指導教授:王振乾
學生:符昌中
教授
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; 2trifluoroethyl 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.
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.
• Odor restricts use of mercaptans to applications
such as rubber compounding where odors are
commonly encountered.
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.
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 Amide
is theblocking
least groups
reactive
of this
amide deblocking
series, with
succimide and acetanilide
being
in order
of decreasing
temperature-caprolactam,
methyl
acetanilide.temperatures 20-30 ℃ below MEKO. The authors
much more acetamide,
reactive,succinimide,
with deblocking
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).
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 crosslinker for hydroxyl groups in epoxy adhesives.
Imidazole urea.
Pyrazoles and 1,2,4-triazoles
times of pyrazole blocked HDI derivatives with polyamines decrease with alkyl
• Gelation
Pyrazoles
and 1,2,4-triazoles have low deblocking
substitution on the pyrazole ring; pyrazole<3-methylpyrazole <3,5-dimethylpyrazole,
and
the reactions are inhibited not catalyzed by DABCO.
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.
3,5-Dimethylpyrazole and 1,2,4-triazole
Hydrogen bonding with urea NH
in pyrazole blocked isocyanate.
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 Nmethylaniline, diphenylamine, and N-phenylnapthalene
blocked TDI increases in the order given and similarly the
cure rate in crosslinking hydroxyfunctional polybutadiene
increases in that order.
Decomposition routes of primary ureas.
Secondary amine
blocking groups.
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.
Formation of malonate blocked isocyanates.
Reaction of malonate blocked isocyanate with hydroxyl
functional substrate.
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