What Is UV Curable Adhesive?
UV curable adhesives are adhesives that cure by irradiating with ultraviolet (UV) light. They cure through a chemical reaction initiated by the application of activation energy using ultraviolet light, and are characterized by their ability to bond quickly.
They can also bond a wide range of materials, including electronic components, metals, glass, and plastics. However, UV irradiation lamps are required and cannot be used on parts that cannot be irradiated.
Uses of UV Curable Adhesives
UV curable adhesives can bond a wide variety of materials, including glass, plastics, metals, and ceramics, and can shorten working hours through high-speed bonding. They are used in many industrial fields, including automobiles, electronic components, assembly of smartphones, digital cameras, and liquid crystal displays, and optical components. Approved ones are also used in medical applications, and the adhesives are used in dental fillers and cements, as well as in the assembly and repair of medical equipment.
Principles of UV Curable Adhesives
1. Bourdon Tube Gauge
UV curable adhesives are activated by irradiation of ultraviolet light as an energy source, causing a photopolymerization reaction to proceed. The main components are a photoinitiator, a polymer or monomer as the main ingredient, and additives (stabilizers, etc.). The initiation and progression of the polymerization reaction are as follows:
1. The photoinitiator absorbs UV light and generates the active species: radicals, cations, or anions.
2. The active species generated by the initiator cause the polymerization reaction of the adhesive’s main component to proceed. Monomers are converted into polymers with larger molecular weights, and components that were originally polymers are converted into larger polymers.
3. The melting point rises as the molecular weight increases due to the polymer formation, and the adhesive goes from a liquid to a gel-like state, then quickly becomes solid and cures.
Characteristics of UV Curable Adhesives
One of the characteristics of UV curable adhesives is that the curing reaction proceeds quickly, from a few seconds to a few minutes, and the working time can be shortened compared to other adhesives. However, it should be noted that UV irradiation lamps are always required and that they cannot be used on parts that cannot be irradiated with UV light. The chemical bonds created by UV irradiation are strong bonds such as covalent bonds, and they also have excellent chemical resistance, heat resistance, and moisture resistance.
In the past, mercury lamps were used for UV irradiation, but UV LED products with high output and long life are now commercially available, and are now regarded as an environmentally friendly method.
Types of UV Curable Adhesives
The types of UV curable adhesives are mainly divided into acrylic resin-based adhesives using a radical polymerization reaction and epoxy resin-based adhesives using a cationic polymerization reaction.
In acrylic resin-based adhesives, radicals are generated from the polymerization initiator by UV irradiation. This radical reacts with the acrylic resin, causing a radical polymerization reaction to proceed, resulting in the formation of a polymer and curing. In epoxy resin-based adhesives, UV irradiation generates acid from the polymerization initiator, which reacts with the acrylic resin. As a result, a cationic polymerization reaction proceeds, resulting in polymerization and curing.
In general, acrylic resins are less expensive and have the advantage of faster curing speed. On the other hand, cationic polymerization type resins are not inhibited by oxygen and are less prone to shrinkage during curing. Therefore, epoxy resin-based adhesives may be more suitable for applications such as lenses, where positional accuracy is required. Curability, hardness, and viscosity also vary depending on the product, and some can be adjusted. Appropriate selection should be made according to the application.