Advancements in Light-Curing Technology and Future Trends

Reposted from: Adhesive Industry News

　　Photocuring (UV) technology is an efficient, environmentally friendly, energy‑saving, and high‑quality emerging technology for the 21st century, widely used in coatings, adhesives, inks, optoelectronics, and other fields. Since 1946, when the U.S. company Inmont obtained the first patent for UV‑curable ink, and in 1968 when Germany’s Bayer developed the first generation of UV‑curable wood coatings, photocurable coatings have experienced rapid global growth. Over the past few decades, a large number of novel, highly efficient photoinitiators, resins, monomers, and advanced UV light sources have been incorporated into UV curing systems, driving the continued expansion of the UV‑cured coatings industry.

　　 Photocuring technology continues to advance.

Photocuring technology uses light as an energy source: irradiation causes photoinitiators to decompose, generating reactive species such as free radicals or ions. These active species initiate the polymerization of monomers, enabling a rapid transformation from a liquid to a solid polymer. Owing to its low energy consumption—only one-fifth to one-tenth that of thermal polymerization—its fast curing speed—completing the process in seconds to tens of seconds—and its environmental friendliness—eliminating solvent volatilization—it is regarded as a green technology.

At present, China has become one of the largest consumers of photopolymerizable materials, and its progress in this field has attracted significant international attention. As environmental pollution continues to worsen, the development of environmentally friendly, zero‑pollution photopolymerization technologies has become increasingly critical. According to statistics, approximately 20 million tons of hydrocarbons are released into the atmosphere worldwide each year, with the majority originating from organic solvents used in coatings. During coating production, about 2% of the total coating mass is emitted as volatile organic compounds (VOCs), while during application, an additional 50% to 80% of the coating’s VOC content evaporates. To reduce pollutant emissions, UV‑curable coatings are gradually replacing traditional thermally cured and solvent‑based coatings.

As photocuring technology continues to advance, its range of applications will gradually expand. In its early stages, photocuring was primarily used in coatings, as challenges related to light penetration and absorption in pigmented systems remained unresolved. However, with the development of photoinitiators and the increasing power of light sources, photocuring has become increasingly adaptable to the needs of various ink formulations, leading to rapid growth in the field of photocurable inks. In recent years, ongoing advances in photocuring technology have enabled it to penetrate into other domains. Driven by progress in fundamental research—resulting in a deeper understanding of the underlying mechanisms—and by evolving societal and environmental demands, photocuring technology has continued to evolve and innovate.

　　Photocurable coatings are being used more and more widely.

　　UV-curable coatings include:

　　Light-cured bamboo and wood coating: As a distinctive product of China, bamboo furniture and bamboo flooring are currently predominantly coated with UV-curable finishes. The proportion of UV coating applied to various types of flooring in the domestic market is very high, making it one of the key applications of UV coatings.

　　Photocurable paper coating: As one of the earliest UV coating formulations to be developed, UV paper varnishes are widely used across various printed materials, particularly for advertising and publication covers, and remain one of the largest segments within the UV coatings market.

　　Light-curable plastic coatings: To achieve both aesthetic appeal and durability, plastic products typically undergo coating. UV‑curable plastic coatings come in a wide variety, with diverse performance requirements, though they are predominantly used for decorative purposes. The most common applications include the housings of household appliances, mobile phones, and other consumer electronics.

　　Photocurable vacuum coating paint: To enhance the tactile quality of packaging, the most common approach today is to metallize plastic via vacuum deposition. This process requires products such as UV primers and topcoats, with cosmetic packaging being its primary application.

　　Photocurable Metal Coatings: Metal UV coatings include UV‑resistant primers, UV‑curable temporary protective coatings for metals, metallic UV decorative coatings, and UV‑curing surface protection coatings for metals, among others.

　　 Light-curable fiber-optic coating: The production of optical fibers requires 4 to 5 coating steps, from the core to the cladding. Today, nearly all of these coatings are applied using UV curing. UV‑curable fiber coatings represent one of the most successful applications of this technology, with curing speeds reaching up to 3,000 meters per minute.

　　Light-curable conformal coating: For outdoor products, especially electronic devices, it is essential to withstand the rigors of natural environmental conditions such as wind and rain. To ensure long-term reliable performance, electrical components must be protected. UV‑curable conformal coatings were specifically developed for this purpose, aiming to extend the service life and operational stability of electronic equipment.

