Base resins, cation radicals

Adhesive systems based on free-radical curing epoxymethacrylates are cited (16). And Dudgeon (17) has covered cationic, heat-curable rubber-modified epoxy resin systems which very likely have latent-cure, structural adhesive capability. 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

Of the commercially available EB-curable adhesives [9-12], the resins fall within one of two categories based on their curing mechanisms. The majority of EB-curable resins are based on (meth)acrylate-functionalized oligomers involving a free-radical curing mechanism. The second category is the epoxy resins that cure by a cationic mechanism. 

Various bifunctional resins are based on acrylic epoxide monomers. Such systems can photopolymerize by the radical and/or cationic mechanism. With iron arene photoinitiators in the presence of an oxidant, radical as well as cationic photopolymerization of these monomers is possible . Onium -type photoinitiators form radical species upon photolysis, as shown in Figs. 3 and 4. The local radical concentration is, however, too low to permit the polymerization of such systems... 

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. 

Ion exchange reactions are carried out using insoluble polymer resins, activated with various functional groups. The polymerized material is usually based on a mixture of styrene and vinyl benzene (Figure 12.2). The active radical of cation exchangers is generally sulfonic acid (-SO3H),... 

The primary resins used in this market are the radiation-curable epoxy acrylates, accounting for 60% of the resins used. A small amoimt of cycloaliphatic epoxies are also used in UV-curable inks and resists. Phenol and cresol epoxy no-volacs, and bisphenol A based epoxies are used in thermally cured formulations. The epoxy novolacs are used where higher heat resistance is needed such as in solder masks. Both free-radical and cationic-curable UV inks and colored base coats have grown rapidly because of the needs for higher line speeds, faster cleanup or line turnaround, less energy consumption, less capital for a new hne, and fewer emissions. 

Electrons accelerated to 3-10 MeV are traveling at about 99% the speed of light. Approximately 50% of the beam energy is lost in hard collisions that remove electrons from host atoms and thereby produce ionized species. As a result of hard collisions, polymerization can occur by free-radical or ionic mechanisms. Materials that proceed via free-radical polymerization include acrylic/methacrylic systems, maleic and fumaric polyester resins, maleimides, and thiole-ene systems. Of these systems, resins based on acrylic and methacrylic ftinctionalities have been studied the most (116). The other major material system to be studied is that of epoxy polymerized via a cationic mechanism, which requires a diaryliodonium or triaryliodonium salt catalyst. 

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