Photopolymers applications

Photopolymer technology, which encompasses the action of light to form polymers and light initiated reactions in polymeric materials, is an immense topic. Previous papers in this symposium have described some of basic chemistry utilized in photopolymer technology. The primary objectives of this paper are a) to develop the connections between basic photopolymer chemistry and practical uses of the technology and b) to provide an overview of the wide variety of photopolymer applications that have been developed since the 1950 s. Every attempt has been made to make this review as inclusive as possible, but because of the extensive nature of this topic, there are many applications of photopolymer chemistry that have not been included. In addition, only limited representative references are provided since the patent and open literature for this technology are quite vast (7). 

Additional groupings within each category have been made where appropriate. Every attempt has been made to make this review as inclusive as possible, but because of the extensive nature of this topic, there are many photopolymer applications that have not been included. 

The photopolymer office copy system, however, had shown that HABIs could be effective photoinitiators in polymeric systems and provided stable, almost colorless and readily modifyable components. Exploratory work by G. R. Nacci, J. E. Pazos, and others showed just what possible ways there were to utilize effectively these then novel photoinitiators. This work inevitably focused on the use of o-Cl-HABI, which was the least expensive hexaarylbiimidazole that possessed the requisite stability and absorption spectrum. By the time that the photopolymer applications became commercially important, the Dylux Venture had been terminated, and there was essentially little interest in synthetic work to produce still more effective photoinitiators of this family. 

This review will highlight the interrelationships between basic photopolymer science and practical applications of this technology. Each application of photopolymer technology can be described in terms of three primary descriptors the mode of exposure, the mechanism of the photopolymer reaction employed and the visualization method used. Using this foundation, the widely diverse applications of photopolymer technology to electronic materials, printing materials, optical and electro-optical materials, the fabrication of devices and polymeric materials, adhesives and coating materials will be discussed. 

The basic light initiated reactions used in the diverse applications of photopolymer technology can be classified into 5 categories based on the chemical and physical processes utilized. 

Photocrosslinking. The second class of photopolymer chemistry that is used in some commercial products is based on the reaction of unsaturated moieties attached to an organic polymer. These photopolymer materials include the [2+2] cycloaddition of the ethylenic groups in poly(vinyl cinnamate) polymers and in the newer styryl pyridinium (10) and thiazolium (77) derivatives of poly(vinyl alcohol). The main advantage of this chemistry is that, unlike free-radical photopolymerization, they are insensitive to the presence of oxygen. This photopolymer mechanism is principally used in applications employing a washout development process (e.g. resists). 

Photo/Thermal Reactions. The fifth basic class of photopolymer chemistry that can be used in commercial applications is based more on physical changes in a polymer-based matrix than on chemical reactions. A recent application of this technology is the laser ablation (77) of an organic coating on a flat support to directly produce a printing plate. The availability of newer high energy lasers will allow more applications to be based on the photo/thermal mechanism. 

In the previous section, five basic mechanisms of photopolymer reactions were identified. In order to utilize these photopolymer mechanisms in practical systems, sensitive visualization methods must be used to develop the latent images. Six different classes of visualization techniques are used in most commercial applications of photopolymer technology. 

Adhesion. The modulation of adhesion with light is used in some important commercial applications of photopolymer technology. The adhesion and cohesion of a photopolymer to other polymeric materials can either be increased or decreased upon the application of light, depending upon the chemistry of the photopolymer formulation and surface properties of the films in the product structure. The... 

During the past three decades there has been a rapid increase in the number of commercial applications of photopolymer technology. To facilitate this brief survey of the wide variety of imaging and non-imaging uses of photopolymer chemistry, the practical applications of this technology have been classified into 6 general categories ... 

Electronic Materials. Applications of photopolymer technology to electronic products is probably one of the largest imaging categories in terms of sales and total number of patents and publications. A wide variety of products have been developed for the production of printed wiring boards (PWB) and the manufacture of integrated circuits (IC). 

Printing Materials. The application of photopolymer technology to a wide variety of imaging applications in the printing industry is a quite important. Most printing plates and pre-press proofing products used today are based on, or utilize some element of, photopolymer chemistry. 

Other Printing Applications. There are a number of lower volume applications of photopolymer chemistry to the printing industry. Some of these include the production of tactile images (raised Braille patterns) (47) and screen stencil resists... 

Liquid Crystal Displays (LCD). Liquid crystal displays, once limited to small devices such as calculators, are now displacing color CRT (cathode ray tube) displays in commercial quantities. The ability to fabricate these display devices at high quality and at low cost is partially due to the wider spread use of photopolymer-based materials. Photopolymer technology is being used for the alignment of liquid crystal (LC) elements (49), the orientation of ferroelectric materials (50), the synthesis of LC polymers (57) and the manufacture of color filters for liquid crystal display applications (52). 

Fabrication of Devices and Materials. Many of the previously discussed applications of photopolymer technology have utilized photopolymer materials to manufacture a part that is then incorporated into commercial products (e.g. film resists used to manufacture printed circuit boards). One area that has been increasing very rapidly in the past 5-6 years has been the use of photopolymer chemistry to fabricate devices and materials that are used directly in the final product. This development has been made possible, in part, by the design of new photopolymer chemistry that can produce polymerized materials with enhanced physical properties. 

Dental Materials. One very practical application of photopolymer materials is in dental restoratives. Photopolymer chemistry has been adapted for use in composite fillings (67), surface coatings (68) and in the formation of impressions (69). 

Membranes. Photopolymer chemistry is being applied to the design and manufacture of a variety of membrane materials. In these applications, photopolymer technology is used to precisely define the microscopic openings in the membrane as it is being formed or to modify an existing membrane. Some of the applications of photopolymer chemistry to membranes include the modification of ultrafiltration membranes (78) and the manufacture of amphiphilic (79), gas permeable (80), untrafiltration (81), ion-selective electrode (82) and reverse osmosis membranes. 

Coatings and Surface Modifications. Probably the one application of photopolymer chemistry that has the most worldwide commercial value in terms of product sales is the use of photopolymer materials for curable coatings. Most of the wood paneling and less expensive furniture manufactured today utilize UV or electron-beam curable materials for decorative finishes (e.g. simulation of wood grain) and protective coatings. In addition, the surfaces of many commercially important materials (e.g. textile fibers and polyester films) are being modified by photopolymer processes. 

Coatings. The use of photopolymer materials for the preparation of curable coatings is highly diverse. Many of the products used at home and at work have been coated with a photopolymer material for protective and decorative applications. Just a few of the many applications of photopolymer technology include UV curable pigmented (94) and clear (95-96) protective coatings, protective furniture (97) and... 

Surface Modifications. Basic photopolymer chemistry is also being used for the surface modification of films, textiles fibers, and many other organic-based materials (104). Some of the novel applications of photopolymer technology to surface modification include the design of cell repellent treatments and in photografting of various chemical functionality onto the surface of materials to improve color retention, enhance the adhesion of antistatic chemicals or to improve staining resistance. 

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