Uv Curing Coating Components and Their Industrial Applications (I)

UV curing (Uv curing) is a type of radiation curing technology and is a rapidly developing "green" new technology. UV-cured coating (UvCC) is an energy-saving and environment-friendly coating developed in the 1960s. After exposure to ultraviolet light, it undergoes a photochemical reaction, the coating of liquid oligomers (including monomers), and the formation of a solid coating by cross-linking polymerization. UVCC can be widely applied and developed because of its unique advantages of energy saving and environmental protection, excellent coating performance, and high production efficiency.

1. Composition of UVCC

The main components of UVCC include cross-linkable polymeric prepolymers (photoactive oligomers), reactive diluents (photoactive monomers), photoinitiators, auxiliaries (leveling agents, defoamers, matting agents, surfaces Slip agent). Their respective performance and research progress are as follows.

1.1 Oligomers

Oligomer is one of the largest proportion of light curing products. It is a base resin for light curing formulations and determines the basic properties of the cured product (including hardness, flexibility, adhesion, optical properties, and aging resistance). . It mainly includes unsaturated polyester resins, epoxy acrylates, urethane acrylates, polyesters, and acrylated polyacrylates. In the late 1930s, unsaturated polyester resins were first developed as photocurable oligomers. Epoxy acrylate is obtained by esterification of commercial epoxy resin and acrylic acid or methacrylic acid. It is the largest class of photo-curing oligomers in the domestic photo-curing industry. Its chemical resistance, strong adhesion, and good wetting of pigments make it widely used in paper coatings, wood coatings, and metal primers.

In order to highlight the performance advantages of the oligomerization field, there are many researches on its modification at home and abroad. For example, amine-modified epoxy acrylates introduce quaternary amine groups. Its main features are high curing speed and increased adhesion of cured films. With enhanced toughness, it has important application value in screen printing, plain printing and flexo printing inks. There are also phosphate modification, polyanhydride modification, siloxane modification, long chain fatty acid modified epoxy acrylate, and the like. Polyurethane acrylates are widely used next to epoxy acrylates, especially in the light-curing finishing of soft substrates such as paper, leather and fabrics. However, because of its slow curing and relatively high price, it is less used as a host oligomer in light-cured formulations, but as an auxiliary functional resin.

In the late 1980s, oligomers such as vinyl ether series, epoxy series, and other cation mechanisms cured to form a film, ie non-acrylate oligomers, appeared. The curing of such oligomers was not inhibited by oxygen in the air. The effect of the effect, curing speed, rapid development. There have been synthesized acrylate groups that have a free radical curing mechanism and hybridized oligomers of ethylene groups that have an oxygen ion curing mechanism. With the continuous development of UVCC and people’s growing awareness of environmental protection, a variety of water-based oligomers continue to emerge. Research papers have reported unsaturated polyesters used in light-curable waterborne coatings. Phifips et al. reported on the synthesis of polyester acrylates with different polyols and polybasic acids. It is believed that in order to obtain good water solubility, at least 6% to 7% of the hydrophilic groups must be present, the relative molecular weight is 6403,000, and the cured product has Good solvent resistance and water resistance.

1.2 Photoactive monomers

Traditional solvent-based paints usually require the addition of organic solvents to regulate the viscosity. These organic solvents generally do not participate in the film-forming reaction. Volatilization into the air during film formation can cause environmental pollution. In the photocuring system, the photoactive monomer (reactive diluent) not only plays the role of adjusting the viscosity of the system, but also usually participates in the curing of the film forming reaction and rarely volatilizes into the air, giving the system environmental protection characteristics. According to the reactive groups contained in the molecule, reactive diluents are divided into monofunctional, multifunctional reactive diluents. Young studied the effect of the functionality on its polymerization rate. The results show that the higher the functionality, the faster the cure and the higher the degree of cross-linking.

Currently the most widely used reactive diluents are (meth)acrylates, which have been developed as third generation reactive diluents, with high reactivity, low shrinkage, and the like.

1.3 Photoinitiator

The photoinitiator is a key component of UVCC and its role is to produce reactive groups that initiate the curing reaction. Generally divided into free radical polymerization photoinitiators and cationic polymerization photoinitiators. Anionic photoinitiators are less studied and commercial applications have not been reported. Free radical photoinitiators are divided into two major categories: cracking and hydrogen abstraction. Cracking type free radical photoinitiators are mainly based on aryl alkyl ketone derivatives, such as benzoin derivatives, benzyl ketals derivatives, dialkoxy acetophenone, a-hydroxyalkyl benzophenone, A monoamine alkyl benzophenone, among which benzoin is a photoinitiator developed earlier, due to its shortcomings of yellowing, so it is rarely used. A-hydroxyalkyl benzophenone is currently the most successful type of photoinitiator for application development. The most widely used domestic ones are CIO's I~OCUY1173 and Irgacure 184. Hydrogen abstracting initiators use a tertiary amine photosensitizer to form an initiator/photosensitizer complex initiation system that suppresses oxygen inhibition and increases cure speed. The cationic initiator, represented by the cartridge salt, can quickly initiate the curing of oligomers and monomers to form a film, and is not affected by the inhibition of O in the air. Several onium salts have been synthesized. The initiating behavior of Weng salt has been studied in depth and their photoinitiation efficiencies have been compared. It was found that not all salt extractions have cation-initiating properties. Effective onium salts include: thiosulfuric acid salts, iodonium salts, phosphonium salts, phenathrene iron salts, etc. The anions are [B(PhF5)4]-, PF6-, SbF6-, and AsF6-, etc., depending on the curing rate. For the activity of these anions, it was found that their activity size was SbF6≥AsF6≥PF6≥BF4[B(PhF5)4]-. This indicates that [B(PhF5)4]-one is more active than SbF6, and 0.4% of [B(PhF5)4]-iodium salt is used to initiate the functionalized siloxane polymer and its curing line speed. Up to 457m/rain. Since SbF has a certain toxicity, [B(PhF5)4]-salt tends to replace the commonly used sb-salt. In addition, a polymeric photoinitiator and a polymerizable photoinitiator have also been studied to overcome the degrading effect of residual small molecules or fragments on the cured film after the decomposition of the small molecule photoinitiator.

1.4 Other additives In practical applications, light curing systems often need to add various additives to meet other requirements. For example, in a colored system (paint, ink, etc.), pigments need to be added; in order to achieve good flow smoothness, leveling agents need to be added; in order to suppress the formation of bubbles in the system, defoamers are often added; addition of matting agents It is to reduce the gloss of the cured film to obtain a low gloss or matt coating. on