Process for the production of polyisocyanate mixtures containing uretdione and isocyanurate groups

The present invention is directed to a process for the production of polyisocyanate mixtures containing isocyanurate groups and uretdione groups in a molar ratio of about 1:9 to 9:1 by oligomerizing a portion of the isocyanate groups of hexamethylene diisocyanate using trialkyl phosphines and/or peralkylated phosphorus acid triamides as the catalysts which accelerate the dimerization and trimerization of isocyanate groups, terminating the reaction at the desired degree of oligomerization by the addition of a catalyst poison and removing unreacted hexamethylene diisocyanate to a residual content of at most 0.5% by weight, characterized in that, before addition of the catalyst, the hexamethylene diisocyanate starting material is freed from carbon dioxide to a residual content of less than 20 ppm (weight). The present invention is also directed to the polyisocyanates containing uretdione groups and isocyanurate groups obtainable by this process and their use, optionally blocked by blocking agents for isocyanate groups, as the isocyanate component for the production of polyisocyanate polyaddition products.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a new process for the production of 
polyisocyanates containing uretdione and isocyanurate groups, i.e. 
polyisocyanate mixtures, by the oligomerization of a portion of the 
isocyanate groups of hexamethylene diisocyanate (hereinafter referred to 
as "HDI") using catalysts which accelerate both dimerization (uretdione 
formation) and also trimerization (isocyanurate formation), to the 
products obtained by this process and their use, optionally in blocked 
form, as the isocyanate component in polyurethane lacquers. 
2. Description of the Prior Art 
The production of polyisocyanate mixtures containing uretdione and 
isocyanurate groups by the dimerization and/or trimerization of a portion 
of the isocyanate groups of organic polyisocyanates using 
phosphorus-containing catalysts is known. These mixtures are known to be 
valuable starting materials for the production of polyurethane plastics. 
However, known processes for the production of such polyisocyanate 
mixtures using HDI as the starting material (cf. for example DE-OS 
1,670,720 and DE-OS 3,432,081) are not optimally suited to large-scale 
production. The disadvantages of the known processes are the long reaction 
time and the relatively large quantities of catalyst which result in the 
need for a correspondingly large quantity of deactivators such that the 
end product contains a relatively high percentage of unwanted foreign 
components which adversely affect the properties of the polyurethane 
plastic. 
In the process according to DE-OS 3,437,635, considerable quantities of 
alcohols are used as co-catalysts which means that valuable isocyanate 
groups are consumed, i.e. destroyed, by the addition reaction which takes 
place between isocyanate groups and hydroxyl groups. 
Accordingly, an object of the present invention is to provide a new process 
for the production of HDI-based isocyanate mixtures containing uretdione 
and isocyanurate groups which is not attended by any of the disadvantages 
mentioned above. 
According to the invention, this object is achieved by using HDI which is 
substantially free from carbon dioxide as the starting material. It is 
possible in this way to limit the reaction time to less than one working 
day, i.e., to less than 10 hours, and at the same time to carry out the 
process using minimal quantities of catalysts and without using large 
quantities of co-catalysts which consume isocyanate groups. 
SUMMARY OF THE INVENTION 
The present invention is directed to a process for the production of 
polyisocyanate mixtures containing isocyanurate groups and uretdione 
groups in a molar ratio of about 1:9 to 9:1 by oligomerizing a portion of 
the isocyanate groups of hexamethylene diisocyanate using trialkyl 
phosphines and/or peralkylated phosphorus acid triamides as the catalysts 
which accelerate the dimerization and trimerization of isocyanate groups, 
terminating the reaction at the desired degree of oligomerization by the 
addition of a catalyst poison and removing unreacted hexamethylene 
diisocyanate to a residual content of at most 0.5% by weight, 
characterized in that, before addition of the catalyst, the hexamethylene 
diisocyanate starting material is freed from carbon dioxide to a residual 
content of less than 20 ppm (weight). 
