UV-curable systems

Traditionally, UV curable polymers have been utilized as coatings for wood and vinyl floors, but their applications have increased dramatically over the last twenty years to encompass many diverse areas, including optical fiber coatings (7), adhesives (2), disc replications (3-5), and microelectronics (6). This widespread use of UV cross-linked systems is attributed to their rapid, energy efficient curing and their solvent free, one piece formulations. Typically, UV curable systems require only a small fraction of the power normally utilized in thermally cured systems and their solvent free nature offers an environmentally safer alternative. 

Many novel coatings technologies are discussed in the review by McGinniss (McGinniss, 1996) who discusses advances made in environmentally benign coatings that include silicon oxides, sihca fillers, and hybrid waterborne/UV-curable systems. 

Visconti M, Cattaneo M. (2000) A highly efficient photoinitiator for water-borne UV-curable systems. Prog Org Coatings 40 243-251. 

MAJOR PRODUCT APPLICATIONS paints, coatings, inks, UV-curable systems, powder coatings, adhesives, insulating and sealing compounds, fibers, paper, sealants, mastics, lubricants... 

Quartz was used as a filler in the manufacture of optical devices from epoxy in a UV-curable system. Figure 6.22 shows that addition of a filler can substantially reduce curing shrinkage which is highly desirable in the precise manufacture of these materials. Reduced shrinkage is the result of a low thermal expansion coefficient of quartz in comparison with the resin. 

Photopolymer systems are photocurable resins incorporating reactive liquid monomers, photoinitiators, chemical modihers and hhers. Typically stereolithography utilizes UV radiation, so UV-curable systems are used. Free-radical-photopolymerizable acrylate systems were originally used however, newer cationic epoxy-resin and vinyl ether systems (based on iodinium- or sulfonium-salt cationic initiators) are now being utilized. 

Vicure . [Akzo] Benzoin ether < r methyl phenylglyoxylate photo sensitizer ex photoinitiator ftnr uv-curable systems. 

EINECS 245-039-6 Ethanone, 2-(2-methylpropoxy)-1,2-diphenyl- Wcure . Photosensitizer for UV curable systems, e.g., coatings, inks, graphic arts, bpo.5 = 133° d = 0,9850. Akzo,... 

Phenylglyoxylic acid, methyl ester Vicure 55. High purity photoinitiator for UV curable systems, especially xrylate-based formulations. Liquid bp = 246-248 d = 1.1550. Akzo Chemie. 

With the basic mechanism of free-radical and cationic photocuring presented in Schemes II and III, we turn to consideration of each of the individual components in UV curable systems. It is this aspect over which the formulator has control in producing a resin system which will cure in an appropriate environment (temperature, air, etc.) at reasonably fast rates to generate the type of film desired. 

All UV curable systems have four basic components which must be included in order to develop a successful coating. They are the photoinitiator(s), oligomer(s), monomer(s), and additive(s). Table II lists the properties of each component which make it essential to the UV curable formulation. In the next few pages, we summarize each of the components and their properties. The photoinitiator section covers both radical and cationic type photoinitiators while the oligomer and monomer sections are restricted to components used in free radical systems. 

Monomers. As shown in Table H monomers are used in UV curable systems to provide final film properties and viscosity control of the resin. They are also important in determining the speed of cure, crosslink density, and final surface properties of the cured film. Properties which dictate the selection of one monomer over another are listed below ... 

In accordance with the above requirements, several monomer types employed in UV curable systems are listed below ... 

By far the acrylates are the monomers of choice in UV curable systems. Not only do they cure at extremely rapid rates compared to other monomer systems (acrylic > methacrylic > vinyl > allylic), but they are also available in a wide range of structures which are monofimctional, difunctional, trifunctional, and tetrafunctional. Additionally, as shown in the oligomer section, acrylates can be used to derivatize oligomers or pre-polymers. Commonly in UV curable formulations it is necessary to use a number of monomers in order to achieve a balance between speed of cure and properties of the final film. It is not unheard of to use four or five monomers in a single UV curable formulation. For instance, tri- and tetra-functional acrylates result in highly crosslinked films when incorporated into UV curable resins however, they severely limit the extent and rate of the curing process. Thus, one often combines a tetrafunctional acrylate to increase crosslink density with a mono and/or difunctional acrylate to increase the cure rate. 

The exotherm apparatus (either DSC or thin-foil) is a highly useful tool for evaluation of the efficiency and speed of UV curable systems. It can be applied as a screening mechanism for UV curable formulations or as a highly sophisticated scientific tool. Its use has led, and will no doubt lead in the future, to the development and improvement of photocurable coatings. 

Therefore the phenones Id-g were compared in their curing behaviour with the unsubstituted parent compound la in an isual acrylate based UV curable system (experimental conditions see ref. From the results (Table I) it is concluded that the phenones Id-f have a similar activity as the parent compound la, although differences became obvious at short irradiation time. The initiator activity of the azide Ig was extremely low compared to the phenones la and Id-f. This unexpected result might be explained by the effect of the azido group. To get more insight to the photoreactivity of the azide Ig an ethanolic solution was irradiated for one hour. A new crystalline compound was isolated and characterized as 4-(2-azidoethoxy)benzoic acid. 

Temperature Effect on the Phase Transformation of UV-Curable Systems... 

A system that has recently been receiving attention Is radiation induced cationic curing. The interest in cationic curing has been Inspired by the development of onlum salt catalysts. Strong acids are liberated when the coatings are Irradiated In the presence of certain onlum salts. These acids are capable of catalyzing cationic polymerization reactions (7). Onlum salts have been used mostly In UV curable systems. However, It has been shown recently that they may also be used for electron beam Induced curing (8,9). 

Recent efforts have been directed towards an improvement of the performance of UV-curable systems, frying to overcome some of the most severe limitations by acting at different levels ... 

In this paper we report some of the progress recently made in these various areas, concentrating on the light-induced polymerization of multiacrylic monomers which are today the most widely used UV-curable systems. We also describe here a new analytical method, based on IR spectroscopy, that permits the kinetics of photopolymerizations, which develop extensively in a fraction of a second, to be followed quantitatively and in real time. 

Table 1. Performance analysis of UV-curable systems Photoinitiator [IrgacuretiSl] =5% Film thickness 30 pm...

Though potential UV-curable system [90] are many, only two have found significant commercial use in adhesives, namely, free-radical polymerization of acrylates and cationic polymerization of epoxies. 

Photoinitiators are essential for UV-curable systems and can be classified into two groups ... 

Chem. Descrip. 1-Hydroxycyclohexylphenyl ketone, benzophenone Uses Photoinitiator tor UV curable systems, clear coatings tor paper, plastics, and metal... 

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