Note: Descriptions are shown in the official language in which they were submitted.
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Cyclic ketones as blocking agents
The present invention relates to new blocked polyisocyanates, to a method of
producing them and to the use thereof in single-component polyurethane
systems.
Blocking of polyisocyanates to effect temporary protection of the isocyanate
groups
thereof is a procedure which has long been known, and is described, for
example, in
Houben Weyl, Methoden der organischen Chemie XIV/2, pages 61-70. Hardenable
compositions which contain blocked polyisocyanates are used in polyurethane
lacquers, for example.
Single-component (1C) polyurethane systems are widely used in the field of
industrial stoving lacquers such as mass-production automobile coating and
coil
coating, and are distinguished by their very good film properties, such as
resistance
to chemicals, scratch-resistance and resistance to weathering. These lacquer
films are
hardened by thermal activation (by a stoving operation) of the blocked poly
isocyanates with polyols, optionally in the presence of a suitable catalyst. A
review
of blocking agents which are suitable in principle here is given by Wicks et
al. in
Progress in Organic Coatings 1975, 3, pages 73-79, 1981, 9, pages 3-28 and
1999,
36, pages 148-172, for example.
For use in the field of automobile coating, the blocked polyisocyanates must
be
crosslinkable at maximum stoving temperatures of 140°C, and must only
exhibit
very slight yellowing, and preferably no yellowing, during the stoving
operation. The
stoving temperature is mainly controlled via the reactivity of the blocked
poly-
isocyanate.
Most stoving systems, such as melamine-formaldehyde and urea-formaldehyde
resins, for example, are characterised by the release of volatile constituents
during
hardening, which increase the VOC value.
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Moreover, a certain proportion of the blocking agent remains in the lacquer
film
which is formed and has a disadvantageous effect on the properties thereof.
Due to
the blocking agent which remains, properties such as the scratch-resistance
and acid-
resistance of single-component lacquer films are not comparable with those of
what
are termed two-component (2C) polyurethane lacquer coatings (e.g. T. Engbert,
E.
Konig, E. Jiirgens, Farbe&Lack, Curt R. Vincentz Verlag, Hannover 10/1995).
Furthermore, separation of the blocking agent and the escape thereof in
gaseous form
from the lacquer film can also lead to bubble formation in the lacquer film.
Subsequent incineration of the emitted blocking agent can sometimes be
necessary.
Isocyanates blocked with diethyl malonate have mainly been used recently for
particularly low stoving temperatures within the range from 90 to 120°C
(e.g. EP-A
0947531). In contrast to blocking procedures which employ heterocyclic N
compounds, such as caprolactam or butanone oxime, for example, the blocking
agent
as a whole is not split off or separated here; rather, this blocking agent
results in a
transesterification reaction on the isocyanate which is blocked with diethyl
malonate.
Ethanol is separated during this transesterification. This method can be
employed at
relatively low stoving temperatures, since the second, adjacent ester function
is an
activated ester. The disadvantage of this method is that systems such as these
are
extremely susceptible to the effect of acids, because the labile ester bond
can be
rapidly cleaved. The possibilities for the use of these products are thereby
restricted.
The object of the present invention was to provide new blocked polyisocyanate
systems which react without separation of the blocking agent, i.e. free from
emissions, and which exhibit low crosslinking temperatures. The object was
also that
these blocked polyisocyanate systems should be stable on storage at ambient
temperature, and that they should be suitable, particularly in combination
with
suitable polyol components, for the production of single-component stoving
lacquers.
Surprisingly, it has now been found that acidic CH compounds which possess the
basic structure of an activated cyclic ketone, particularly that of
cyclopentanone-2-
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carboxymethyl ester, are particularly suitable for blocking polyisocyanates in
order to
obtain emission-free coatings with a reduced tendency to exhibit yellowing.