The present invention is also directed to the polyisocyanates containing 
uretdione groups and isocyanurate groups obtainable by this process and 
their use, optionally blocked by blocking agents for isocyanate groups, as 
the isocyanate component for the production of polyisocyanate polyaddition 
products.

DETAILED DESCRIPTION OF THE INVENTION 
The use of HDI which is substantially free from carbon dioxide as starting 
material is critical to the present invention. The HDI used in accordance 
with the invention contains less than 20 ppm (weight), preferably less 
than 10 ppm (weight) and more preferably less than 5 ppm (weight) of 
carbon dioxide. 
Technical HDI purified by distillation, of the type previously used for the 
production of polyisocyanates containing isocyanurate groups, contains 
considerable quantities (approx. 20 ppm to 100 ppm (weight)) of carbon 
dioxide. Carbon dioxide can enter the HDI during the production process, 
for example during the phosgenation of carbonic acid salts of 
hexamethylene diamine. It can be taken up from the air during storage and 
can be formed by chemical reaction of the NCO groups to form carbodiimide 
groups or by reaction with a trace of moisture. After 24 hours in a sealed 
container HDI, which has been freshly purified by vacuum distillation 
contains, for example, 40 ppm carbon dioxide. HDI stored over a period of 
approximately 6 months can contain up to 0.6% by weight of carbon dioxide 
when the container has been opened during the period of storage. 
Carbon dioxide is known to react with uretdione groups to form unwanted 
secondary products containing oxadiazinone groups. The formation of 
secondary products with carbon dioxide is mentioned in DE-OS 1,670,720. 
However, there was no recognition of the considerable influence carbon 
dioxide has on the catalyst and on the reaction time, nor that standard 
distillation is unable to adequately reduce the carbon dioxide content in 
HDI. 
Carbon dioxide can be removed from HDI by blowing out with ultrapure 
nitrogen or with a noble gas, for example argon, for example at about 
0.degree. to 120.degree. C., preferably about 0.degree. to 70.degree. C. 
and more preferably about 30.degree. to 50.degree. C. Although higher 
temperatures may also be applied, this does not provide any advantages. 
Carbon dioxide can also be removed by distillation in a stream of nitrogen 
or noble gas. The way in which the carbon dioxide is removed is not 
crucial to the process according to the invention. As mentioned, however, 
substantially complete removal of carbon dioxide to a residual content of 
less than 20 ppm is generally not possible by standard distillation under 
reduced pressure. 
Tertiary phosphines or peralkylated phosphorus acid triamides are used as 
catalysts in the process according to the invention. Mixtures of tert. 
phosphines and peralkylated phosphorus acid triamides may of course also 
be used, although this is less preferred. Suitable tert. phosphines 
include, in particular, aliphatic, araliphatic or mixed aliphaticaromatic 
tert. phosphines having a molecular weight of 76 to about 500. Examples 
include compounds corresponding to the formula 
##STR1## 
wherein R', R" and R'" may be the same or different and represent C.sub.1 
-C.sub.10, preferably C.sub.2 -C.sub.8 alkyl groups; C.sub.7 -C.sub.10, 
preferably C.sub.7 aralkyl groups: or C.sub.6 -C.sub.10, preferably 
C.sub.6 aryl groups, provided that at most one of the substituents is an 
aryl group and preferably at least one of the substituents is an alkyl 
group; two of the substituents may form (with the phosphorus atom) a 4- to 
6-membered ring containing phosphorus as hetero atom, in which case the 
third substituent is a C.sub.1 -C.sub.4 alkyl group. 
Examples of suitable tert.-phosphines are triethyl phosphine, dibutyl ethyl 
phosphine, tri-n-propyl phosphine, triisopropyl phosphine, tri-tert.-butyl 
phosphine, tribenzyl phosphine, benzyl dimethyl phosphine, dimethyl phenyl 
phosphine, tri-n-butyl phosphine, triisobutyl phosphine, triamyl 
phosphine, trioctyl phosphine or butyl phosphacyclopentane. 