　　 Photocurable Glass Coating: Glass itself has limited decorative appeal; to achieve colored effects, it must be coated. UV‑curable glass coatings have thus emerged, meeting stringent requirements for aging resistance and chemical resistance, and representing a high‑end UV‑curing product.

　 　Photocurable Ceramic Coating: To enhance their aesthetic appeal, ceramics require surface coating; currently, the main UV coatings used for ceramics include ceramic inkjet coatings and ceramic decal coatings.

　　Light-curing stone coating: Natural stone inherently exhibits various defects; to enhance its aesthetic appeal, it must be treated. The primary purpose of UV‑curable stone coatings is to repair these defects, while meeting stringent requirements for strength, color, wear resistance, and aging resistance.

　　UV-curable leather coating: UV leather coatings fall into two main categories: one is UV‑curable release coatings for artificial leather patterned paper, which are used in very large quantities; the other is decorative coatings for leather, designed to alter the appearance of natural or synthetic leather and enhance its aesthetic appeal.

　　Light-Curing Automotive Coatings: Light‑curing technology is employed throughout automotive lighting, from the interior to the exterior: lamp bowls and lampshades are both coated using this process. In both interior and exterior trim, numerous components—such as instrument panels, rearview mirrors, steering wheels, gearshift knobs, wheel hubs, and decorative trim strips—are manufactured with light‑curing techniques. Automotive bumpers are also produced via light curing, with their surface coatings applied through photopolymerization. A wide array of electronic parts, including in‑vehicle displays and center consoles, rely on light‑curable materials during fabrication. Even today’s popular car wraps feature weather‑resistant topcoats that are cured using light‑curing technology, while automotive body paints have already transitioned to light‑cured formulations. Furthermore, light‑curing technologies are increasingly used for repairing paint films and fixing cracked glass.

　　 Photocurable waterborne coatings: To address the environmental pollution caused by the use of solvents in UV coating application, an important current trend is the development of waterborne UV coatings, which employ water as the solvent to enhance their applicability. At present, both domestic and international waterborne coatings are still in their early stages of development.

　　Photocurable powder coatings: By combining conventional powder coatings with photopolymerization technology, a new type of photocurable powder coating has been developed, characterized by low curing temperatures, excellent product quality, and broad application versatility. While this coating is still in the research and development stage in China, it has already been commercialized abroad.

　　Photocurable Antistatic Coating: Photocurable antistatic coatings are a specialized type of UV‑curable coating that incorporates antistatic additives to enhance the substrate’s antistatic performance. Although their usage volume remains relatively small, they possess distinct characteristics and applications.

　　 Photocurable Flame-Retardant Coating: Photocurable coatings sometimes require flame-retardant performance; therefore, this can be achieved by incorporating specialized flame retardants. Although certain conventional flame retardants can be used in UV‑curable coatings to impart flame resistance, the unique characteristics of UV coatings—such as stringent light transmission requirements—dictate that flame retardants for UV‑curable systems must meet specific structural criteria.

　　 Photocurable fluorocarbon coating: Fluorocarbon coatings are widely used due to their excellent weather resistance, and the application of UV‑curable fluorocarbon coatings is becoming increasingly common. The key challenge lies in achieving mutual solubility among the various components; this requires starting with material‑structure design to develop UV‑curable fluorocarbon coating formulations that meet performance requirements.

　　 Enhancing photopolymerization technology from the perspectives of raw materials and processes.

　　As for photopolymerization technology itself, in order to preserve its inherent advantages and enhance its competitiveness, it is essential to continuously upgrade the technology by making advancements in raw materials, new techniques, and other areas. The main aspects include the following:

　　 Photocuring Surface Modification

　　Due to the limitations of light propagation, photopolymerization technology cannot penetrate deep into materials; therefore, its applications are primarily confined to surface‑level chemical reactions. In the realm of general‑purpose material surfaces, photopolymerization leverages its advantages across a wide range of uses, from conventional printing varnishing to home décor, building materials, automotive interiors, and outdoor protective coatings. For certain specialized environments and time‑critical applications—such as renovations in schools, hospitals, indoor retail spaces, and parking garages—photopolymerization occupies an irreplaceable niche. Constraints on time, such as schools needing to complete repairs during brief holiday periods or hospitals requiring nighttime closures to renovate operating rooms, demand a fast yet safe solution, making photopolymerization the optimal choice. Moreover, because solvent‑free photopolymerizable coatings produce no volatile organic compound emissions, their safety profile is significantly enhanced.

