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This introductory chapter appends various growing commercially important areas of photocured materials. The first section briefly mentions the use of this technique in areas that are either completely developed or at the stage of intense research. In the second section, the studies accomplished and articles contributed by authors are abstracted. This will provide readers with a concise overview of the contributions in this book.

Photocured materials are obtained by photoinduced hardening or crosslinking of various monomer-, oligomer-, and polymer-based compositions. Most commonly these materials are cured by UV, visible light, and electron beam (EB). A wide range of inexpensive UV photoinitiators coupled with the compactness, simplicity, and relatively low cost of UV equipment allows UV-cured materials to be used much more often and the interest in photocured materials has continued to grow1  in recent years.

Photocured materials are commonly used as coatings, inks2  and adhesives. The global consumption of photocurable coatings, inks and adhesives was 868 million pounds in 2012 worth $4.94 billion, according to a recent study from Kusumgar, Nerlfi & Growney Inc.3  Photocured materials have also been the mainstay of photolithographic applications playing an important role in the creation of microchips and printed circuit boards. Due to the very rapid and low-temperature cure these single-component systems also found uses in biomedical applications from nail polish and dental restorations to providing scaffolds for tissue and organ regeneration. Increasing demands for higher productivity and lower emissions of volatile organic chemicals (VOC) continue to support the expansion of photocured materials into various areas of human activities.

As the printing market declined due to reduced use of newspapers, magazines, and paper books the growth in the UV and electron beam (EB) -curable printing inks and coatings continued to be in the packaging applications. Especially attractive has been the use in food packaging where there is a continued need for safer, faster, and cheaper inks, functional coatings, and overprint varnishes.

Recent research in the photocurable inks area has been focused on replacing mercury-vapor-based UV lamps with UV LED cure sources and developing better performing systems with low migration and low odor for use in food packaging. Acrylate oligomers developed for use in UV/EB curable inks have to be fast curing, disperse multiple pigments well, and provide suitable rheology to the ink formulations. Most commonly these are polyester acrylates,4  though aliphatic epoxy acrylates and polyurethane acrylates are also used. Various special effect inks continue to be developed.5 

UV inkjet inks are being used to make packaging labels and typically suffer from fewer delamination issues than conventional inks. These inks also allow for easy inline processing such as hot stamping, die cutting, embossing and others.6  Plastic films are well suited for UV inkjet printing.7 

EB cure often provides a viable alternative to UV for inks and overprint varnishes.8  EB cure has been found to be especially effective in food-packaging applications where it leads to higher conversions of photocurable materials, while also removing the need for migrating photoinitiators and cure accelerators. Being an ionizing radiation with higher depth of penetration EB allows for much better through cure of pigmented coatings and inks. EB also enables cure through opaque to UV and visible-light multilayered substrates. EB coatings can be sufficiently durable to replace more expensive film lamination in paper-bag packaging.9 

Photocured materials are being used in laminating and pressure-sensitive adhesives and even as structural adhesives.10  However, they still face a tough competition with the often less expensive on the per weight unit basis solvent, waterborne, and 100% solids two-component systems. Providing a low migration bonding layer for substrates used in food packaging presents an additional level of challenge due to the lower functionality of the oligomers and monomers used in adhesives than in coatings. This is often coupled with the difficulties in curing through opaque bonded substrates. EB-curable adhesives have recently seen better growth due to superior properties and better process control of EB cure.11 

Due to the speed of cure and ease of application UV-curable sealants found use in manufacturing of ammunition rounds.12  In this application UV-curable sealant is applied to the joint between the brass casing and the projectile of the ammunition cartridge after the cartridge has been fully assembled and crimped. The sealant penetrates into the joint by capillary action and is then UV cured to form a waterproof joint. A specially designed LED UV source is used to obtain a narrow strip of UV exposure with well-controlled uniformity.

UV-curable hot melt adhesives continue to be developed and often have superior properties to the conventional systems. UV-cured hot melt adhesives find uses in specialty pressure sensitive tapes, construction, and medical applications.

