone resists]] is the presence of [[functional group|multifunctional]] branches on the [[polymer]]s used. Radiation of the [[polymer]]s in the presence of an [[photoinitiator|intiator]] results in the formation of chemically resistant [[branching (polymer chemistry)|network polymer]]. A common [[functional group]] used in [[photoresist|negative resist]] is [[epoxy]] [[functional group]]s. An example of a widely used [[polymer]] of this class is [[SU-8]]. [[SU-8]] was one of the first [[polymer]]s used in this field, and found applications in wire board printing.<ref>{{cite web|title=SU-8 Photosensitive Epoxy|url=http://www.cnm.es/projects/microdets/su8.htm|accessdate=2014}}</ref> In the presence of a [[cationic]] [[photoinitiator]] photopolymer [[SU-8]] forms [[branching (polymer chemistry)|networks]] with other [[polymer]]s in solution. Basic scheme shown below.

[[File:SU8.jpg|center|SU-8 photopolyermization]]

[[SU-8]] is an example of an [[intramolecular]] [[polymerization|photopolymerization]] forming a matrix of [[branching (polymer chemistry)|cross-linked]] material. [[photoresist|Negative resists]] can also be made using co-[[polymerization]]. In the event that you have two different [[monomer]]s, or [[oligomer]]s, in solution with multiple [[functional group|functionalities]] it is possible for the two to [[polymerization|polymerize]] and form a less soluble [[polymer]].

===Positive Resists===
As mentioned, [[photoresist|positive resist]] exposure 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 specific [[cross-link|linkers]] in the [[polymer]]. Once irradiated the "decomposed" [[polymer]]s can be washed away using a developer [[solvent]] leaving behind the [[polymer]] that was not exposed to [[light]]. This type of technology allows the production of very fine stencils for applications such as [[microelectronics]].<ref>{{cite book|last=Allcock|first=Harry|title=Introduction to Materials Chemistry|date=2008|publisher=Wiley and Sons|isbn=9780470293331|pages=248-258}}</ref> In order to have these types of qualities, [[photoresist|positive resist]] utilize [[polymer]]s with [[labile]] linkers in their back bone that can be cleaved upon irradiation or using a [[photoinitiator|photo-generated acid]] to [[hydrolyze]] bonds in the [[polymer]]. A [[polymer]] that decomposes upon irradiation to a liquid, or more soluble product is referred to as a [[photoresist|positive tone resist]]. Common [[functional group]]s that can be hydrolyzed by [[photoinitiator|photo-generated acid]] catalyst include [[polycarbonate]]s and [[polyester]]s.<ref>{{cite book|last=Thompson|first=Larry|title=Polymers for Microelectronics|date=1993|publisher=American Chemical Society}}</ref>

=References=
{{reflist}}

Version vom 14. März 2014, 17:25 Uhr

A photopolymer is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum.[1] These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing [2],[3]. A wide variety of technologically useful applications rely on photopolymers, for example some enamels and varnishes 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, and photoresist technologies.

Photopolymer scheme1
Photopolymer scheme1

Changes in structural and chemical properties can be induced internally by chromophores that the polymer subunit already posseses, or externally by addition of photosensitive 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 can polymerize in the presence of light either through internal or external initiation. Photopolymers undergo a process called curing, where oligomers are cross-linked upon exposure to light, forming what is known as a network polymer. The result of photo curing is the formation of a thermoset network of polymers. One of the advantages of photo-curing is that it can be done selectively using high energy light sources, for example lasers, 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 activate polymerization of specific functional 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 monomeric styrene and oligomeric acrylates.[6]

intro scheme for photopolymerization
intro scheme for photopolymerization

Most commonly, 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.[7] 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.[1]

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

Mechanisms

Ionic Mechanism

In ionic curing process, an ionic photoinitiator is used to activated 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 compunds, 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 specie 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. [9]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 of cationic photoinitiator
Photolysis of cationic photoinitiator

Cationic Photoinitiators

The proposed mechanism for cationic photopolymerization begins with the photoexcitation of the initiator. Once excited, both homolytic cleavage and dissociation of a counter anion takes place, generating cationic radical (R), an aryl radical(R') and unaltered counter anion (X). The abstraction of a lewis acid, in figure above a hydrogen, by the cationic radical produces a very weakly bound hydrogen and a free radical. The acid is further deprotonated by the anion(X) in solution generating a lewis acid with the starting anion (X) as a counter ion. It is thought that the acidic proton generated is what ultimitely initiates the polymerization.[10]

Onium Salts

Since there discovery in the 1970's aryl onium salts, more specifically iodonium and sulfonium salts, have received much attention and have found many industrial applications.Other, less common, onium salts not mentioned here include ammonium and phosphonium salts.[11]

Onium Salts
Onium Salts

The 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 225-300nm.[12] 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: Vorlage:Chem, Vorlage:Chem, Vorlage:Chem, Vorlage:Chem. There is a indirect relationship between the size of the counter ion and percent conversion.