　 　Photocurable patterning

　　Photocuring, owing to its spatiotemporal controllability, can be employed for the fabrication and transfer of patterns. Lithography leverages this controllable temporal and spatial nature of photocuring. Through photopolymerization, lithographic applications spanning multiple scales—ranging from chip manufacturing to LCD displays and printed circuit board production—can be realized, enabling the transfer of features of varying dimensions onto diverse substrates and facilitating the creation of high‑precision patterns. Today, as microelectronic components continue to shrink in size while their performance steadily improves, a key factor is the advancing sophistication of lithographic techniques, which yield ever finer lines, thereby making the miniaturization of microelectronic devices feasible and reducing their power consumption. Moreover, photocuring technology finds applications in microfluidic channel fabrication, three‑dimensional image patterning, and the machining of complex structures. These precision‑oriented processes impose extremely stringent requirements on photocurable materials, whose purity differs markedly from that of conventional inks and coatings.

　　Light-curing 3D printing

　　Due to its rapid curing capability, photopolymerization is particularly well suited for fast additive manufacturing processes such as 3D printing, enabling the swift fabrication of complex parts. Among current 3D‑printing technologies, photopolymerization is the most widely used; for example, stereolithography, which employs a laser as its light source, forms the foundation of 3D printing and represents the first generation of this technology. It uses a laser to perform rapid scanning, thereby solidifying three‑dimensional structures. Today, photopolymer‑based 3D printing has given rise to a wide array of applications, and the light sources have evolved from early ultraviolet illumination to visible light.

　　Light-curable biomaterials

　　The applications of light‑curing technology in biomedicine primarily include dental restorative materials, bone regeneration, rapid tissue wireless suturing, surgical clinical simulation models, cardiac surgical fixation, repair of tissue defects, and the fabrication of soft‑tissue hydrogels. The earliest light‑curable biomaterials developed were light‑curing dental restoratives; today, light‑curing 3D printing for orthodontic models is widely adopted. Light‑curing orthopedic materials are mainly used to replace traditional stainless‑steel implants for bone repair, enabling rapid healing while minimizing the pain associated with a second surgery to remove the fixation devices. Light‑curing techniques for cardiac surgical fixation and tissue defect repair are similar to those for bone repair, differing only in anatomical location and material requirements: because the heart beats, the materials must be elastic rather than rigid like bone, and since different human tissues have distinct functions and structures, the reparative materials must likewise mimic these characteristics; otherwise, the repaired tissue cannot function properly. Wireless suturing technology leverages light curing to promote rapid wound closure without the need for traditional sutures, and these light‑curable adhesives are biodegradable, eliminating the requirement for removal. This approach reduces the burden of suture removal, which is particularly important in intra‑body procedures. However, in clinical practice, light‑curing wireless suturing still faces numerous challenges.

　　 Photocurable bulk material

　　With advances in photocuring technology, hybrid processes that combine photopolymerization with other techniques have begun to see practical application, including photo‑thermal and photo‑moisture curing, front‑face photopolymerization, and cationic photopolymerization. Photocuring is gradually shifting from surface modification toward bulk material processing, enabling the fabrication of a wide range of bulk materials such as photocurable composites, solid‑state photocurable blocks, and photocured components for automobiles, aircraft, and spacecraft. For example, by using light as the driving force to initiate polymerization on the material’s surface, the heat released during this process can become sufficient to trigger conventional thermal polymerization; at that point, illumination is no longer required, and the heat generated by thermal polymerization can further sustain subsequent reactions. Similarly, after initiating surface polymerization via photopolymerization, if moisture‑induced curing can follow, atmospheric water can continuously permeate the material, allowing moisture‑curing to proceed until all the material has fully cured—thus facilitating the production of very thick sections. As for cationic photopolymerization, once cations are generated, they remain stable over long periods; hence, light can first initiate cationic polymerization, while areas inaccessible to light can rely on pre‑existing cations and be cured further through thermal activation. These technologies have already been applied in the manufacture of automotive bumpers, interior trim, aerospace components, and aircraft parts. In particular, with the growing emphasis on vehicle lightweighting, carbon‑fiber composites are increasingly being adopted in automotive applications, and the use of photocuring is steadily expanding.