Films and plastic containers with gas barrier coatings found multiple uses in food packaging where protection of flavor and extension of the life of packaged food and beverages is of great importance. Photopolymerizable thiol–ene systems were shown to form films with oxygen permeability that can be dialed from very low to very high depending on the glass-transition temperature of the created polymer network. Modification of these networks with amine functional groups was found to create very good oxygen barrier films.13  Surface modification of clays with acrylate or thiol-functional groups coupled with the selection of suitable exfoliating surfactants was shown to lead to UV-curable formulations with improved mechanical properties. Such formulations were also shown to result in better gas and water barrier properties.14 

Various other barrier and sensor applications became possible via printing of the functional components using UV- and EB-cure processes.15  The opportunity in gas and especially oxygen barrier coatings remains to be open for photocured coatings with high flexibility and adhesion to various plastic substrates.

Release coatings are usually applied either on film or paper16  substrates and used as removable liners for pressure-sensitive adhesive films or as protective films for various displays. Modern UV-curable release coatings are based on acrylated or epoxy functional polysiloxanes.17 

UV powders are typically based on methacrylates and unsaturated polyesters. These formulations are applied by electrostatic spray to a substrate followed by melting of the powder in an oven and UV cure of the melt. The temperature required for melting is lower than what is required for thermally cured powder coatings and is typically around 120 °C.18  UV powder coatings are used on a variety of substrates such as metals, medium density fiberboard (MDF), structural foams, plastics, composites and other heat-sensitive materials.19  Creating cost-effective photocurable polymer powders with better resistance to sintering during storage and shipment remains an industry challenge.

UV-cure technology capabilities have been extended to the high-performance corrosion protection of steel and aluminum surfaces in industrial applications. Thinner and faster to cure UV coatings were shown to have corrosion protection similar to solvent based two-component urethane and epoxy systems.20  Adhesion is often a challenge for UV-cured protective coatings on metal and some materials were suggested to address this issue.21  Very large commercial opportunities for coatings with corrosion protection for metal remain in marine, automotive and aerospace uses, where there is a strong desire to move to rapidly curing, solvent-free and isocyanate-free systems. One of the key challenges remains to be the need for a well-working system that would combine exceptional durability and weatherability of the cured coatings with easy to use and relatively inexpensive curing and application devices. Recent developments in robotic UV systems have been made that allow precision application and cure of UV coatings on surfaces with various curvature and complexity including large vertically positioned surfaces.22 

UV-curable coatings based on halogenated phenyl acrylates were shown to inhibit biofouling and biocorrosion on plastic panels immersed in waste water.23 

UV coatings are slowly making way into the automotive refinish area.24  Among the challenges is the need to cure around corners and in cracks. Dual cure and photolatent catalyst approaches have been used to address these issues.25  Recent advances in UV LED resulted in creation of hand-held cure devices that do not emit in the UVB and UVC range of the spectrum and do not generate ozone, which is expected to accelerate the adoption of the UV-cure technology by the automotive refinish and other applications where the materials are field applied and cured.

UV coatings have found uses in aircraft exterior applications where faster process flow for coatings is often sought.26  Since the aircraft exterior finishing takes place at the end of the build process the urgency to avoid delivery delays while holding a very expensive inventory pushes the manufacturers to selection of faster curing coatings and UV is one of the options being considered.27  UV coatings are also being considered as replacement for thermally cured coatings for the exhaust vent areas,28  where the coatings can be subjected to temperatures as high as 150 °C. Recent developments in UVA LED cure sources and matching photoinitiating systems resulted in increased interest in field repair and stencil coatings for aircraft. These UVA-curable coatings have been shown to have performance that rivals that of the conventional 2K urethane systems.29 

The idea of creating shark-skin-inspired microscale patterns on the surface of aircraft to improve fuel economy by reducing drag was recently tested using UV-curable materials.30 

Creation of stable dispersions based on submicrometer scale particles of alumina and silica31  along with the improvements in polyurethane acrylates led to creation of a new generation of scratch-resistant coatings with exceptional wear resistance. More recent developments in functionalized silicas, hybrid alkoxysilane-epoxy and acrylate systems,32  and waterborne polyurethane acrylates33  made UV-curable coatings especially useful for such easy to scratch plastics like polycarbonate, which is widely used in automotive headlamps. Various consumer electronics devices, optical discs, eye glasses, and even vinyl flooring have also benefited from UV-cured scratch- and abrasion-resistant coatings.