Tri-(n-butyl)-phosphine is a particularly suitable catalyst for the 
process according to the invention. 
Peralkylated phosphorus acid triamides suitable for use as catalysts 
include organic compounds corresponding to the formula 
EQU P(NR.sub.2).sub.3 
wherein the individual substituents R may be the same or different, 
preferably the same, and represent C.sub.1 -C.sub.10, preferably C.sub.1 
-C.sub.4 alkyl radicals: C.sub.7 -C.sub.10, preferably C.sub.7 aralkyl 
radicals: or C.sub.6 -C.sub.10, preferably C.sub.6 cycloalkyl radicals. It 
can be seen from this definition of the substituents R that the expression 
"peralkylated" should be broadly interpreted to include not only alkyl 
radicals but also cycloalkyl and aralkyl radicals as possible substituents 
of the nitrogen atom. However, the peralkylated phosphorus acid triamides 
to be used as catalysts in accordance with the invention are preferably 
those corresponding to the above general formula in which all the 
substituents R are C.sub.1 -C.sub.4 alkyl radicals, preferably methyl 
radicals. Permethylated phosphorus acid triamide, i.e., 
tris-(dimethylamino)phosphine, is the preferred catalyst from the group of 
phosphorus acid triamides which may be used in the process according to 
the invention. 
The catalysts mentioned are used in a quantity of about 0.01 to 2% by 
weight, preferably about 0.1 to 1% by weight and more preferably about 0.1 
to 0.5% by weight, based on the total quantity of HDI. 
The process according to the invention is preferably carried out in the 
absence of solvents, although this does not preclude the presence of 
standard lacquer solvents during the oligomerization reaction. Examples of 
these solvents include esters such as butyl acetate or ethoxyethyl 
acetate; ketones such as methyl isobutyl ketone or methyl ethyl ketone; 
hydrocarbons such as xylene; and mixtures of such solvents. However, since 
unreacted HDI is removed after the reaction, the presence of such solvents 
during the reaction causes unnecessary additional expense. 
The process according to the invention may be carried out, for example, as 
described in DE-OS 1,670,720, DE-OS 3,432,081 (U.S. Pat. No. 4,614,785, 
herein incorporated by reference) or DE-OS 3,437,635. For example, the HDI 
which is substantially free from carbon dioxide may initially be 
introduced into a suitable stirred reactor, followed by the addition of 
the catalyst at about 0.degree. to 100.degree. C., preferably about 
20.degree. to 70.degree. C., after which the reaction mixture is kept at a 
temperature of about 0.degree. to 100.degree. C., preferably about 
20.degree. to 70.degree. C. by cooling or heating, preferably with 
stirring. It is advantageous to pass a stream of dry nitrogen or noble 
gas, for example argon, through the reaction mixture throughout the 
reaction. The progress of the reaction may be followed by determination of 
the NCO content of the reaction mixture. The reaction is generally 
terminated after reaching a degree of oligomerization of about 5 to 70, 
preferably 15 to 40%. By "degree of oligomerization" is meant the 
percentage of isocyanate groups which react to form dimers or trimers. 
This degree of oligomerization corresponds to an NCO content of the 
reaction mixture of 15 to 47.5% by weight, preferably 30 to 42.5% by 
weight. 
The reaction is terminated by the addition of a catalyst poison which 
neutralizes the effect of the catalyst. Suitable catalyst poisons include 
the alkylating or acylating agents mentioned in DE-OS 1,670,720, sulfur 
and, in particular, the sulfonyl isocyanates recommended for this purpose 
in DE-OS 3,432,081 (U.S. Pat. No. 4,614,785, herein incorporated by 
reference). Trialkyl siloxy sulfonyl isocyanates, such as trimethyl siloxy 
sulfonyl isocyanate, are also suitable. The catalyst poison is used in an 
at least a half-molar quantity, based on the catalyst. The molar ratio of 
catalyst to catalyst poison is preferably 1:0.5 to 1:2. 
