Cationically cured systems

In comparison with cationic curing systems, hybrid curing systems increase the rate of cure, produce cured films with improved solvent resistance, and offer a greater formulation latitude. When compared to free radical cure, better... 

The chemistry involved depends on the oligomer used silicone acrylates cure by radical mechanism epoxy-silicones require cationic curing systems. Silicone... 

Cationic cured systems have traditionally been aimed at niche applications or where ink based on free radical acrylate curing is not suitable. The raw materials used in cationic systems are generally different than those used in free radical cured systems. ... 

It is frequently observed that the properties of uv-cured films and coatings change as a function of time after cure. This process can be accelerated by a brief thermal treatment, in the case of cationically-cured systems this phenomenon is generally attributed to the reaction kinetics of the system. The initiation step is photochemically-activated and is very rapid. The propagation step is slower, and requires time and/or thermal activation to proceed to completion. Since there are few chain termination reactions which affect cationic systems, cure can proceed for extended periods after UV exposure, as long as there are still reactive species present. 

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... 

While water is typically considered to be harmful to cationically curing systems, this is not always the case. Cationic dionato (from acetylacetone, acetylcyclohexanone, etc.) complexes of Si, B, Ge, and P have been prepared and studied as hydrolytically activated Bronsted acid precursors (31). ... 

UV-curing adhesives are now well established. There are two kinds of UV-curing systems radical curing systems, most of which are based on acrylates and cationic curing systems, which are mostly based on epoxides. 

The cationic curing systems can impart requisite barrier and electrical properties unlike the free radical initiated polymerisation of acrylates. In the cationic photopolymerisation, the process involves the ring opening of oxiranes and/or oxetanes and is initiated by strong protonic acids. The use of photosensitisers like thioxanthone and anthracene derivatives and photoinitiators further enhances the polymerisation process. 

However, the development of cationic curing systems has been constrained by the limited availability of raw materials, which can be used for the process. 

Better flexibility can be obtained in cationic curing systems because of the polyol component of the cationic formulation. 

The reaction rate at the initial temperature must be vanishingly small but rapid at the front temperature. The front temperature is determined by the enthalpy of the reaction, heat capacity of the product and the amount of heat loss. Free-radical polymerization is ideal because for most peroxide and nitrile initiators the rate of polymerization at ambient temperature is low but high at elevated temperatures. Amine-cured epoxies suffer from the problem of short pot life but cationic cured systems are very similar to free-radical systems (10). 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

Cationic systems are generally slower than free radical curing systems. 

Epoxy acrylates are also commonly used as oligomers in radiation-curing coatings and adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation. These resins are formulated with photoinitiators to cure via uv or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. The uv/EB cured epoxy formulations are discussed in Chap. 14. 

Photoinitiators are perhaps the most important component in uv cured radiation coatings. The photoinitiator is an ingredient that absorbs light and is responsible for the production of free radicals in a free radical polymerized system or cations in a cationic photoinitiated system. The photoinitiators are usually added to the reactive coating formulations in concentration ranges from less than 1 to 20 percent by weight based on the total formulation. The absorption bands of the photoinitiators should overlap the emission spectra of the various commercial light sources. 

Cationic Cure Epoxies. As with free radical uv chemistry, the same principles of formulation component selection apply to cationic cured uv epoxy adhesives. Although different monomers and oligomers are normally required for this type of chemistry, the main difference lies with the photoinitiator system. 

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