Photocured coatings are typically praised for their high gloss. Matte finish or low gloss photocured coatings have been less common due to the challenges in their formulation.34  Recently, there has been an increased interest in matte UV coatings driven in part by the increasing demand for matte finishes in plastic packaging. There is also a need, particularly for the automotive and consumer electronics markets, for “soft touch” rapidly cured at low-temperature coatings where despite multiple attempts the balance of the soft to touch, velvety texture with high chemical resistance has not yet been attained.

UV-cured coatings are also used as base and top coats for metallization of substrates using physical vapor deposition techniques.35  The “chrome look” has found uses in automotive, cosmetics and home-appliances markets.

UV coatings found use in protection of beverage can rims from abrasion damage. The coating also allows for easier movement of the cans on conveyors during manufacturing and filling.36  UV inks are also used in metal container decoration, although to a smaller extent than solvent-based inks.

Modern coatings for parquet flooring are based on UV-curable formulations. These coatings are multilayered systems consisting of the bottom layer, which is typically a water borne UV-curable formulation, followed by 100% solids UV-curable formulations layers to provide the necessary abrasion and scratch resistance.37  UV-cured coatings are still used in multiple furniture and kitchen cabinetry applications. 100% solids UV lacquer was found to be the best alternative from an environmental point of view as a surface coating for wood furniture.38 

UV-curable formulations have been developed for coating concrete floors.39  While these coatings provide almost instantaneous cure and are zero to low VOC they continue to face tough competition with the two-component rapid-cure technologies. Repair of rail seat abrasions on concrete ties has been shown to be very effective when using UV-cured materials.40 

Alkyd paints are based on oxidative drying of the unsaturated fatty acid esters and are used in households to paint doors, trim, and cabinets. Cobalt salts are typically used to accelerate the drying that otherwise can take more than 24 h. Recent studies showed that cobalt salts could be linked to adverse health and environmental effects, which forced the paint industry to seek alternatives to cobalt-based accelerators of the drying. A photochemical system was shown to be a viable candidate for a photoassisted autoxidation drying process in alkyds.41 

A full-color imaging system based on photopolymerizable pressure-sensitive microcapsules was invented in 1984 for use in color copying.42  The system was later developed into a visible light and pressure-sensitive single-sheet full-color printing medium.43  More recent systems were developed to color irreversibly directly upon the exposure of photopolymerizable materials to UV and even EB without requiring secondary mechanisms such as heat or pressure for color development.44  These systems found uses as cure indicators in photopolymerizable systems, as sensors and dosimeters for UV and EB radiometry,45  and presented an opportunity for various photocured formulations to develop color as part of the curing process. Photocurable color imaging systems based on structural color using magnetically tunable photonic crystals are also being developed.46 

Photocured materials can also be used as parts of the image forming or transferring devices. UV-curable formulations containing carbon nanotubes found uses in electrostatographic devices as transfer belts.47  Such formulations were shown to have better control over the uniformity of electrical resistivity and superior mechanical properties to solvent-based thermoplastic and thermosetting systems.

Photocure can be used to prepare negative-tone photoresists, where photopolymerization or crosslinking can be used to render the material insoluble in the developer. Positive-tone photoresists, where the polymer is rendered soluble in the developer after the exposure, are typically not prepared by photopolymerization and can be classified as photosensitive, rather than photocurable materials.