After termination of the reaction, the free, unreacted HDI present in the 
reaction mixture is removed by suitable means, for example by extraction 
(for example using n-hexane as extractant) or preferably by thin-layer 
distillation in a high vacuum at about 110.degree. to 180.degree. C., 
preferably (due to the heat sensitive uretdione groups) at 110.degree. to 
130.degree. C. to a residual content of at most 0.5% by weight. 
The products obtained by the process according to the invention are 
distinguished from known products by less coloration because less catalyst 
is required in the process according to the invention. They are generally 
colorless to faintly yellow-colored liquids having a color value (HAZEN) 
according to DIN 53,409 of less than 200 and generally less than 100. They 
have a viscosity at 23.degree. C. of about 50 to 3000 mPa.s. They have an 
NCO content of about 10 to 24% by weight, preferably 18 to 23% by weight. 
In accordance with the invention both dimerization and trimerization 
reactions occur. The products obtained by the process according to the 
invention are mixtures of diisocyanates containing uretdione groups and 
polyisocyanates containing isocyanurate groups and, because the two 
reactions take place simultaneously, small quantities of modified 
polyisocyanates containing both uretdione groups and isocyanurate groups. 
The molar ratio of uretdione to isocyanurate groups in the products 
according to the invention is about 1:9 to 9:1, generally about 1:3 to 
3:1. It may be influenced within these limits during the process according 
to the invention by the choice of catalyst and reaction temperature and 
may be quantitatively determined, for example, by hot titration with 
dibutylamine solution or IR spectroscopy. The advantage of the process 
according to the invention over known processes is that it can be carried 
out in a short time, for example in less than one working day, under very 
mild conditions and is also very easy to carry out continuously. A major 
technical advantage is that a high throughput of product can be rapidly 
obtained in small apparatus. 
Since only small quantities of catalyst are used in the process according 
to the invention, the quantity of deactivator, i.e. the catalyst poison 
can also be kept correspondingly small. Therefore, the products obtained 
by the process according to the invention contain very small quantities of 
secondary products formed from catalyst and catalyst poison which 
generally remain dissolved and do not affect the subsequent use of the 
products. Even when technical HDI is used (i.e., HDI which has not been 
subjected to standard purification by distillation in the presence of 
weakly basic compounds, such as metal oxides or sodium hydrogen carbonate, 
to remove traces of chlorine-containing compounds) the products obtained 
by the process of the present invention are clear and colorless. By virtue 
of their low viscosity, the products obtained by the process according to 
the invention are particularly suitable for the production of solventless 
or low-solvent polyisocyanate polyaddition products, preferably 
polyurethane lacquers by a reaction with compounds containing at least two 
isocyanate-reactive groups, preferably hydroxyl groups. 
When the products obtained by the process according to the invention are 
used in accordance with the invention, they may be blocked by blocking 
agents for isocyanate groups. Suitable blocking agents include the 
compounds mentioned by way of example in EP-A-10,589, page 15, lines 14 to 
26 (U.S. Pat. No. 4,324,879, herein incorporated by reference). 
The products obtained by the process according to the invention may be used 
for the production of high-quality two-component polyurethane lacquers, 
preferably in combination with the polyhydroxy polyesters, polyhydroxy 
polyethers and, more preferably, polyhydroxy polyacrylates known from 
polyurethane lacquer technology. In addition to these relatively high 
molecular weight polyhydroxyl compounds, these lacquers may also contain 
low molecular weight polyols, preferably aliphatic polyols. Combinations 
of the products obtained by the process according to the invention with 
polyhydroxypolyacrylates represent particularly valuable two-component 
binders for high-quality, highly weather-resistant lacquers. 
Polyamines, preferably in blocked form as polyketimines or oxazolidines, 
may also be used as reactants for the products obtained by the process 
according to the invention. 
