Фотополимеры: Свойства, Применение и Перспективы Развития

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. 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. 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 the potential advantages of being simpler and safer 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 higher rates of polymerization and environmental benefits from elimination of volatile organic solvents.

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.

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. The absorption band for ferrocinium salt derivatives are in a much longer, and sometimes visible, region. Upon radiation the metal center loses one or more ligands and these are replaced by functional groups that begin the polymerization. One 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 is followed by either heterolytic bond cleavage or electron transfer generating the active anionic initiator.

Photocurable adhesives are also used in the production of catheters, hearing aids, surgical masks, medical filters, and blood analysis sensors. 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.

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. In the presence of a cationic photoinitiator photopolymer, SU 8 forms networks with other polymers in solution. Basic scheme shown below. 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.

Exposure of a positive 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 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. In order to have these types of qualities, positive resists utilize polymers with labile linkers in their back bone that can be cleaved upon irradiation, or use 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 a photo generated acid catalyst include polycarbonates and polyesters.

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

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.

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.

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 VCT.

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.