UV adhesive is a specially formulated resin with a photoinitiator (or photosensitizer) added to it. After absorbing the high-intensity ultraviolet light (300nm-400nm) in the UV light curing equipment, it produces active free radicals or ion radicals, thereby initiating polymerization, cross-linking and grafting reactions, so that the resin is converted from liquid to solid within seconds, thereby achieving bonding of the substrate.
Curing principle of UV glue
1. UV adhesive curing principle
UV glue is made of prepolymer, active monomer, photoinitiator (photosensitizer), crosslinking agent and other materials. It is generally in the state of viscous liquid before being irradiated by ultraviolet light. When ultraviolet light irradiates UV glue, the bonds of the photoinitiator absorb energy and break, decomposing into active free radicals. The active free radicals initiate monomer polymerization and crosslinking reactions, making the UV glue transform from liquid to solid within a few seconds. It is the process of resin polymerization and molding.
2. UV light curing components
Prepolymer: 30~50% acrylate, prepolymers include: epoxy acrylate, polyurethane acrylate, polyether acrylate, polyester acrylate, acrylic resin, etc.
Monomer: 40~60%, monomers include: monofunctional (IBOA, IBOMA, HEMA, etc.), difunctional (TPGDA, HDDA, DEGDA, NPGDA, etc.), trifunctional and multifunctional (TMPTA, PETA, etc.)
Photoinitiator: 1~6%. Initiators include: 1173, 184, 907, benzophenone, etc.
Additives: 0.2~1%, additives include: surfactants.
3. Advantages of UV light curing
Safety: Made of low-toxic acrylic materials, no solvents, no VOC volatiles, no pollution to the ambient air;
Economical: Compared with thermal curing, UV curing has a high energy utilization rate and can save up to 90% of energy; Fast: The curing speed is fast, and the curing can be completed in a few seconds to tens of seconds, which is conducive to automated production lines and improves labor productivity;
Easy to use: The equipment is simple and easy to use, suitable for both large-scale factory production and small-scale home use;
Compatibility: Good adhesion with materials, can be combined with materials that are sensitive to temperature and humidity;
Adjustable: The molecular weight and cross-linking degree of the resin can be adjusted by raw materials and light.
Six main application categories of UV adhesive
1. UV pressure sensitive adhesive (PSA)
At present, PSA is produced through solution coating, drying and chemical reaction. More than 60% of the solvent must be volatilized and burned, resulting in high emissions, long production process, low efficiency, and residual solvents and other harmful chemicals in the product. PSA is produced by UV curing. After the solvent-free liquid component is coated on the substrate, it is photopolymerized by UV light. No hot drying is required, the production line is simplified, the production process is environmentally friendly, and there is almost zero emission. The production efficiency is also greatly improved.
PSA products come in a variety of forms, from decorative films, various protective films to engineering structural tapes, and have a wide range of uses, including in advertising boards, electronic products and high-end packaging boxes.
2. UV composite adhesive
Food, beverages and some biopharmaceutical products require high-barrier flexible packaging. The barrier properties of packaging materials determine and affect the edible time. High-barrier packaging materials are made of plastic film, paper and metal film. They have the characteristics of high strength, good oxygen and water insulation, safety, hygiene and non-toxicity, and are suitable for printing. They can withstand freezing and high temperatures of 120°C.
At present, the composite adhesives in the flexible packaging industry are almost all solvent-based adhesives, with the mass of solvents greater than 70%. The composite process involves the steps of coating, solvent volatilization, composite, storage and aging. In addition to being flammable and explosive, with high emissions, a long production process and low efficiency, there are also residual solvents in the composite film. During the holiday season when the demand for exquisite packaging is strong and delivery is urgent, many composite packaging films are delivered for use without fully reacting and completely evaporating the solvents. The residual solvents and unreacted chemicals may penetrate and contaminate food, which is a major reason why many medical and food packaging products still fail to meet health standards. Although water-based adhesives are solvent-free, they dry slowly, have low production efficiency, and are not ideal for bonding plastics.