The present invention relates to organic polyisocyanates comprising at least
two
isocyanate groups, the isocyanate groups of which are blocked with acidic CH
cyclic
ketones of general formula (I),
O
X
H
(I)
R2 R'
wherein
X is an electron-attracting group,
R' and RZ, independently of each other, represent the radicals H, a C,-C2o
(cyclo)alkyl, a C6-C24 aryl, a C1-GZO (cyclo)alkyl ester or amide, a
C6-C24 aryl ester or amide, or mixed aliphatic/aromatic radicals
1 S comprising 1 to 24 carbon atoms, which can also form part of a 4 to 8-
membered ring,
n is an integer from 0 to 5,
and which have a content of blocked isocyanate groups (calculated as NCO) of
20 to
0 % by weight in total.
A content of blocked isocyanate groups (calculated as NCO) ranging from lS.Sb
to
0% by weight is preferred. A content of blocked isocyanate groups (calculated
as
NCO) ranging from 14 to 0 % by weight is particularly preferred. Partial
blocking of
the polyisocyanate can optionally be effected; the non-blocked isocyanate
groups can
then be used for further reactions. Typically, all the isocyanate groups are
blocked.
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The electron-attracting group X can comprise any substituent which results in
the a-
terminal hydrogen exhibiting an acidic CH character. This substituent can
comprise
ester groups, sulphoxide groups, sulphone groups, nitro groups, phosphonate
groups,
nitrite groups, isonitrile groups, polyhatogen alkyl groups, fluorine,
chlorine or
carbonyl groups. Nitrite and ester groups are preferred, whilst methyl
carboxylate
and ethyl carboxylate groups are particularly preferred.
Compounds of general formula (I), the ring of which optionally contains hetero
atoms such as oxygen, sulphur or nitrogen atoms, are also suitable.
The activated cyclic ketone of formula (I) preferably has a ring size of 5 (n
= 1) or 6
(n = 2).
Preferred compounds of general formula (I) include cyclopentanone-2-
carboxymethyl ester and -carboxyethyl ester, cyclopentanone-2-carboxylic acid
nitrites, cyclohexanone-2-carboxymethyl ester and -carboxyethyl ester, or
cyclopentanone-2-carbonylmethyl. Cyclopentanone-2-carboxymethyt ester and -
carboxyethyl ester, as welt as cyclohexanone-2-carboxymethyl ester and -
carboxyethyl ester, are particularly preferred. The cyclopentanone systems can
readily be obtained industrially by the Dieckmann condensation of dimethyl
adipate
or of diethyl adipate. Cyctohexanone-2-carboxymethyl ester can by be obtained
by
the hydrogenation of methyl salicylate.
The polyisocyanate to be blocked can be any organic polyisocyanate which is
suitable for the crosstinking of compounds comprising active hydrogen, i.e.
aliphatic
polyisocyanates, including cycloaliphatic polyisocyanates, as well as aromatic
and
heterocyclic polyisocyanates comprising at least two isocyanate groups and
mixtures
thereof. Typical examples of potyisocyanates include aliphatic isocyanates
such as
di- or triisocyanates, e.g. butane diisocyanate (BDI), pentane diisocyanate,
hexane
diisocyanate (HDI), 4-isocyanatomethyl-1,8-octane diisocyanate (triisocyanato-
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nonane, TII~, or cyclic systems such as 4,4'-methylene-bis(cyclohexyl
isocyanate)
(Desmodur W , Bayer AG, Leverkusen), 3,5,5-trimethyl-1-isocyanato-3-
isocyanatomethyl-cyclohexane (IPDI), as well as w,w'-diisocyanato-1,3-
dimethylcyclohexane (HMI). Examples of aromatic polyisocyanates include 1,5-
S naphthalene diisocyanate, diisocyanato-diphenylmethane (MDI) or crude MDI,
diisocyanatomethylbenzene (TDI), particularly the 2,4- and the 2,6-isomers
thereof
and industrial mixtures of the two isomers thereof, as well as 1,3-
bis(isocyanato-
methyl)benzene (XDI). Polyisocyanates which are also very suitable are those
which
can be obtained by the reaction of di- or triisocyanates with themselves via
their
isocyanate groups, such as uretdiones or carbodiimide compounds, or such as
isocyanurates or iminooxadiazinediones which are formed by the reaction of
three
isocyanate groups. The polyisocyanates can also contain monomeric di- and/or
triisocyanates and/or oligomeric polyisocyanates comprising biuret,
allophanate and
acylurea structural elements, triisocyanates which have a low monomer content
or
partially modified monomeric di- or triisocyanates, as well as any mixtures of
the
aforementioned polyisocyanates. Polyisocyanate prepolymers which on average
contain more than one isocyanate group per molecule are also very suitable.
