Photopolymer

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A photopolymer or light-activated resin 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.

Changes in structural and chemical properties can be induced internally by

Most commonly, photopolymerized systems are typically cured through UV radiation, since

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

Cationic photoinitiators

The proposed mechanism for

A typical

Organometallic

Although less common,

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

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]

Benzoin ethers,

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

Dentistry is one field in which 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. Resin cements are utilized in luting cast ceramic, full porcelain, and veneer 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]

Conventional

Stereolithography, digital imaging, and 3D inkjet printing are just a few 3D printing technologies that make use of photopolymerization pathways. 3D printing usually utilizes CAD-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 liquid polymer, 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 upon polymerization in order to avoid distortion of the solid object. Common monomers utilized for 3D imaging include multifunctional acrylates and methacrylates, 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 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 acrylic 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-fabricated chips. The ability to pattern the resist using a focused light source has driven the field of photolithography.

Negative resists

As mentioned,

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

Positive resists

Exposure of a

Photopolymers can be used to generate printing plates, which are then pressed onto paper-like

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]

Fishing

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]

Floor refinishing

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 for

Environment Pollution

Washing the polymer plates after they have been exposed to ultra-violet light may result in^[