Organometallic

Although less common, transition metal complexes can act as cationic photoinitiators as well. In the general the mechanism is more simplistic than the onium 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 attentions from commercial applications. [13] The absorbtion band for ferrocinium salt derivatives are in a much longer, and sometimes visible, region. Upon radiations the metal center loses a ligand(s) and the ligand(s) are replaced by functional groups that begin the polymerization. Some of the drawbacks of this method is a greater sensitivity to oxygen. There are also several organometallic anionic photoinitiators which react through a similar mechanism. For the anionic case, excitation of a metal center followed by either by heterolytic bond cleavage or electron transfer generating the active anionic initiator.[5]

Pyridinium Salts

Generally pyridinium photoinitiators are N-substituted pyridine derivatives, with a positive charge placed on the nitrogen. The counter ion is most cases is a non-nucleophilic anion. Upon radiation, homolytic bond cleavage takes place generating a pyridinium cationic radical and a neutral free radical. A hydrogen, most cases, is abstracted from the oligomer by the pyridinium radical. The free radical generated from the hydrogen abstraction is then terminated by the free radical is solution. This results in a strong pyridinium acid that can initiate polymerization.[14]

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

Free rad mech1
Free rad mech1

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.[5] Typically these oligomers and monomers alone do not absorb sufficient energy for the commercial light sources used, therefore photoinitiators are included.[15],[4]

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.[15]

Free-rad types of photoinitiators1
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 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 and Monomers

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.[4]

Acrylated epoxy oligomer
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, 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.[4]

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.[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 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 polymerizations 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. [1]

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 fabicated chips. The ability to pattern the resist using a focused light source has driven the field of photolithography.

Differences between negative and positive photoresist
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 characterisitc 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 intiator results in the formation of chemically resistant network polymer. A common functional group used in negative resist 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 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 you have two different monomers, or oligomers, in solution with multiple functionalities it is possible for the two to polymerize and form a less soluble polymer.

Positive Resists

As mentioned, positive resist exposure 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 specific linkers in the polymer. Once irradiated the "decomposed" polymers can be washed away using a developer solvent leaving behind the polymer that was not exposed to light. This type of technology allows the production of very fine stencils for applications such as microelectronics.[18] In order to have these types of qualities, positive resist utilize polymers with labile linkers in their back bone that can be cleaved upon irradiation or using a photo-generated acid to hydrolyze bonds in the polymer. A polymer that decomposes upon irradiation to a liquid, or more soluble product is referred to as a positive tone resist. Common functional groups that can be hydrolyzed by photo-generated acid catalyst include polycarbonates and polyesters.[19]

References

Vorlage:Reflist

  1. a b c d e f g Elsa Reichmanis, Crivello, James: Photopolymer Materials and Processes for Advanced Technologies. In: Chem. Mater. 26. Jahrgang, 2014, S. 533–548.
  2. Roger Phillips: Photopolymerization. In: Journal of Photopolymerization. 25. Jahrgang, 1984, S. 79–82.
  3. Jeff Burton: A Primer on UV-Curable Inkjet Inks. Specialty Graphic Imaging Association;
  4. a b c d e f A. Ravve: Light-Associated Reactions of Synthetic Polymers. Springer Science+Business Media, LLC, Spring Street, New York, NY 10013, USA 2006, ISBN 0-387-31803-8.
  5. a b c d e f Jean Pierre Fouassier: Photoinitiators for Polymer Synthesis: Scope, Reactivity and Efficiency. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany 2012, ISBN 978-3-527-64824-5.
  6. Radiation Chemistry in EB-and UV-Light Cured Inks. Paint & Coatings Industry;
  7. a b J.P. Fouassier, Allonas, X., Burget, D.: Photopolyermziation reactions under visible lights: principle, mechanisms and examples of applications. In: Progress in Organic Coatings. 47. Jahrgang, 2003, S. 16–36, doi:10.1016/S0300-9440(03)00011-0.
  8. J.M.G. Cowie: Polymers: Chemistry and Physics of Modern Materials. CRC Press: Taylor and Francis Group, 2008, S. 76.
  9. J. Crivello, E. Reichmanis: Photopolymer Materials and Processes for Advanced Technologies. In: Chemistry of Materials. 26. Jahrgang, 2014, S. 533–548.
  10. Viktor Zhdankin: Hypervalent Iodine Chemistry: Preparation, Structure, and Synthetic Applications of Polyvalent Iodine Compounds. John Wiley & Sons Ltd, 2013, S. 427.
  11. J. Crivello, E. Reichmanis: Photopolymer Materials and Processes for Advanced Technologies. In: Chemistry of Materials. 26. Jahrgang, 2014, S. 533–548.
  12. Jean Fouassier: Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency. John Wiley & Sons Ltd, 2012, S. 293.
  13. K Meier: Proceedings of the RadCure Europe. Basle Technical Paper, 1985.
  14. EIJI TAKAHASHI, FUMIO SANDA, TAKESHI ENDO: Novel pyridinium salts as cationic thermal and photoinitiators and their photosensitization properties. In: Journal of Polymer Science Part A: Polymer Chemistry. 40. Jahrgang, Nr. 8, 2002, S. 1037.
  15. a b c Charles Hoyle: Radiation Curing of Polymeric Materials. Am. Chem. Soc., Washington, DC 1990, S. 1–15.
  16. Biancamaria Baroli: Photopolymerization of biomaterials. In: J. Chem. Technol. Biotechnol. 81. Jahrgang, 2006, S. 491–499, doi:10.1002/jctb.1468.
  17. SU-8 Photosensitive Epoxy. Abgerufen im Jahr 2014.
  18. Harry Allcock: Introduction to Materials Chemistry. Wiley and Sons, 2008, ISBN 978-0-470-29333-1, S. 248–258.
  19. Larry Thompson: Polymers for Microelectronics. American Chemical Society, 1993.