Relatively simple photopolymerizable systems can be used to make low-resolution stencils and masks for etching glass and ceramics. Flexography is based on the use of the printing plates prepared from photocurable materials image wise exposed by UV.48  Stencils for screen printing can also be prepared using photoresists.

High-resolution nanolithography and patterning is obtained using immersion 193 nm systems in making microchips.49  EUV (extreme UV), EB, X-ray, and other imaging technologies are under different stages of evaluation and development for fabricating even smaller nanometer scale patterns for next-generation microchips.

UV-curable formulations find use in stereolithography – a process of printing 3D objects using a computer-controlled laser layer-by-layer using photocurable materials. The photopolymerization takes place using a 355 nm laser and features as small as 40 μm in the lateral direction can be obtained.50  Two-photon absorption-induced photopolymerization allowed to further expand the capabilities in creating 3D microstructures using photocured materials – features as small as 120 nm can be created.51 

UV-curable coatings for optical fiber have very stringent requirements for resistance to humidity and temperature variations. Acrylate monomers used in such formulations also have very high purity requirements to help minimize potential data transmission losses. As optical fiber began to be used in subterranean exploration for natural resources additional requirements for the coatings were added such as resistance to high pressure and corrosive environment of the drilling wells.52 

Radiation-curable materials are used in integrated optical and optoelectronic applications (optical discs, diffraction gratings, antireflective coatings, imaging sensors and photonic devices).53  UV-curable conductive inks are very suitable for making printed electronics for uses in RFID tags and OLED displays.54 

UV-curable formulations found use in liquid-crystal displays.55  Antireflective coatings for displays can also be obtained by using UV-cured coatings and films.56 

UV-curable materials were shown to be capable of meeting the performance requirements for use in photovoltaic structures as adhesives.57  UV-curable hard coatings found use in silvered films used in production of lead-free mirrors, where they greatly reduced the weight and cost of the system.58 

Low-energy EB (120 keV to 300 keV) is typically used to cure inks, coatings and adhesives. Medium-energy EB (300 keV to 5 MeV) is used to crosslink wire and cable insulation and to modify and cure fiber and composites.59  High-energy (10 MeV) EB crosslinking of high density polyethylene was shown to result in superior dielectric properties vs. thermal crosslinking using peroxides.60  Although, low-energy EB has also been found to be useful in crosslinking low-density polyethylene.61  EB crosslinked wood–plastic composites are being developed to present a sustainable and nontoxic alternative to pressure-treated lumber for building materials.62 

UV-curable coatings have been a common alternative to solvent-based formulations in nail-polish applications. Polyurethane methacrylates are typically the basis of these formulations.63  Renewable UV-curable coating materials are sought for this use.64 

Photocured materials have been used in restorative dentistry since 1969.65  The cure is now done using visible light, typically the blue region of the electromagnetic spectrum and often blue 450 nm LEDs are used.66  Camphorquinone-based photoinitiating systems are most common, though other photoinitiating systems are constantly being developed in order to obtain faster and better curing composite systems with minimum shrinkage.67 

Photocured materials can be used in stereolithography based construction of artificial joints68  or as scaffolds supporting tissue repair. Hydrogels are used as scaffolds for tissue engineering and can be obtained from photocured materials.69  The use of UV-cured materials for creation of nerve-implanting surfaces with high spatial resolution may lead to creation of prosthetic materials with better outcomes for patients.70 

Section 1.1 described several interesting applications of photocured materials. It was difficult to cover all the mentioned areas due to scarce of available literature and proprietary nature of technologies. This section briefly details the technical articles included in this book.

The area of UV light-cured coatings has been well established, and the coating industry was probably the beginner to adopt the use of photocuring. Soucek et al., in Chapter 2 describes the fundamentals, the chemical components and their use in the development of photocured coatings. The authors also detail the mechanism of cationic photopolymerization and free-radical photopolymerization along with the advantages and disadvantages of both the methods. Novel synthesis of photocrosslinkable polymers containing pendant chalcone moieties with different substituents has been described by Kumar et al., in Chapter 3. The authors utilized the free-radical polymerization method for the development and claimed that polymers containing pendant chalcone moiety exhibits liquid-crystalline behavior.

