Isocyanates oxime blocked

Steric crowding on either oxime or isocyanate enhances the dissociation rates. Isocyanates blocked with diisopropyl and diisohutyl ketoximes dissociate faster than the same isocyanates blocked with MEKO (15). Oxime blocked TMXDI dissociates faster than other less sterically crowded aliphatic isocyanates and with comparable rates to aromatic isocyanates. Blocked TMI cross-linkers can be synthesized by first copol5merizing the TMI with acrylate esters, and then reacting the polyfimctional acrylate with an oxime such as MEKO. Alternatively, one can react TMI with the oxime and then copolymerize the blocked monomer with acrylate esters (16). 

Tris(alkoxycarbonylamino)triazine (TACT), a mixed methyl and butyl carbamate derivative of melamine, acts as an alcohol blocked isocyanate (21). TACT is much more reactive than conventional alcohol blocked isocyanates, since it is activated by the triazine ring. It cross-links hydroxy-functional acrylics at 125°C in 30 min, comparable to (or lower than) oxime blocked isocyanates. The reaction is not catalyzed by tin compounds it is catalyzed to a small extent by dodecylbenzene sulfonic acid. 

The hydroxy-functional polyurethane dispersions can be cross-linked with MF resins or blocked isocyanates. Blocked isocyanates can also be used with water-reducible anionic acrylic or polyester resins instead of an MF resin oxime blocked isocyanates are commonly used, owing to their high reactivity. Oxime blocked aliphatic isocyanates are more stable to hydrolysis than oxime blocked aromatic isocyanates (32). As mentioned earlier alcohol blocked isocyanates are used in cationic electrodeposition coatings. 

One-part polyurethane adhesives can also correspond to prepolymers with isocyanate functions blocked reversibly (due to reaction with oximes, phenols, etc.). The isocyanate groups are then regenerated, either by heating to a more or less high temperature, or by substitution of the blocking agent by a diol (oligomer, at least partly) (O Fig. 14.18). 

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

Masked or blocked diisocyanates are used in coatings applications. The blocked diisocyanates are storage-stable, nonvolatile, and easy to use in powder coatings. Blocked isocyanates are produced by reaction of the diisocyanate with blocking agents such as caprolactam, 3,5-dimethylpyrazole, phenols, oximes, acetoacetates, or malonates. 

Blocking. A 50% solution (in the above solvent mixture) of a slight equivalent excess of 2-butanone oxime and 0.2 wt % dibutyltin dilaurate (T-12) were added to the prepolymer solutions in a three-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet. The reaction was carried out under nitrogen at 80 C until the isocyanate content reached zero (complete blocking). 

The authors recognized the grafting reaction between the isocyanate and the pendant hydroxyls on the polyacrylate, and blocked the isocyanate with 2-butanone oxime. However, all of these materials apparently formed... 

Blocked isocyanates permit making coatings that are stable at ambient temperature when baked, the monofunctional blocking agent is volatilized and the coreactant is cross-linked. An extensive review of blocked isocyanates, their reactions, and uses is available (127). The blocking agents most widely used are phenols, oximes, alcohols, e-caprolactam (hexahydro-2ff-azepin-2-one) [105-60-2], 3,5-dimethylpyrazole, 1,2,4-triazole, and diethyl malonate (propanedioic acid diethyl ester) [105-53-3]. A variety of catalysts are used DBTDL is most widely used but many other catalysts have also been used. Bismuth tris(2-ethyl hexanoate) has been particularly recommended (128). In electrodeposition primers, DBTDL has insufficient hydrolytic stability, and tributyltin oxide is an example of an alternate catalyst (129). Cyclic amidines, such as l,5-diazabicyclo[4.3.0]non-5-ene, are reported to be superior catalysts for use with uretdione cross-linkers in powder coatings (130). 

Methyl ethyl ketone oxime (MEKO) (2-butanone oxime) [96-29-7] blocked isocyanates are more reactive than phenols. One package coatings formulated with hydroxy-functional resins cure in 30 min at 130°C. In automotive clear coats, MEKO leads to yellowing. Use of a combination of 3,5-dimethylpyrazole and 1,2,4-triazole has been recommended for clear coats that are non-yellowing... 

Isocyanates form addition products reversibly with compounds which contain moderately reactive hydrogen, such as phenol, oximes, lactams and malonates. Fig. 14 is an example of a typical blocking-unblocking reaction. These addition compounds, called blocked isocyanates, are stable to water. They have been used in place of free isocyanate. The phenol-blocked isocyanates are the most common. In 1957 the DuPont Co. developed an aqueous dip based on blocked isocyanate. This process, called D-417, used phenol-blocked methylene bis(4-phenyl isocyanate) and a water soluble epoxide, the reaction product of glycerine and epichlorohydrin. RFL is used as the topcoat or second step dip ... 

Both aliphatic and aromatic isocyanates can be blocked by a variety of blocking agents. These include alcohols, phenols, oximes, lactams, j8-dicarbonyl compounds, bisulfite addition compounds, hydroxylamines and esters of p-hydroxybenzoic acid and salicylic acid. Perhaps the most widely used blocking agents at present are phenol, branched alcohols, 2-butanone oxime (methyl ethyl ketoxime) and 8-caprolactam. The use of blocked isocyanates in PU coatings has been comprehensively reviewed. ... 

Witzeman (37) studied the effect of resin and crosshnker types on the cure behavior of polyester resins cured with blocked isocyanates. It was found that cure temperatures as low as 129°C to 140°C could be achieved with blocked isocyanates based on the oximes of dusobutyl ketone and diisopropyl ketone. The polyester resin utilized contained stericaUy unencumbered hydroxyl groups to allow unique flow properties at the low curing temperatures. 

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