　　Other potential applications of light curing

　　During the fabrication of solar panels, photopolymerization technologies are employed, such as crosslinking of EVA encapsulant films, anti-fouling coatings for solar‑cell surfaces, and roll-to-roll photopolymerization coating processes for organic solar cells.

　　The fabrication of wind turbine blades can now be carried out using light curing, and when it comes to repairing damaged wind turbine blades, light curing is one of the simplest, most effective, and most cost‑efficient methods.

　　In addition to the aforementioned applications in automotive and aerospace sectors, photocuring technology is also extensively employed in high-speed rail interior components, high-speed rail composite materials, and ship interior materials. For example, photocurable fire‑retardant interior panels are used in high-speed trains and cruise ships, as well as for coating entire high-speed train restrooms.

　　Light-curing technology is used to repair damaged road surfaces; its performance is comparable to that of concrete, enabling rapid completion in as little as 30 minutes and thereby avoiding large-scale traffic congestion.

　　For highway signage, which is exposed for extended periods to harsh environmental conditions—including high temperatures, high humidity, extremely low temperatures, and weathering from wind and sunlight—and must withstand infrequent replacement, stringent performance requirements are essential. Abroad, electron-beam (EB) curing technology has already been employed to apply surface coatings to expressway signs, achieving superior resistance to aging, high‑temperature and high‑humidity exposure, as well as rain and snow.

　　In recent years, with the advancement of microelectronic fabrication technologies, photopolymerization has become increasingly mature in its applications to optical films. From conventional hard-coat films to brightness‑enhancing films, and from polarizing films to diffuser films, photopolymerization plays a pivotal role in their production; moreover, photoresists used in chip manufacturing are of paramount importance.

　　Future Trends in Photocuring Technology

　　The development of photopolymerization is inextricably linked to advances in raw materials, equipment, and technology. Looking ahead, the future of photopolymerization will encompass the following aspects:

　　 Development of Functionalized Resins

　　Resins containing low‑surface‑energy functional groups are employed in anti‑fouling coatings; these include silicon‑ and fluorine‑containing structural units. The siloxane–fluorine architecture effectively reduces the surface energy of the system, thereby imparting anti‑fouling and self‑cleaning properties.

　　Photocurable waterborne resins are primarily resins containing cationic, anionic, or nonionic functional groups; they can dissolve or disperse in water, allowing water to be used as a diluent and reducing the reliance on organic solvents, thereby lowering VOC emissions. At present, the main challenge with waterborne UV‑curable resins is that the resulting coatings often fail to meet performance requirements, such as water resistance, acid–alkali resistance, solvent resistance, and scratch resistance.

　　Inorganic–organic hybrid resins are used to prepare high-performance surface coatings, enhancing hardness and scratch resistance. These resins are typically synthesized via the sol–gel process, which disperses nanoscale inorganic particles uniformly within an organic matrix; the organic phase imparts polymerization properties, while the inorganic particles confer additional functional characteristics.

　　In recent years, driven by advances in photopolymerization technologies such as 3D printing, inkjet printing, and solvent-free spraying, the demand for low-viscosity resins has been steadily rising, making the development of ultra‑low‑viscosity resins an urgent priority. As modern photopolymerizable materials increasingly stringent performance requirements for cured coatings, enhancing material properties necessitates the use of high‑functionality resins to improve polymer characteristics. A particularly promising approach is to modify these resins with hyperbranched polyesters and similar compounds, thereby synthesizing polymerizable resins.

　　The development of resins based on renewable resources is currently a major research focus. For instance, substantial fundamental research is underway to synthesize resins derived from natural oils, natural saccharides, natural polymers, and plant and animal extracts. Several products, such as soybean‑oil‑modified acrylates and furfural‑based acrylic resins, have already been commercialized.

