指導教授:王振乾 教授 學生:符昌中

Blocking groups The structure of blocking groups has a major effect on deblocking temperatures and cure rates of coatings. There are other important aspects in addition to reactivity involved in the choice of blocking groups, these generally are related to a particular application as will be discussed in the sections on uses of blocked isocyanates in the subsequent second paper.

Phenols, pyridinols, thiophenols and mercaptopyridines Phenols react more slowly with isocyanates than alcohols, however phenol blocked isocyanates deblock at lower temperatures than aliphatic urethanes, in line with the slower rate of the reverse reaction. The effect of single methyl groups is small but o-cresol gives more rapid deblocking than p-cresol, which has been attributed to a steric effect. 2,6-dimethylphenol deblocks at significantly higher temperatures, it is suggested that the electronic effect of two methyl groups overshadows the steric effect. Para and ortho substitution of phenol.

Phenols, pyridinols, thiophenols and mercaptopyridines Aside from having an aromatic leaving group the presence of the amine group presumably further reduces the deblocking temperature. 2-Pyridinol and possible hydrogen bonding in blocked isocyanate. The gel time for thiophenol blocked isocyanates with polyamines is shorter than for phenol blocked isocyanates. Thiophenol and 2-mercaptopyridine.

Alcohols, other hydroxy-functional agents, and mercaptans Many alcohols have been used as blocking agents, generally they give high deblocking temperatures. Another exception is trihaloethyl alcohols; 2-trifluoroethyl and 2-trichloroethyl alcohol blocked phenyl isocyanate are reported to have deblocking rates almost two orders of magnitude greater than n-butyl alcohol blocked phenyl isocyanate. Alcohols for blocking.

Alcohols, other hydroxy-functional agents, and mercaptans Hexyl mercaptan blocked TDI is reported to deblock more rapidly than MEKO blocked TDI. Odor restricts use of mercaptans to applications such as rubber compounding where odors are commonly encountered. Decomposition of t-butanol blocked isocyanate. Tertiary alcohol (e.g. t-butyl) urethanes are relatively unstable and may thermally decompose to give alkenes, carbon dioxide, and amines. N,N-dibutylglycolamide N,N-dibutylglycolamide have been patented as a blocking agent permitting lower temperature cure in E-coats as compared with 2-ethylhexyl alcohol.

Oximes Oximes have been widely used due to their low deblocking temperatures compared to alcohols, phenols, and caprolactam. Formation of ketoximes from hydroxyl amine and ketones.

Oximes Among the advantages of the oxime groups is their high reactivity towards isocyanates, which allows the blocked products to be readily made without catalyst. Blocking of an isocyanate with MEKO.

Tetrmethylcyclobutanedione monooxime. Oximes Tetrmethylcyclobutanedione monooxime. Tetramethylcyclobutanedione monooxime has a low deblocking rate, even lower than caprolactam, this result is attributed to the electron-withdrawing nature of the cyclobutyl carbonyl group.

Amides, cyclic amides, and imides Acetanilide blocked HDI isocyanurate has been reported to have a deblocking temperature of 100 ℃ as compared with the corresponding MEKO blocked isocyanate deblocking temperature of 130 ℃. Deblocking is promoted by having the carbonyl oxygen in a position to form an intermediate six-membered ring with the H on the N from the isocyanate. N-Methyl acetamide and formation of the intramolecular hydrogen bond after blocking of an isocyanate.

Amides, cyclic amides, and imides Caprolactam is the least reactive of this amide series, with succimide and acetanilide being much more reactive, with deblocking temperatures 20-30 ℃ below MEKO. The authors attributed this order of reactivity to changes in polarization of the N-H bond. This polarization leads to the succinimide and acetanilide groups reducing the rate of recombination(k-1). Amide blocking groups in order of decreasing deblocking temperature-caprolactam, methyl acetamide, succinimide, acetanilide.

Imidazoles, amidines, and related compounds Imidazole blocked 1,5-naphthalene diisocyanate has been patented for use as a blocked catalyst for epoxy-dicyanamide coatings and adhesives as has 2-methylimidazole. Blocking IPDI isocyanurate with a combination of 2-phenylimidazoline and acetophenone oxime provides for release of both a catalyst and a cross-linker for hydroxyl groups in epoxy adhesives. Imidazole urea.

Pyrazoles and 1,2,4-triazoles Pyrazoles and 1,2,4-triazoles have low deblocking temperatures. Deblocking is promoted by having an amino N in a position to form an intermediate five-membered ring with the H on the N from the isocyanate. Gelation times of pyrazole blocked HDI derivatives with polyamines decrease with alkyl substitution on the pyrazole ring; pyrazole<3-methylpyrazole <3,5-dimethylpyrazole, and the reactions are inhibited not catalyzed by DABCO. Hydrogen bonding with urea NH in pyrazole blocked isocyanate. 3,5-Dimethylpyrazole and 1,2,4-triazole

Amines The reverse reaction is so rapid with primary amines that they are not useful as blocking groups; they also have the distinct disadvantage that the urea bond can cleave on either side of the carbonyl. Secondary amines can be used. The thermal stability of N-methylaniline, diphenylamine, and N-phenylnapthalene blocked TDI increases in the order given and similarly the cure rate in crosslinking hydroxyfunctional polybutadiene increases in that order. Secondary amine blocking groups. Decomposition routes of primary ureas.

Active methylene compounds Several active methylene compounds have been used as blocking agents; the reaction pathway differs from other blocked isocyanates since the dominant reaction with hydroxyl groups is to form esters rather than urethanes. Reaction of malonate blocked isocyanate with hydroxyl functional substrate. Formation of malonate blocked isocyanates. There is a marked advantage that coatings can be made having good package stability combined with low temperature cure by using a monofunctional alcohol as part of the solvent.

Other blocking agents Benzylmethacrylohydroxamate blocked MDI is reported to be desirable for use with maleic anhydride propanediol polyester plastics since the released blocking agent cannot only react with hydroxyl groups but can also copolymerize through the acrylic double bond . As a result, there is no release of volatile blocking agent. Benzylmethacrylohydroxamate.

Uretdiones, carbodiimides, and uretonimines Self-condensation products of isocyanate monomers, such as uretdiones, are attractive because they do not generate volatile blocking groups. Decomposition of TDI dimer into monomer. Decomposition of uretdione group in IPDI dimer urethane.

Uretdiones, carbodiimides, and uretonimines In the presence of methylphosphine oxide, isocyanates self-condense to yield carbodiimides and CO2. Reaction of a blocked carbodiimide with carboxylic acid, with the further breakdown of the expected N-acylurea (bold) into an amide and isocyanate.

Encapsulated particles These materials have been surface reacted so that they are insoluble in the rest of the vehicle at storage temperatures but dissolve in the coating during heating, releasing free isocyanate that reacts with a hydroxyfunctional polymer. Isocyanate encapsulation and breakage.

Comparisons of the different blocking agents Crosslinking temperatures of derivatives