Cationic curable systems

Polymeric cationic curable formulations containing an d -biphenylthioxanthenium salt initiator 638 have been reported to reduce the residual odor and benzene levels associated with other -phenyl cationic photoinitiator systems <2006W02006/060281 >. The mechanism of photo-acid generation from related rS -aryl thiopyranium salts... 

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. 

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. 

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). 

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. 

Chem. Descrip. Polyether triol containing primary hydroxyl groups Uses Flow aid, leveling agent, abrasion resist, aid for radiation-curable monomers, urethane and polyester resins, high-solids coatings, polyester resin synthesis reactive modifier for UV cationic cure systems Features Good compat. with resin systems Properties Lt. yel. clear liq. m.w. 240 sp. gr. 1.1 dens. 9.2 Ib/gal vise. 800-900 cps hyd. no. 680-710 pH 6-8 (2%) 0.5% moisture... 

Uses Flow aid. leveling agent, abrasion resist, aid for radiation-curable monomers, urethane and polyester resins, high-solids coatings, polyester resin synthesis reactive modifier for UV cationic cure systems Trade Names Photonol PHO-7149... 

The compositions of materials photocrosslinkable by cationic mechanism consist of mixtures of various vinyl ethers, or epoxides, or both. Difunctional cycloaliphatic epoxides have been used extensively in some UV curable systems, often as diluents for the various epoxy resins described in Chapter 6. Use of various divinyl ethers is also extensive. Because some cationic photoinitiators also generate free radicals, some compositions may contain mixtures of both types of materials, those that cure by cationic and those that cure by free-radical mechanisms. 

Free-radical, radiation-curable systems generally consist of monomers, oligomers, photoactivators, other resins, and fillers or tackifiers. An ultraviolet source or electron-beam generator is used to cure the systems. As an illustration Stueben (37) describes a typical UV-cured PSA system containing acrylates and polyvinyl ether. McGinniss (38) discusses formulation design related to UV-curable systems. The cationic photoinitiators are shown in Fig. 1. 

No phase changes or segregation phenomena occurred up to ca. 20 % conversion of the UV-curable system, in a wide temperature range The LC phase does not affect the free radical polymerization rate Unlike cationic photopolymerization, the free-radical mechanism promotes rate accelerations at conversions related to compositions, for which changes in medium opacity are observed... 

UV curable systems are not restricted to acrylate technology. Cationically cured UV systems based on cycloaliphatic epoxy resins, vinyl ethers and iodonium salt photoinidators are also used for metal decorating. 

Materials and additives that are chemically basic in nature have a detrimental effect on the curing of cationic-initiated epoxy systems. These substances can either stop the curing mechanism completely or produce under-cured polymers. Therefore such additives as amines or imides that are known to be adhesion promoters cannot be used in the EB-curable epoxy adhesive formulations. 

Janke, C.J., Dorsey, G.F., Havens, S.J. and Lopata, V.J., Toughened epoxy resins eured by eleetron beam radiation. 28th International SAMPE Technical Conference, S71, 1996. Janke, C.J., Electron beam curable cationic epoxy resin systems and composites. 1st Annual Electron Beam Curing of Composites Workshop, September 18-19, Oak Ridge, TN, Ref. ID, 1996. 

There are many possibilities, but only a few systems are being used for practical UV curable formulations using free radical or cationic initiation. These will be discussed in this section. 

Ultraviolet-curable laminating adhesives use cationic chemistry and visible initiating systems to solve laminating problems with polyester films, which absorb most UV radiation.15... 

The generation of a protonic acid (HX) is responsible for the initiation of cationic polymerization. Various monomers are polymerized by sulfonium photoinitiators. Especially this system has been shown to be potentially useful for UV curable coatings of metal and plastics with epoxy resins. 

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