Note: Descriptions are shown in the official language in which they were submitted.
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Mo4317
LeA 30,784 -Aus 1 and
A PROCESS FOR THE PRODUCTION OF POLYISOCYANATES
AT LEAST PARTIALLY BLOCKED WITH 3.5-DIMETHYLPYRAZOLE
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a novel process for the production of
polyisocyanates at least partially blocked with 3,5-dimethylpyrazole
(DMP) using DMP produced in situ from acetylacetone and hydrazine
hydrate.
Background of the Invention
The use of DMP as a blocking agent for isocyanates is known
(US-PS 3,248,398 or EP-A-0,159,117). Lacquer polyisocyanates blocked
with DMP may be used as crosslinking agents in high-grade one-
component polyurethane coating compositions. The advantage of DMP
over other blocking agents, such as butanone oxime, resides in a
substantially lower thermal yellowing of the coatings and a comparatively
low stoving temperature of approximately 130°C.
DMP is prepared by the condensation of acetylacetone with
hydrazine and is a solid which melts at above 100°C. For this reason,
large-scale industrial handling of DMP has been more complicated than,
for example, the use of liquid blocking agents.
An object of the present invention is to provide a novel process for
the production of polyisocyanates blocked with DMP in which the
separate production of the blocking agent and the need to handle it as a
solid is omitted.
Surprisingly, it proved possible to achieve this object with the
process according to the invention described in greater detail below. It
has been found that the crude product produced by the condensation
reaction in an organic solution between acetylacetone and hydrazine
hydrate may be used directly for the blocking reaction for organic
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polyisocyanates without isolation of the DMP present. The use of this
crude product does not result in any appreciable impairment of the
quality of the blocked polyisocyanate obtained in this manner. A further
advantage of the process according to the invention is that the yield of
blocked polyisocyanate relative to the introduced starting materials is
virtually quantitative. To the contrary the previous isolation of DMP and
its subsequent use as a blocking agent inevitably resulted in unavoidable
losses in yield.
SUMMARY OF THE INVENTION
The present invention relates to a process for preparing a blocked
polyisocyanate which comprises reacting in a first reaction stage an
equimolar mixture of acetylacetone and hydrazine hydrate via a
condensation reaction in a water-immiscible solvent with separation and
removal of the introduced and formed water to form a liquid organic
product containing 3,5-dimethylpyrazole and in a second reaction stage
reacting said liquid organic product, without separation of 3,5-dimethyl-
pyrazole as a solid, with an organic polyisocyanate, which is optionally
dissolved in a solvent, to at least partially block the isocyanate groups
with 3,5-dimethylpyrazole.
DETAILED DESCRIPTION OF THE INVENTION
The polyisocyanates to be blocked in the process according to the
invention are selected from organic polyisocyanates having at least two
(cyclo)aliphatically or aromatically bound isocyanate groups. Examples
of these polyisocyanates include 2,4- and/or 2,6-diisocyanatotoluene
(TD/), hexamethylene diisocyanate (HD/), 1-isocyanato-3,3,5-trimethyl-5-
isocyanatomethylcyclohexane (/PD/), 4,4'-diisocyanato-dicyclohexyl-
methane (HMDI) and in particular the polyisocyanates, which have
isocyanurate, urethane, allophanate, biuret and/or uretidione groups and
are prepared from these diisocyanates. Higher molecular weight NCO
prepolymers prepared from the previously disclosed di- and/or
2~.6316~
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polyisocyanates may also be used in the process according to the
invention. Preferred di- or polyisocyanates have a molecular weight of
168 to 1000 and an NCO content of 15 to 50 wt.%. Derivatives of HDI,
IPDI and/or HMDI having isocyanurate groups are especially preferred for
use in the process according to the invention.
The first stage of the process according to the invention is the
condensation reaction between acetylacetone (pentane-1,3-dione) and
hydrazine, which is generally used as hydrazine monohydrate, in a water-
immiscible solvent. Suitable solvents include toluene, xylene, 2-methoxy-
propyl acetate, solvent naphtha 100 and mixtures of these solvents. The
condensation reaction generally takes place at a temperature of 50 to
150, preferably 70 to 110°C, wherein 4 to 7, preferably 5 to 6, moles
of
each of the two starting components are used per liter of the stated
solvent. The exothermic reaction is complete once the water introduced
with the hydrazine hydrate and the water formed during the condensation
reaction has been separated.