Torgersen et al., in Chapter 4 briefly describe laser-assisted two-photon polymerization. Their precise fabrication technique enabled the authors to create three-dimensional micro- and nanostructures. The chapter provides insight into optimization of photoinitiators to absorb two photons for the radical formation leading to photopolymerization. Formation of polymer liquid-crystalline composite material by a photoinduced inhomogeneous curing pattern has been shown by Veltri et al. in Chapter 5. The authors found that two parameters related to diffusion and curing intensity primarily governed the structures of the final product.

It is worth mentioning that photosensitive materials are now playing a vital role in precise fabrication of micro/nanocomponents and devices. Chen, in Chapter 6, comprehensively reviews the fundamental principles and interdisciplinary approach behind the development of miniaturized objects. The author also describes various schemes of microfabrication in photolithography, light stereolithography, soft lithography and inkjet printing. The developments of UV-curable scratch-resistant, functional coatings are detailed by Sangermano et al. in Chapter 7. Several variations of coating containing series of nanoparticles were achieved by photocuring technique. Zhou et al. in Chapter 8 demonstrate a photoimmobilization technology that uses photoreactive and nonbiofouling polymers for the preparation of microarray biochips. The authors expect that use of this technology will assist in understanding the fundamental aspects of biological interactions, along with other useful applications in clinical analysis. Cakmakci et al., study the flame-retardant photocurable coating containing boron and phosphorous in Chapter 9. The effectiveness of various monomers and additives in flame retardancy is discussed in detail. Similar to Veltri et al., in Chapter 10, Sio et al., demonstrated the fabrication of curved periodic microstructures containing self-aligned liquid crystals. The holography and lithography techniques assisted the authors in developing periodic nano-/microcomposite gratings. The diversified nature of polymer liquid-crystal polymer slices (POLYCRIPS) technology is mentioned by Sio et al. as well as in Chapter 11 by Caputo et al. The author's study suggested that POLYCRIPS technology is valuable for switchable diffraction phase grating, switchable optical phase modulator, arrays of mirrorless optical microresonators for tunable lasing effects and a one-step fabrication of fork gratings.

The use of photocuring in fabrication of thick high aspect ratio structures is shown by Chiu et al., in Chapter 12. The authors have selected a negative-tone photoresist to fabricate complex three-dimensional structures. The basic material properties and fundamentals of the fabrication process related to the selected negative photoresists are summarized. In Chapter 13, Morales et al. adopted a unique approach for rapid crosslinking and stabilization of carbon fibers. The authors noticed that with the use of short UV treatment, precursor fibers could be thermo-oxidatively stabilized and successfully carbonized rapidly. The resulted fibers retained a higher extent of molecular orientation and displayed higher tensile modulus.

Although photocured technologies have demonstrated extremely high potential in various commercial arenas, they show limitations when used in food contact. The photoinitiators from packaging materials tend to leach out into food products that poses serious human-health issues. Lago et al., in Chapter 14 review the methods available for the detection and determination of photoinitiators in food packaging and foodstuffs. The authors comprehensively summarize the state-of-the-art systems for the determination of nonintentionally added substances derived from the photoinitiators.

Apart from conventional UV light lamps for photocuring and photopolymerization, LED light curing and polymerization show significant promises. Vallo et al., in Chapter 15 suggest that since spectral output of LED sources is concentrated in a comparatively narrow wavelength range, more efficient curing is possible, resulting in reduced curing time and increased depth of cure compared to conventional light sources. Moreover, with LED curing there will be less heat transfer to the substrate without the possibility of harmful UV rays. Finally, in Chapter 16, Barrera et al., discusses the importance of gamma-radiation technology for the structural and physicochemical modification of waste materials such as PET bottles, TetraPak® packing containers and tire rubbers. Additionally, the utilization of such modified waste as fillers and reinforcement in concrete is emphasized.

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