　　The Development of Light Sources

　　Traditional photopolymerization relies on high-pressure mercury lamps as the light source. During operation, these lamps generate ozone, polluting the environment, and release substantial heat, leading to energy waste. Moreover, mercury itself is a toxic substance, which further restricts the use of mercury lamps. Developing new types of light sources has thus become a critical task; energy‑efficient, safe, and highly effective LED light sources represent an effective alternative.

　　Developing LED light sources with diverse wavelengths—particularly those in the 300 to 365 nm range—is a critical need for photopolymerization technologies, as efficient light emission is key to energy savings. For longer-wavelength LEDs, such as those operating at 385–405 nm, the technology is already quite mature; however, the availability of photoinitiators matched to these wavelengths remains limited, thereby constraining their applications. Moreover, long‑wavelength LED sources have yet to adequately address surface curing challenges, underscoring the necessity of developing short‑wavelength LED light sources. Yet, as wavelength decreases, photon energy increases, and this higher energy can degrade organic molecules, leading to their decomposition. Consequently, encapsulation materials for short‑wavelength LEDs represent the most significant current hurdle. If the challenges associated with encapsulating short‑wavelength LEDs and managing their high photon energy can ultimately be overcome, the adoption of photopolymerization technologies could advance substantially. This is because LED light sources offer long lifetimes, low costs, and reduced energy consumption—factors that would greatly facilitate the broader deployment of photopolymerization techniques.

　　 New Light-Curing Technology

　　Essentially, electron-beam (EB) curing is a form of photopolymerization; the key difference is that EB technology employs shorter wavelengths and higher energy. Currently, EB curing has been applied in areas such as printing inks, surface coatings, pressure-sensitive adhesives, composite materials, release films, and coil‑coating, with processing speeds reaching up to 300 meters per minute. It is both energy‑efficient and environmentally friendly, offering promising prospects for future development. In China, EB curing is still in its early stages, but as domestically produced EB equipment matures, its adoption is expected to expand. In recent years, EB curing has begun to gain traction in the printing industry, owing to its superior energy efficiency, faster processing speeds, and enhanced product quality. Cigarette filter tips are in direct contact with the oral cavity, so they must meet extremely stringent requirements: they must be insoluble in water, prevent any migration of harmful compounds, and remain odorless. However, since filter tips are made of paper—material that is inherently water‑sensitive—a protective coating is necessary to impart water resistance, biocompatibility, and other critical properties. Electron-beam curing stands out as one of the most suitable options for applying such coatings.

　　 EB‑cured release films have also begun to be adopted in China, primarily leveraging the high energy of electron beams to achieve extensive crosslinking. This ensures that no low‑molecular‑weight substances are released from the release layer, thereby guaranteeing the film’s long‑term release performance. For high‑performance materials such as optical films, even trace contamination in the release layer can degrade optical performance to the point where the material becomes unusable; consequently, EB‑cured release films are predominantly used in premium‑grade applications.

　　 The application of electron-beam (EB) curing in coil‑coating has already achieved mass production overseas, but in China it remains in its infancy. Its primary advantage is its rapid curing speed, which can significantly boost production efficiency and reduce energy consumption. Moreover, the resulting coatings exhibit outstanding performance, particularly with outdoor aging resistance that far surpasses both UV‑curable and conventional thermally cured coatings. Solventless spray technology was developed primarily to address the issue of solvent pollution caused by the need to add a certain amount of solvent for dilution during the spraying process.

　　Cationic photopolymerization technology

　　At present, rapidly advancing radical‑based systems, owing to their inherent limitations, fail to meet the demands of certain application areas; thus, the development of cationic photopolymerization offers an effective complementary approach. For instance, in the case of highly flexible coatings, conventional radical photopolymerization often proves impractical due to the intrinsic properties of the materials, whereas epoxy‑based cationic photopolymerization can readily yield coatings with exceptional flexibility. Furthermore, when coating metal substrates, radical photopolymerization typically results in poor adhesion because of its rapid cure and volumetric shrinkage; by contrast, cationic photopolymerization induces ring‑opening of epoxides during polymerization, leading to volumetric expansion that significantly enhances coating adhesion.

The development of photopolymerization technology is driven not only by advances in the technology itself but also by national policies, demands from other sectors, and technological breakthroughs in related industries. Under China’s stringent environmental regulations, solvent emissions are tightly controlled, making pollution-free photopolymerization technologies increasingly favored.