These
are obtained by the preliminary reaction of a molar excess of one of the
aforementioned polyisocyanates, for example, with an organic material which
contains at least two active hydrogen atoms per molecule, e.g. in the form of
hydroxy
groups.
The preferred polyisocyanates are those which contain a uretdione,
isocyanurate,
iminooxadiazinedione, acylurea, biuret or allophanate structure, e.g. those
which are
based on butane diisocyanate (BDI), pentane diisocyanate, hexane diisocyanate
(HDI), 4-isocyanatomethyl-1,8-octane diisocyanate (triisocyanatononane, TIN)
or on
cyclic systems such as 4,4'-methylene-bis(cyclohexyl isocyanate) (Desmodur
W~',
Bayer AG, Leverkusen), 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclo-
hexane (IPDI), as well as w,w'-diisocyanato-1,3-dimethylcyclohexane (H6XDI).
Examples of aromatic polyisocyanates include 1,5-naphthalene diisocyanate,
diisocyanato-diphenylmethane (MDI) or crude MDI, diisocyanatomethylbenzene
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(TDI), particularly the 2,4- and 2,6-isomers thereof and industrial mixtures
of both
isomers thereof, as well as 1,3-bis(isocyanato-methyl)benzene (XDI).
Polyisocyanates which are particularly preferred are those based on hexane
diiso-
cyanate (HDI), on 4,4'-methylene-bis(cyclohexyl isocyanate) or on 3,5,5-
trimethyl-1-
isocyanato-3-isocyanatomethylcyclohexane (IPDI).
The present invention further relates to a method of producing the blocked
organic
polyisocyanates according to the invention, characterised in that
polyisocyanates are
reacted with acidic CH cyclic ketones of general formula (I),
O
X
)n H
(I)
R2 F2~
wherein
X is an electron-attracting group,
RI and RZ, independently of each other, represent the radicals H, a C1-C2o
(cyclo)alkyl, C6-C24 aryl, a CI-C2o (cyclo)alkyl ester or amide, a
C6-C24 aryl ester or amide, or mixed aliphatic/aromatic radicals
comprising 1 to 24 carbon atoms, which can also form part of a 4 to $-
membered ring, and
n is an integer from 0 to 5,
in the presence of a catalyst, wherein 0.8 to 1.2 mol of the cyclic ketone of
formula
(I) are used per isocyanate group equivalent of the polyisocyanate to be
blocked.
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One isocyanate group equivalent of the polyisocyanate to be blocked is
preferably
reacted with 1 equivalent of the blocking agent.
Suitable catalysts include alkali metal and alkaline earth metal bases, such
as
powdered sodium carbonate (soda). Depending on the cyclic ketone used,
trisodium
phosphate or Dabco (1,4-diazabicyclo[2.2.2]octane) can also be used.
Carbonates of
metals of subgroup II are also suitable. Sodium carbonate or potassium
carbonate is
preferably used. Alternatively, the reaction of the cyclic ketone with the
isocyanate
can also be conducted in the presence of zinc salts as catalysts. Reaction
with zinc 2
ethylhexanoate is particularly preferred.
