Principle and application of UV glue

Principle and application of UV glue

2014-10-15 Pushengte Adhesive Scheme Consultation

UV glue knowledge training

Ultraviolet curing is a kind of radiation curing. Ultraviolet curing is a chemical process that rapidly converts low molecular weight substances into high molecular weight products. Curing is carried out on unheated substrates. The system contains no solvent or very little solvent. After irradiation, the liquid film is almost 100% cured, so VOC (volatile organic compounds) emissions are very low. Therefore, since the end of the 1960s, this technology has developed rapidly in the world, and its products have been widely used in many industries.

1、 Overview:

Ultraviolet curing is a type of radiation curing. Radiation curing uses electromagnetic radiation, such as ultraviolet (UV) or electron beam (EB), to irradiate the coating, generating radiation polymerization, radiation crosslinking, radiation grafting and other reactions. A chemical process that rapidly converts low molecular weight substances into high molecular weight products. Curing is carried out directly on the unheated substrate. The system contains no solvent or very little solvent. After irradiation, the liquid film is almost 100% solidified, so VOC (volatile organic compounds) emissions are very low. Therefore, since the end of the 1960s, this technology has developed rapidly in the world, and its products have been widely used in many industries.

1. Classification:

Radiation curing can be divided into radiation curing adhesive, radiation curing coating and radiation curing ink according to application. According to the radiation source used, it can be divided into ultraviolet (UV) curing, electron beam (EB) curing and visible light curing. As shown in Figures (1) and (2) below.

2. Basic knowledge of ultraviolet light:

Ultraviolet (UV for short) is a section of electromagnetic wave radiation. The electromagnetic spectrum includes radio wave, red line, visible light, ultraviolet, X-ray γ Ray, the wavelength range is 10-14m to 106m, as shown in Figure 3. Ultraviolet is only a very narrow segment, with a wavelength range of 10~400nm (nm: nanometer, 1nm=10-9m), which can be divided into long wave ultraviolet (UVA), medium wave ultraviolet (UVB), short wave ultraviolet (UVC) and ultra short wave ultraviolet (UVC). The shorter the wavelength, the stronger the energy and the weaker the penetration ability.

Long wave UVA, with a wavelength of 320~400nm, has a strong penetration ability and can penetrate glass. The ultraviolet energy in this band is equivalent to most chemical bond energies, which is easy to induce light chemical reaction. UVA is usually used for light curing.

Medium wave UVB, the wavelength is between 280~320, the penetration is weak, and the glass has strong absorption to it. The sunlight is rich in UVA and UVB.

The short wave UVC, with a wavelength between 200~280nm, is strongly absorbed by the ozone layer. Therefore, UVC in sunlight is absorbed by the ozone layer before reaching the ground. UVC has a strong destructive effect on organisms, killing bacteria and viruses, so it is often used for disinfection.

Ultraviolet can damage skin and eyes, and UVA can make skin dark, loose and wrinkled; UVB will cause acute dermatitis (i.e. sunburn), skin will become red and painful, and long-term exposure will also easily lead to skin cancer. Therefore, attention should be paid to the protection during the light curing operation:

(1) The curing machine shall be equipped with proper shielding equipment, so that the ultraviolet light is not easily transmitted;

(2) Appropriate personal protection, wear long sleeved clothes, cloth work gloves, wear UV resistant glasses, masks, and do not look directly at the lamps.

(3) Pay attention to ambient ventilation to prevent ozone accumulation.

Generally speaking, there is no need to worry too much, because the equipment manufacturer usually takes the protective equipment into account when designing, as long as it is operated according to the regulations.

3. Characteristics of radiation curing:

Advantages of radiation curing:

(1) Single component system without mixing, easy to use;

(2) The curing speed is fast, and the curing can be completed in a few seconds to dozens of seconds, which is conducive to the automation of the production line and the improvement of labor productivity;

(3) The curing temperature is low, saving energy. It can be cured at room temperature. It can be used for materials that are not suitable for high temperature curing. Compared with the heat curing resin, the energy consumption of UV curing can be saved by~90%;

(4) No pollution, low volatile monomers and oligomers can be used, no solvent is used, and 100% curing is required, so there is basically no air pollution and no waste water pollution;

(5) Excellent performance, abrasion resistance, solvent resistance, impact resistance and high strength.

Disadvantages of radiation curing:

(1) Large equipment investment, especially electron beam curing equipment, the price of radiation curing adhesive is higher than that of conventional adhesive, which can be compensated by low energy consumption and high production efficiency;

(2) Due to the weak penetration of ultraviolet light and limited curing depth, the geometry of curable products is limited, and the opaque parts and dead corners that cannot be irradiated by ultraviolet light are not easy to be cured;

(3) Ultraviolet light can produce ozone, so an exhaust system is required.

2、 Curing mechanism:

UV curing system can be divided into free radical system and cationic system, both of which have different curing mechanism components. The free radical system is generated by the photoinitiator excited by UV irradiation, which initiates the polymerization and crosslinking of monomers and prepolymers; Cationic system is generated by cationic photoinitiator under radiation to produce strong proton acid, catalyze addition polymerization, and solidify the resin. The following is an example of free radical system:

UV free radical curing goes through the following steps:

(1) After UV irradiation, the free radical photoinitiator is excited and decomposed to produce free radicals:

(2) Chain initiation: the free radical generated by the initiator initiates the unsaturated double bond of resin and monomer molecules to generate new free radicals.

(3) Chain growth: the free radicals generated by the resin and monomer can continue to trigger unsaturated double bonds in the resin and monomer molecules to generate free radicals and carry out free radical chain reaction.

(4) Chain termination: In chemical reaction, because free radicals contain uncoupled electrons, which are very active, they are prone to coupling or acidification of other radicals, so that the chain reaction is terminated.

As a result of the above reaction, polymer compounds are generated to transform the glue solution into a solid.