After isolation of the separated water, the organic solution is used
directly in the second stage of the process according to the invention. In
this stage one of the two reactive components (either the polyisocyanate
dissolved in an inert solvent such as methoxypropyl acetate or the
above-stated solution of the blocking agent produced in situ) is initially
introduced and the other component is then mixed into the starting batch.
The blocking reaction generally proceeds at 50 to 110, preferably 70 to
90°C. The quantities of the reaction components used in the second
stage of the process according to the invention are selected such that 0.3
to 1.05 moles (30 to 105 equivalent percent) of DMP are available for
each equivalent of unblocked isocyanate groups. If blocked
polyisocyanates are to be produced in which DMP is present as the sole
blocking agent, 0.95 to 1.05 moles (95 to 105 equivalent percent) of the
DMP are used per equivalent of isocyanate groups. In this case an
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equivalent quantity of the blocking agent is preferably used. The course
of the reaction may be monitored by the decrease in NCO content.
The process according to the invention is also very well suited to
the production of co-blocked polyisocyanates, i.e. polyisocyanates having
isocyanate groups blocked with different blocking agents. During
production of such co-blocked polyisocyanates, at least one other
blocking agent is used in addition to DMP. Other suitable blocking agents
include butanone oxime, 1,2,4-triazole, diisopropylamine, ethyl malonate
and ethyl acetoacetate.
For the production of co-blocked polyisocyanates using the
process according to the invention, at least 0.3 moles (30 equivalent
percent), preferably 0.3 to 0.9 moles (30 to 90 equivalent percent) and
more preferably 0.4 to 0.5 moles (40 to 50 equivalent percent) of DMP
are used per equivalent of isocyanate groups of the unblocked
polyisocyanate.
The co-blocked polyisocyanates may be produced in accordance
with either of two embodiments. In the first embodiment isocyanates
already partially blocked with at least one "other" blocking agent are used
in the second stage of the process according to the invention. The
quantity of the DMP previously produced in situ is calculated such that
0.95 to 1.05 moles of DMP are present for each equivalent of unblocked
isocyanate groups. It may be seen from these and previous ratios that
the "other" blocking agent may be used in an amount of up to 75
equivalent percent, based on the isocyanate groups originally present in
the starting polyisocyanate.
According to the second embodiment for the production of
co-blocked polyisocyanates, unblocked starting polyisocyanates and a
subequivalent quantity of DMP, based on the free isocyanate groups
present, are used for the blocking reaction according to the invention.
The quantity of DMP is at least 0.3 moles per equivalent of free
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isocyanate groups. Subsequent to the partial blocking of the starting
polyisocyanate, blocking with the "other" blocking agent proceeds in
known manner. The quantity of the "other" blocking agent is also
calculated such that 0.95 to 1.05 moles of this blocking agent are present
per equivalent of the remaining unblocked isocyanate groups.
The blocked polyisocyanates produced using the process
according to the invention are valuable starting materials for the
production of one-component polyurethane lacquers for any heat
resistant substrates. Particularly preferred reaction partners for the
products according to the invention are the known polyhydroxyl
compounds, preferably polyacrylate resins having hydroxyl groups.
All percentages in the following examples are weight percentages.
EXAMPLES
Example 1 (according to the invention)
Starting materials:
85.0 g (0.85 moles) acetylacetone
42.5 g (0.85 moles) hydrazine hydrate
170.0 g (0.85 NCO equiv) polyisocyanate having isocyanurate
groups and prepared from
1,6-diisocyanatohexane, NCO content:
21%
100.0 g xylene
70.0 g 2-methoxypropyl acetate
467.5 g
-45.9 g (2.55 moles) water
421.6 g (0.85 equiv blocked blocked NCO content: 8.4%, solids
NCO groups) content 60%, viscosity at 22°C: 225
mPa~s
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Stage 1: Production of 3,5-dimethylpyrazole solution
Acetylacetone and the two solvents were initially introduced into a
stirred apparatus fitted with a water separator, reflux condenser and
dropping funnel, and heated to 70°C. Hydrazine hydrate was added
dropwise to this initial mixture, which caused the temperature of the
reaction mixture to rise to 90 to 100°C, i.e., weak refluxing. After
the
addition of hydrazine hydrate was complete, the mixture was heated such
that it was constantly refluxing (bottom temperature 120 to 140°C).
