UV-cure of epoxy resins

Abadie et al. (2002) showed that the following auto-catalytic kinetic model was useful for modelling the UV cure of epoxy-resin systems ... 

Figure 13. UV picture of epoxy resin cured with TETA hardener at 50 °C. Dark streaks are DNFB staining of unreacted amine groups.

Numerous published reports describe how polyol or alcohol additives Improve elongation. Impact resistance, and general toughness of brittle. Inflexible UV-cured cycloaliphatic epoxy resins (16,17),... 

Ultraviolet curing of epoxy resins has become important. Cycloaliphatic epoxy resins are combined with substances that induce polymerization under the influence of UV radiation or an electron beam. Industrially important products include tri-arylsulfonium salts (UVE 1014, 1016, General Electric) and arene-ferrocenium compounds (CG 24-061, Ciba-Geigy). To ensure good film flexibility and chemical resistance, polyols should be added. The films should be postcured for 30-120 s at ca. 100"C [2.126]. 

The development since the 1960s of extremely rapid methods of curing these coatings with ultraviolet and electron beam radiation has led to an extension of the chemistry to compositions based on resins and monomers containing acrylic unsaturation, and to the cationic UV curing of epoxy compositions. All of these coatings will also be discussed in this chapter. 

Besides the potential irritants, such as cleansing agents and inks, the classic allergens, such as rubber chemicals, paraphenylenediamine and its derivatives, and preservatives (Adams 1983), new allergens have added to the spectrum of allergens in silk-screen printers. They are the acrylate components of the UV-curing compounds, epoxy resin, diaminodiphenylmethane, and triglycidyl isocyanurate. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

The second patent [94] does not give further information about the synthesis but describes an application of these products — UV curing of coatings for optical fibers. Telechelics diols are used as coreagents for cationic polymerization of epoxy resins. Such coatings are composed of ... 

The formation of networks by addition polymerization of multifunctional monomers as minor components included with the monofunctional vinyl or acrylic monomer is industrially important in applications as diverse as dental composites and UV-cured metal coatings. The chemorheology of these systems is therefore of industrial importance and the differences between these and the step-growth networks such as amine-cured epoxy resins (Section 1.2.2) need to be understood. One of the major differences recognized has been that addition polymerization results in the formation of microgel at very low extents of conversion (<10%) compared with stepwise polymerization of epoxy resins, for which the gel point occurs at a high extent of conversion (e.g. 60%) that is consistent with the... 

Recently [Sangermano et al., 2008] reported for the first time the paeparation of antistatic epoxy coatings via cationic UV curing of an epwxy resin in the presence of a very low content of carbon nanotubes (CNT). After dispersing the CNT into the epexy resin, in the range between 0.025-0.1 wt.-%, the formulations were cured by means of UV light in the presence of a sulfonium salt as cationic photoinitiator. 

The Photo-Differential Scanning Calorimetry (photo-DSC) [10] was used to study the cure kinetics of UV-initiated photo-polymerization of epoxy resin monomers and vinyl ether in presence of cationic photo-initiator, mainly Cyracure UVI-6976, former Cyracure UVI-6974, which consists of a mixture of dihexafluoroantimonate of S, S, S, S -tetraphenylthiobis (4,1-phenyllene) disulfonium and hexafluoroantimonate of diphenyl (4-phenylthiophenyl) sulfonium (CAS no. 89452-32-9 and 71449-78-0) at 50 wt. % in propylene carbonate - Figure 44 [11]. 

The viscoelastic and thermal properties of fully and partially cured DGEBA epoxy resin composites were smdied modified with montmorillonite nanoclay exposed to UV radiation. Samples were fabricated and cured to 80 % conversion (partially cured) based on isothermal cure kinetic smdies. Influence of 1-3 wt% loading of montmorillonite nanoclay on the cure behavior and development of physical properties of these composites were evaluated. Results of the smdy revealed that for optimization of modified epoxy composite properties, a different curing cycle was necessary due to interaction of different amounts of nanoclay and epoxy molecules. Addition of nanoclay increased the viscoelastic properties, storage modulus and activation energy of decomposition of partially cured samples evolved over exposure time, while fuUy cured samples degraded over the same period. 

The most commonly used materials for permanently filhng through holes include singlecure (thermal) resins, photoimageable dielectrics, conductive pastes, and the dual-cure (UV -i-thermal) epoxy resin utilized in the Noda screen flat plug process. Following are brief descriptions and comparisons of the characteristics of each of these types of via-fill materials. 

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