Nowadays, most hard coatings are made of acrylate monomers and acrylic oligomers with higher functional groups. Roller coating is applied to the surface of the film. The curing method is usually UV curing. People in the industry believe that only UV curing is the way. In fact, the curing of hard coatings can also be achieved by thermal curing.

UV curing

Ultraviolet irradiation is used to initiate free radicals and to stimulate covalent bonds of organic molecules to produce chemical reactions, resulting in crosslinking and polymerization. This method has lower curing energy but less environmental impact, so its practicability is relatively strong.
The problem with ultraviolet curing is oxygen inhibition. In fact, the radiation curing reaction of all radiation curing materials is affected by oxygen in the air. Because the concentration of oxygen in the surface layer is the highest, the inhibition of oxygen will cause the lower layer to be solidified and the surface to stick without solidification. Experiments show that the energy consumption of coatings cured in air with a thickness of 1 micron is 20 times more than that of coatings with a thickness of 1 micron. The inhibition of oxygen not only prolongs the curing time, but also impairs the important properties (hardness, wear resistance and scratch resistance) of the cured surface.

Inert gas protection is the best method for oxygen inhibition. At present, most domestic equipments do not have such devices. Usually, increasing the concentration of initiator and radiation energy are considered. The combination of initiator (184 and 819, 1173 and TPO) is adopted to promote surface-dry and deep initiation.

Heat curing
Heating initiates free radicals and stimulates covalent bonds to produce chemical reactions, resulting in crosslinking and polymerization. Thermal curing is a method that is easily neglected in the production stage of protective film.

The problems of thermal curing process are: higher energy requirement, worse environmental applicability and longer drying path requirement; longer wire is unfavorable for tension control and coating accuracy control in production stage; considering the high temperature requirement of thermal curing, the polyester film currently widely used is also sensitive to temperature, and can not be stored for a long time above 120 C, but it needs thermal curing. When the temperature is higher than 135 C, the cooling of the film is considered in the curing process. The curing time window is very small, and the response time should be carefully allocated in the efficiency, leveling and drying stages. The effect of thermal effect on the surface hardness should be taken into account in the thermal curing process. Excessive temperature will make the monomer or resin volatile and affect the quality of the film; after evaporation, the initiator used in the thermal curing process will be used. Crystallization on the inner surface of the drying channel will cause secondary pollution to the product.

Thermosetting has the advantage of bottom-free coating and curing; if UV curing, the bottom coating is absolutely indispensable; the potential advantage of thermal curing is that the coating edge interference (rainbow pattern) can be greatly reduced when the formulation is appropriate. The mechanism is that under thermal conditions, the crystallization order of the coating material changes, resulting in refractive index changes that are conducive to reducing edge interference.
The potential advantage of thermal curing is that most of the coatings which produce silica gel protective film in China can directly produce hardened layer without modification.

Stretch Coating Curing

The manufacturing process is direct on-line coating during biaxial stretching of PET film. Hardening layer is directly produced by stretching and heat setting. At present, only a few manufacturers have mastered this process, and the materials used are still being explored.