The curing mechanism of UV primer is a complex and delicate process. It relies on ultraviolet (UV) irradiation to trigger a series of chemical reactions, so that the liquid primer is quickly transformed into a solid state.
The curing process of UV primer is mainly based on photochemical reactions, that is, when ultraviolet light is irradiated on the primer containing photoinitiator, the photoinitiator absorbs light energy and transforms into an excited state, which in turn triggers a series of chain reactions, so that the resin molecules in the primer are rapidly cross-linked and cured. This process is usually completed within a few seconds to a few minutes, with the characteristics of high efficiency and energy saving.
Photoinitiator is a key component in the curing mechanism of UV primer. It can absorb energy under the irradiation of ultraviolet light and transition from the ground state to the excited state. In the excited state, the photoinitiator is unstable and prone to decomposition reactions to generate active intermediates such as free radicals or cations. These active intermediates are highly reactive and can quickly induce the resin molecules in the primer to polymerize or cross-link.
Free radical UV curing is one of the most common curing methods in UV primer. During the free radical curing process, the free radicals generated by the decomposition of the photoinitiator will attack the unsaturated double bonds in the primer to form monomer free radicals. These monomer free radicals will continue to react with other unsaturated double bonds to form a chain reaction, causing the resin molecular chain to continue to grow. Eventually, these long-chain resin molecules will cross-link with each other to form a three-dimensional network structure, allowing the primer to cure into a film.
In addition to free radical curing, cationic curing is also an important curing method for UV primer. In the cationic curing process, photoinitiators usually use iodonium salts or sulfonium salts. These photoinitiators photolyze under ultraviolet light to produce cation sources and proton acids to form cationic active centers. These cationic active centers can initiate the ring-opening polymerization of resin molecules in primers such as epoxy resins. Unlike free radical curing, the cationic curing process can also be carried out in a dark environment, so it has the characteristics of post-curing.
Whether it is free radical or cationic curing, the curing process of UV primer involves chain reactions. In the chain reaction, the active intermediates (free radicals or cations) produced by the decomposition of the photoinitiator will continuously trigger new reactions, causing the resin molecular chain to continue to grow. In this process, chain initiation, chain growth and chain termination are three important stages. Active intermediates are produced in the chain initiation stage; in the chain growth stage, active intermediates continuously trigger new reactions, causing the resin molecular chain to continue to grow; in the chain termination stage, the reaction is terminated to form a cured coating.
The curing speed of UV primer is affected by many factors, including the intensity, wavelength, irradiation time and primer formula of ultraviolet light. The intensity and wavelength of ultraviolet light determine the efficiency of photoinitiator in absorbing energy, thereby affecting the curing speed. The longer the irradiation time, the higher the degree of curing. In addition, the type of resin in the primer, the type and concentration of photoinitiator and the use of additives will also affect the curing speed.
After UV curing, the primer coating will show excellent physical and chemical properties. The cured coating has the characteristics of high hardness, high gloss, good wear resistance and weather resistance. The improvement of these properties is mainly due to the cross-linking of resin molecules during the curing process and the three-dimensional network structure formed by curing. In addition, there is no emission of volatile organic compounds (VOCs) during UV curing, which meets environmental protection requirements and is also one of the important reasons for its wide application.
The curing mechanism of UV primer is a complex and delicate process involving photochemical reactions, chain reactions and a variety of influencing factors. Through reasonable formula design and process control, an efficient and environmentally friendly curing process can be achieved to obtain excellent coating performance.