After
approximately 30 minutes, 45 ml of water had collected in the water
separator. The clear, almost colorless reaction solution was cooled to
approximately 70°C and further processed in stage 2.
Stage 2: Production of the blocked polyisocyanate
The liquid polyisocyanate based on 1,6-diisocyanatohexane
(viscosity at 23°C approximately 3000 mPa~s) was initially introduced,
heated to approximately 70°C and combined in portions with stirring
with
the reaction solution from stage 1. On completion of addition, the reaction
was continued for a further 1 hour at 100°C. No NCO content was then
detectable by IR spectroscopy.
An almost colorless, 60% solution of a blocked polyisocyanate was
obtained having a blocked NCO content of 8.4% and a viscosity at 22°C
of 225 mPa~s. Yield: 100%, based on the polyisocyanate.
~1.6~169
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Example 2 (according to the invention)
Starting materials:
85.0 g (0.85 moles) acetylacetone
42.5 g (0.85 moles) hydrazine hydrate
79.4 g (1.15 moles) 1,2,4-triazole
700.0 g (2.0 equiv) polyisocyanate having isocyanurate
groups prepared from isophorone
diisocyanate (IPDI), and present
as a
70% solution in 2-methoxy-propyl-
acetate/xylene (1/1 ), NCO content:
12.0%
124.0 g 2-methoxypropyl acetate
100.0 g xylene
1130.9 g
-45.9 g (2.55 moles) water
1085.0 g (2.0 equiv blocked blocked NCO content: 7.7%, solids
NCO groups) content 60%, viscosity at 22°C: 8000
mPa~s
Stage 1: Production of 3,5-dimethylpyrazole solution
Acetylacetone, xylene and 70 g of methoxypropyl acetate were
initially introduced into a stirred apparatus fitted with a water separator,
reflux condenser and dropping funnel, and heated to 70°C. Hydrazine
hydrate was added dropwise to this initial mixture, which caused the
temperature of the reaction mixture to rise to 90 to 100°C, i.e., weak
refluxing. After addition of the hydrazine hydrate, the mixture was heated
such that it was constantly refluxing (bottom temperature 120 to
140°C).
After approximately 30 minutes, 45 ml of water had collected in the water
separator. The clear, almost colorless reaction solution was cooled to
approximately 70°C and further processed in stage 2.
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Stage 2: Production of the blocked pol iii soc ay nate
The polyisocyanate based on IPDI and 54 g of methoxypropyl
acetate were initially introduced and heated to 60°C. The white,
crystalline flakes of 1,2,4-triazole were added to this stirred solution and
heated in stages to 100 to 105°C, wherein the 1,2,4-triazole dissolved.
No significant exothermic reaction was detectable at this point. After
reacting for 30 minutes at 100 to 105°C, an NCO content of 4.3% was
measured (calculated value 4.2%). The solution was allowed to cool to
approximately 70°C and the solution of 3,5-dimethylpyrazole from stage
1
at approximately 70°C was added in portions. The temperature was
raised to 100°C and the reaction continued until no NCO content was
detectable by IR spectroscopy (approximately 1 hour).
A pale yellow solution of a polyisocyanate co-blocked with both
triazole and dimethylpyrazole was obtained for use in yellowing-resistant
one-component clear coating compositions. The blocked NCO content of
this solution was 7.7% (calculated), the solids content was 60%
(calculated) and the viscosity at 22°C was approximately 8000 mPa~s.
Yield: 100%, based on the polyisocyanate.
Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood that such
detail is solely for that purpose and that variations can be made therein
by those skilled in the art without departing from the spirit and scope of
the invention except as it may be limited by the claims.
