The UV coatings with the fastest light curing rate

Drill a small hole in a wooden barrel filled with water and then use a light to illuminate the water surface from above the barrel. Next, an amazing scene happens as the water flows out of the hole, the light also presents a curved trajectory, as if the light is ‘captured’ by the water flow.

This seemingly counterintuitive phenomenon raises a question: shouldn't light travel in a straight line? However, science provides us with the answer. The principle behind this phenomenon is the total internal reflection effect of light. When light is emitted from a high-density medium (such as water) to a low-density medium (such as air), if the angle of incidence is greater than a certain angle, the light will not refract into the low-density medium but will be completely reflected into the high-density medium. Therefore, the light propagates along the curved path of the water flow, giving people the illusion that the light is bending forward in the water flow.

Later, people created a kind of glass fiber with high transparency and thickness like hair: glass fiber. When light enters the glass fiber at a suitable angle, the light moves along the winding glass fiber. Since this fiber can be used to transmit light, it is called optical fiber.

There are two types of optical fibers: quartz glass fiber and plastic fiber. Quartz glass fiber is made of quartz glass with a particularly high purity (the main component is SiO2). It has excellent light transmission performance, a small refractive index, low attenuation, long signal transmission distance, and fast transmission rate. It can be used in long-distance, high-speed, and large-capacity communications, broadcasting, data transmission, and other fields. However, it has the defects of high processing costs, strict quality control requirements, and brittle materials, easy to break and difficult to repair. In sharp contrast to quartz glass fiber is plastic fiber, which is made of high molecular polymer materials, which is soft, and easy to process and connect. Plastic optical fiber has a large attenuation and a slow transmission rate, so it is more suitable for short-distance communications, sensing applications, industrial automation control, and household appliances. Therefore, in optical fiber communications, quartz optical fiber occupies an absolute advantage.

Quartz optical fiber generally consists of a five-layer structure, with a core made of high-refractive-index quartz in the center, and a bare quartz fiber with a diameter of about 125 microns made of low-refractive-index quartz; a soft coating is applied on the outside, followed by a hard coating, and finally a colored ink. This clearly layered structural design enables quartz optical fiber to work stably under various environmental conditions, while ensuring high-speed and long-distance transmission of signals.

The production process of quartz optical fiber is a precise and complex process. Firstly, the specially doped prefabricated quartz rod is melted in a high-temperature graphite furnace at up to 2000℃. The molten quartz material is then drawn into fibers to form thin bare fibers. However, although these bare fibers have excellent performance, they are also extremely fragile and easily broken under the influence of the external environment. They may be scratched, dusty, absorb moisture, or even oxidized, all of which may directly affect the transmission quality of the optical signal. Therefore, coating and protecting bare fiber is a crucial step.

The coating process starts immediately after the bare fiber is pulled out. The bare fiber is first cooled to below 150°C, and then vertically passes through the UV coating tank and is evenly coated with UV fiber coating using a dip coating process. This coating not only provides protection but also enhances the durability of the optical fiber.

The double-layer coating process used in optical fiber production consists of an inner soft coating and an outer hard coating to ensure the transmission performance and mechanical strength of the optical fiber. The inner soft coating has a low glass transition temperature (Tg), exhibits high flexibility and low modulus in the temperature range of -60°C to 100°C, and has anti- oxidation and anti-hydrolysis properties, as well as a high refractive index, ensuring that the optical signal can achieve total reflection during transmission and reduce losses.

The outer hard coating has a higher Tg and modulus, providing sufficient mechanical strength and good aging resistance. It has strong resistance to substances such as acids, alkalis, solvents and salt water, and protects the optical fiber from erosion by the external environment.

Early fiber optic coatings were mostly solvent-based and heat-curing, which limited the efficiency of fiber optic production. With the introduction of light-curing technology, the curing rate has increased significantly, greatly improving the production efficiency of optical fibers. In the 1980s, the production speed of optical fibers exceeded 100 meters per minute. In modern times, the development of new oligomers and photoinitiators, combined with inert gas protection during the light-curing process, has enabled the coating rate to reach up to 2,500 to 3,000 meters per minute, making it the fastest light-curing material currently available. This technological breakthrough not only improves production efficiency but also provides a solid foundation for the rapid development of optical fiber communications.

The existence of optical fiber coating enables optical fiber to work over long distances and in complex environments while maintaining long-term stable performance and low signal quality loss. The importance of optical fiber coating to the overall performance of optical fiber can be said to be without high-quality optical fiber coating, there would be no high-quality optical fiber network, and thus there would be no high-speed Internet today.