0.05 to 10 % by weight, preferably 0.1 to 3 % by weight of a catalyst, is
added when
conducting the method according to the invention. 0.2 to 1 % by weight of
catalyst is
most preferably used.
The reaction can be conducted at room temperature or at higher temperatures up
to
140°C. A temperature range from 40 to 90°C is preferred.
Blocking can be effected free from solvents or in the presence of suitable
solvents.
Suitable solvents comprise customary lacquer solvents such as butyl acetate,
methoxypropyl acetate or the solvent naphtha supplied by Exxon-Chemie (Esso
Deutschland GmbH, Hamburg) as solvents which contain aromatic compounds, as
well as mixtures of the aforementioned solvents. Blocking is preferably
effected in
the aforementioned solvents, wherein the solids content should be adjusted so
that it
ranges between 10 and 90 %.
In addition to the cyclic ketones of general formula (I) which are used
according to
the invention, mixtures of any blocking agents can also be used in conjunction
in the
method according to the invention in order to achieve the lacquer properties
which
are required in each case, wherein the proportion of compounds of formula (I)
is at
least 30 % by weight, preferably 50 % by weight, most preferably 100 % by
weight.
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Finally, the present invention also relates to a method of producing 1-C PUR
stowing
lacquers, characterised in that organic polyisocyanates according to the
invention are
used as a crosslinking component for organic polyhydroxyl compounds.
The blocked polyisocyanates according to the invention are distinguished in
that, in
combination with a suitable organic polyhydroxyl compound and in the presence
of
suitable catalysts, they harden at stowing times of 15 to 30 minutes and at
temperatures from 110 to 140°C, preferably from 120 to 140°C.
The stowing times
depend in particular on the amount of catalyst used. Stowing is preferably
conducted
for a period of 30 minutes at a temperature of 120-140°C.
Examples of suitable catalysts for crosslinking include DBTL (dibutyltin
dilaurate),
zinc-2-ethylhexanoate and bismuth 2-ethylhexanoate. The preferred catalysts
are zinc
2-ethylhexanoate and bismuth-2-ethylhexanoate.
Suitable polyhydroxyl compounds for this purpose of use, as well as further
details
with regard to the production and use of stowing lacquers of this type, can be
taken
from the literature. The most preferred field of application for the products
according
to the invention is the use thereof as crosslinking agents in automobile
primer
surfacers.
High-grade, emission-free coatings or lacquer coatings with reduced yellowing
values can be obtained by using the blocked polyisocyanates according to the
invention.
In addition, the blocked polyisocyanates according to the invention can be
hardened
with di- or polyamines. This reaction is preferably conducted at room
temperature. It
can be used for the production of lacquer coatings or workpieces.
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Examples
The polyisocyanate used was an HDI polyisocyanate with an isocyanurate
structure,
an NCO content of 21.8 %, and a viscosity of 3200 mPa.s (Desmodur N3300, Bayer
AG, Leverkusen).
The cyclopentanone-2-carboxymethyl ester and cyclohexanone-2-carboxymethyl
ester which were used as blocking agents were ordered from the Fluka company
and
were used without further purification.
Preparation of~olyisoc~anates blocked with acidic a cyclic ketones
Example 1
A solution of 58.5 g (0.3 equivalent) Desmodur~ N3300 in 81 ml butyl acetate
was
added, slowly and with intensive stirring, to a solution of cyclopentanone-2
carboxyrnethyl ester (42.7 g, 0.3 equivalent) dissolved in 20 ml butyl
acetate. 1.02 g
zinc 2-ethylhexanoate was added as a catalyst. The batch was heated to a
temperature
of 50°C, (for about 8 hours) until a determination of the NCO value
gave a value of
about 0.2 %. The theoretical blocked NCO content was 6.2 %.