Using UV adhesive to replace solvent-based adhesive will not only greatly improve the production efficiency of composite film, but also reduce the production process from several hours to one minute. UV adhesive does not contain solvent, no volatility, no harmful residue, and can ensure that the composite film meets the health and hygiene requirements of food, medicine and other packaging materials. Waiting for the solvent to evaporate for several or even dozens of hours, during which time the construction site cannot be used. If light-curing adhesive is used, there will be no such problem. The light-curing adhesive can be applied to the ground first, and then cured with ultraviolet light after self-leveling. The surface is sticky, and then the PVC floor can be laid and used immediately. Not only does it greatly shorten the construction period, but there is no solvent volatilization in the entire production process, and it does not pollute the indoor air.
3. UV building adhesive
Safety glass mainly uses polyvinyl butyral film as bonding material, and the glass sheets are bonded together through vacuum degassing and hot pressing processes. The total production time takes about ten hours. The production of laminated safety glass using liquid bonding methods, which involves edge sealing, glue pouring, and photopolymerization under natural light for dozens of minutes, has also been around for a long time, but it is still a non-mainstream product and is mainly used in the low-end safety glass market. The main reason why liquid bonding methods are not widely accepted is that the performance of UV glue is not enough and there is a lack of reliable data on safety. UV glue has a good growth momentum in the construction field. Store windows, decoration, and colored glass are all using UV glue. With improved performance, UV construction adhesives will have great development.
In recent years, the application of PVC stone plastic floor has been rapidly developed in China. The construction method is to first scrape a layer of adhesive on the construction ground, and then bond the PVC floor. PVC plastic floor adhesive can be mainly divided into three categories: water-emulsion type, solvent-based type, and reactive type. Water-emulsion adhesive uses water as the dispersion medium, the product is non-toxic and low in cost. However, it has poor water resistance. Generally, after being soaked in water, the bonding force decreases significantly, such as water-based acrylic. Solvent-based adhesives are mainly polyvinyl acetate and chloroprene rubber adhesives. Reactive types include polyurethane series adhesives. These adhesives are required after construction.
4. UV electronic glue
UV adhesives have been widely used in the field of electronic products, including cable positioning, pin sealing, preparation of LCD panels, mobile phone buttons, etc. UV curing bonding is a necessary technical means in the manufacture of electronic products. With the thinning of electronic products and the emergence of organic optoelectronic devices and flexible display devices, there is a huge demand for UV adhesives that adapt to roll-to-roll processes. The high efficiency, rapidity, timeliness and controllability of UV curing will be fully demonstrated.
5. UV optical adhesive
Touch screens are currently the largest single application of UV optical adhesives. The UV optical adhesives in touch screens have three functions: bonding, increasing light transmittance, and improving impact resistance. They need to meet strict performance requirements, including color, weather resistance and environmental stability, electrical performance, optical performance, etc. The bonding materials involved in touch screen bonding mainly include: glass, ITO conductive layer, PET, PMMA, PC, etc. When sapphire is used in the touch screen, UV adhesives with higher refractive index will be required to improve light transmittance.
At present, there are liquid (LOCA) and solid (OCA) optical adhesives used for touch screens. The former is dispensed by a program-controlled dispensing machine, covered with a glass cover, leveled and filled, and then cured through the cover. It has the advantages of good filling and convenient glue application, but it needs to be cleaned up for overflow glue; OCA is a double-sided PSA without a substrate, with release films on the top and bottom. When using it, first remove the light release film for bonding, then remove the heavy release film and bond it with the other bonding surface. The liquid glue used in the early touch screen is heat-cured, with a long production process and low efficiency. Now it has all been converted to UV-cured glue. The current OCA is still produced by traditional solvent-based glue through coating, drying, and thermal cross-linking processes. The drying tunnel is long and there are many defects caused by dust. Switching to UV curing technology will greatly improve production efficiency, reduce equipment investment and production costs.
6. UV medical glue
UV adhesive has great room for growth in the medical field. For example, medical pressure-sensitive adhesives or tapes can be converted to UV-curable preparation to form a series of products.
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