Photocured prepolymers are the main body of UV curable adhesives. The general formula of the prepolymer is shown in Fig. 6. By changing the intermediate oligomer portion, various properties of the photoactive prepolymer can be obtained. Common prepolymers include epoxy acrylates, urethane acrylates, polyether acrylates, and silicone acrylates. Different prepolymer structures can be designed according to the required performance requirements of the adhesive. Therefore, the design and formulation of photo-curing prepolymers are often the core of patented UV-adhesive technology.
Polyurethane Acrylate: In the polyurethane block structure, there is an ether bond, which can rotate freely. The cured product has good toughness and elongation. Such prepolymers are generally prepared by reacting di- or polyisocyanates with di- or polyhydroxy compounds of different structures and molecular weights to produce isocyanate-terminated intermediate compounds, which are then reacted with hydroxyl-containing acrylic or methacrylic acid to obtain acrylic groups. Polyurethane.
Epoxy acrylates: Epoxy resin backbones give adhesives adhesion and chemical stability. Such prepolymers are usually prepared by the ring-opening addition reaction of epoxy resins with acrylic acid or methacrylic acid. Sometimes, in order to further improve the performance of the prepolymer, the acrylic acid chloride or acid anhydride is added to block the hydroxyl groups therein, that is, the hydrophobicity is increased, and more photosensitive groups can be introduced.
Silicone Acrylates: Polyurethane acrylate prepolymers based on siloxane block copolymers. Due to the introduction of siloxane blocks, cured products have good toughness, aging resistance, chemical resistance, and heat resistance. . Such prepolymers can be formed by reacting diisocyanate, 2-hydroxyethyl acrylate, and alkoxylated silicone oils.
1.2.3 Acrylate monomers
The monomer mainly acts as a diluent to reduce the viscosity of the system; the multifunctional monomer can simultaneously act as a crosslinker; and the physical properties of the monomer can be changed by changing the ester moiety of the acrylate. Because the polymerization rate of acryloyl group (CH2=CHCO-) ​​is faster (about 3 times) than methacryloyl group (CH2=C(CH3)CO-), the former is more commonly used. When selecting monomers, volatility and skin irritation are important considerations. For monofunctional monomers, the ester moiety is usually bulky (longer carbon chains, etc. are more common) to reduce the volatility of the monomer. The polyfunctional acrylate monomer is not limited thereto. For higher curing speeds, polyfunctional acrylates are preferred.
To reduce monomer irritation and improve the overall performance of the adhesive, alkoxylated polyfunctional monomers are often used, such as ethoxylated TMPTA and ethoxylated NPGDA monomers.
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