Example 2
A solution of 42.6 g (0.25 equivalent) Desmodur~ N3300 in 71.4 ml butyl
acetate
was added, slowly and with intensive stirring, to a solution of cyclohexanone-
2-
carboxymethyl ester (42.6 g, 0.25 equivalent) dissolved in 20 ml butyl
acetate. 0.9 g
zinc 2-ethylhexanoate was added as a catalyst. The batch was heated to a
temperature
of 80°C, until a determination of the NCO value gave a value of about
0.3 % (after
about 6 hours). The theoretical blocked NCO content was 5.75 %.
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Production of polyurethane lacquers according to the invention
The polyisocyanates listed in the following Table were processed in
stoichiometric
amounts with polyols to form clear lacquers according to the formulations
listed
below, and with the addition of the customary additives Baysilone~ OL 17
(Bayer
AG, Leverkusen (flow enhancer), 0.1 % solid with respect to solid binder
vehicle)
and Modaflovv~ (Monsanto Corp., Solutia Inc., USA; 0.01 % solid with respect
to
solid binder vehicle).
Example 3
Lacquer formulation A
Desmodur N3300 (Bayer AG, Leverkusen), blocked with cyclopentanone-2
carboxymethyl ester (supplied as an approximately 50 % solution in butyl
acetate;
blocked NCO content: 6.2 %) (SN = solvent naphtha):
by weight
Desmophen~ A 870 (Bayer AG, Leverkusen), 70 % in BA 35.94
Desmodur~ N3300. blocked with cyclopentanone
2-carboxymethyl ester (SO % in BA from Example 1) 34.82
Baysilone OL 17, 10 % in xylene 0.48
Modaflow , 1 % in xylene 0.48
Tinuvin~ 292 (Ciba AG, Basle, Switzerland), 10 % in xylene 4.78
Tinuvin- 1130 (Ciba AG, Basle, Switzerland), 10 % in xylene 9.56
bismuth 2-ethylhexanoate, 10 % in MPA 7.17
MPA/SN 100(1:1) 6.77
Total 100.00
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ratio of blocked NCO/OH: 1.0,
solids content: about 45 %,
catalyst content: 1.5 % (solid with respect to solid binder vehicle).
The system exhibited only very slight yellowing. This system could also be
used
successfully when the NCO/OH ratio was 1:1.5.
Lacquer formulation B (comparison)
% by weight
Desmopheri A 870, 70 % in BA 37.15
mixed trimer of hexamethylene diisocyanate and IPDI,
blocked with diisopropylamine (50 % in BA) 33.88
Baysilone OL 17, 10 % in xylene 0.48
Modaflow , 1 % in xylene 0.48
Tinuviri 292 (Ciba AG, Basle, Switzerland), 10 % in xylene 4.80
Tinuvin~ 1130 (Ciba AG, Basle, Switzerland), 10 % in xylene 9.61
DBTL, 10 % in xylene 4.80
MPA/SN 100(1:1) 8.80
Total 100.00
Stoving conditions: 30 minutes at 140°C.
In solvent-containing lacquers, even at relatively low stoving temperatures,
this
system exhibited a clear yellow coloration. The delta b value from 140 to
160°C (30
minutes) was 3.2, and was thus about four times higher than that of a system
which
exhibited only slight yellowing (e.g. dimethylpyrazole), when applied over a
base
lacquer which contained a white solvent in each case.
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Measurement of yellowing due to overstoving: after stoving the lacquers for 30
minutes at 140°C, a first colour measurement was made using what is
termed the
CIELAB method. The higher the positive b value which is determined in this
manner, the more yellow the clear lacquer has become. This was followed by
0
overstoving for 30 minutes at I60 C. The increase in yellow coloration was
subsequently measured, namely what is termed the Ob value according to the
CIELAB colour system (DIN 6174, "Colorimetric determination of colour
separations for body colours according to the CIELAB formula" (Edition 01.79).
For
non-yellowing clear lacquers, this value should be as close as possible to 0.
I0
