Free radical curing

Formulation Design for Free-Radical Cured Systems... 

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. 

Coating materials may be based on short or medium-oil alkyds (e.g. primers for door and window frames) nitrocellulose or thermoplastic acrylics (e.g. lacquers for paper or furniture finishes) amino resin-alkyd coatings, with or without nitrocellulose inclusions, but with a strong acid catalyst to promote low temperature cure (furniture finishes) two-pack polyurethanes (furniture, flat boards) unsaturated polyester resins in styrene with free-radical cure initiated by peroxides (furniture) or unsaturated acrylic oligomers and monomers cured by u.v. radiation or electron beams (coatings for record sleeves paperback covers, knock-down furniture or flush interior doors). 

Organosiloxane Containing Networks via Free-Radical Curing Process... 

Free-radical cure mechanism, silicone network preparation via, 22 568-569 Free-radical cyclization process, 27 147 Free-radical-initiated polymerization, 20 211... 

Alternate Crosslinking Modes. In addition to the crosslinking modes previously described, (co)polymers containing 1 and 2 may be cured by other means. For example, under appropriate acidic conditions with limited availability of active hydrogen species cyclic hemiamidals 2 will lose ROH to form the enamide 9 (Scheme 5). This has been demonstrated on model systems, e.g., 2 where vinyl is replaced by methyl ). The product, N-acetylpyrroline, has in turn been converted to nonvolatile products (oligomers) under free radical catalysis. These systems may thus be considered for application in the UV/EB or catalyzed free radical cure field. 

Alternatively, 1 or 2 could be Incorporated Into a polymer not by radical copolymerization but rather by reaction with, e.g. diol functionality. The pendant acrylamide portion of the structure would then provide a site for light or Initiator catalyzed free radical cure to give a hydrolysis resistant crosslink. These are areas of ongoing research and the results will be reported In detail at a later date. 

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... 

Cationic systems are generally slower than free radical curing systems. 

Large-mass parts Conventional coating of large-mass parts requires exceedingly long times in convection ovens. A considerable reduction of times required for cure is to melt the powder with IR and use fast UV-initiated free radical curing reactions. 

The free-radical cure mechanism of the vinyl ester resins is well understood. In most respects, it is similar to that of the unsaturated polyester resins. To initiate the curing process, it is necessary to generate free radicals within Ike resm mass. Organic peroxides are tlie most common source of free radicals. These peroxides will decompose under the influence of elevated temperatures or chemical promoters, e.g., organometallics or tertiary amines, to form free radicals. Generation of free radicals also can be effected by ultraviolet or high-energy radiation applied directly to the resin system. The free radicals thus formed react to open the double bond... 

Vinyl Free radical cure systems crosslinked polyethylene, peroxide cured elastomers, polyesters. Polyethylene. Polypropylene. 

Chrome complexes have been developed as adhesion promoters by the reaction of chromium chloride with methacrylic acid. The chromium oxide portion of the adhesion promoter reacts with a substrate while the methacrylic portion reacts with a free radical curing outer layer. Chrome-based adhesion promoters are commonly used as a primer for aluminum foil to increase the strength and durability of aluminum/polyethylene interfaces.26... 

Free Radical Cure UV Adhesives. As with any adhesive, formulation variables are critical to the processing and performance characteristics. Variables such as oligomer selection, modifiers and additives, monomer structure, molecular weight, and glass transition temperature directly affect application and performance properties. 

The cationic photoinitiators are sensitive to moisture, and the acid species formed can promote corrosion. Frequently the formulation for these curing methods contains solvent, which evaporates during curing, so that an important environmental advantage is lost. For these reasons cationic uv cure is usually preferred over free radical cure only when the higher-performance properties are justifiable. 

Shirane, K., Radiation-Induced Free Radical Cure of Epoxy Resin, Journal of Polymer Science, vol. 17, 1979, p. 139. 

Several different methods have been developed to cure silicones. Free-radical cures are possible for those polymers containing vinyl groups. The largest-volume process, however, involves room-temperature vulcanizations, which can employ either a one-component or a two-component system. In the one-component system, a cross-linking agent such as methyltriacetoxysilane is used. 

The predominant chemistry for formulating UV curable inks is acrylate-based free radical curing. Most UV inkjet formulations are mixtures of several different acrylates chosen for their individual properties which, when combined with photoinitiators, achieve the required specification. Because of the extremely low viscosity of the... 

UV technology utilizes the same building blocks in all the formulations for various applications. These building blocks are known by several different names — for example monomers, prepolymers, and oligomers. For free radical curing, they are generally based on acrylate chemistry and these are blended together to achieve the final specification required by the end use application. One of the... 

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. ... 

The cationic process is not inhibited by oxygen. Curing can be initiated in air under natural sunlight or low-intensity mercury arcs whose useful lifetime is several times that of the high-intensity arcs used in free-radical curing processes. 

Catalysis of cationic polymerization requires formation of Bronsted acid, while the product of interest in connection with free radical cures is an aryl radical. Early results [69] suggested that the photogeneration of acid, followed by an indicator technique, might be more efficient than formation of the radical products, estimated as Phi and Ph2S, for photolysis of Ph2I+ and... 

The free radical cure of acrylate resins is expected to be far more mechanistically complex than the cationic case described above since the free radical initiator can also function as a chain terminator in the system. It can also recombine to give the original activator structure. Furthermore, each of these reactions will give different size exotherms per reaction unit. For example, the formation of a single carbon-carbon bond from radical recombination gives an... 

Radiation curable polymer systems are based on the same chemical structural design as the conventional polymer systems, but certain modifications are made in order to accommodate reactive unsaturation sites necessary for a radiation-induced free radical curing mechanism. Examples of these modifications of conventional polymer structures to form radiation curable polymers are as follows ... 

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