The quantitative ratios in which the optionally blocked polyisocyanates 
according to the invention and the reactants mentioned are used in the 
production of polyurethane lacquers are generally selected so that there 
are about 0.8 to 3, preferably about 0.9 to 1.8 hydroxyl, amino and/or 
carboxyl groups for every (optionally blocked) isocyanate group. 
It is known that, under certain conditions, the uretdione group may also be 
considered as a reactive group in the same sense as a blocked NCO group. 
In stoving lacquers which are hardened at elevated temperature, the 
uretdione group is included as a blocked NCO group in the quantitative 
ratio of polyisocyanate and reactant. 
The hardening reaction may be accelerated with the catalysts known from 
isocyanate chemistry. Examples include tertiary amines such as 
triethylamine, pyridine, methyl pyridine, benzyl dimethyl amine, 
N,N-dimethylaminocyclohexane, N-methyl piperidine, pentamethyl diethylene 
triamine, N,N'-endoethylene piperazine or N,N'-dimethyl piperazine; and 
metal salts such as iron(III) chloride, zinc chloride, zinc-Z-ethyl 
caproate, tin(II)-2-ethyl caproate, dibutyl tin(IV) dilaurate or 
molybdenum glycolate. 
In blocked form the products obtained by the process according to the 
invention are used in combination with polyhydroxyl compounds of the type 
mentioned, particularly for the production of stoving lacquers which may 
be hardened at temperatures of about 80.degree. to 180.degree. C., 
depending on the blocking agents used, to form high-quality lacquer 
coatings. 
To produce the ready-to-use lacquers, the optionally blocked 
polyisocyanate, polyfunctional reactant, optional isocyanate polyaddition 
catalyst and known additives (such as pigments, dyes, fillers and 
levelling agents) are thoroughly mixed with one another and homogenized in 
a standard mixing unit, for example in a sand mill, either with or without 
solvent. 
The paints and coating compositions may be applied to the article to be 
coated in solution, from the melt or in solid form by standard methods 
such as spread coating, roll coating, casting or spraying. 
The lacquers containing the polyisocyanates according to the invention 
provide films which adhere surprisingly well to metal substrates and are 
particularly light-stable, color-stable under heat and highly 
abrasion-resistant. In addition, they are distinguished by considerable 
hardness, elasticity, very good resistance to chemicals, high gloss, 
excellent weather resistance and good pigmentability. 
In the following examples, all percentages and all quantities in "ppm" are 
based on weight. 
EXAMPLES 
Example 1 
In a stirred reactor, 1200 g HDI were degassed in about 10 minutes at about 
20.degree. C. by application of a vacuum (50 mbar) and vigorous stirring. 
The gas space of the reactor was then filled with pure nitrogen. A 
vigorous stream of pure, dry nitrogen was then passed through the liquid 
for about 1 hour at around 25.degree. C. Whereas the HDI used had an 
initial CO.sub.2 content of 56 ppm, the CO.sub.2 content had fallen to 5 
ppm after the treatment mentioned. Nitrogen was passed through the 
reaction mixture for the remainder of the reaction. 
1.5 g (approx. 0.007 mol or 0.125%, based on HDI) of tributyl phosphine was 
then introduced into the liquid heated to 60.degree. C., followed by 
stirring at that temperature. The progress of the reaction was followed by 
determination of the isocyanate content. After 2 hours, the NCO content 
had fallen to 38.6% (degree of oligomerization approx. 22.8%). 
The reaction was then terminated by the addition of 1.4 g (approx. 0.007 
mol) of trimethyl siloxy sulfonyl isocyanate. After stirring for 30 
minutes, monomeric HDI was distilled off in a short-path evaporator at 
120.degree. C./0.01 mbar. 
474 g of a monomer-free sump product having the following properties were 
obtained: 
______________________________________ 
Viscosity: 130 mPa.s/20.degree. C. 
color value (HAZEN), DIN 53,409 
90 
NCO content 21.4% 
free HDI content 0.2% 
molar ratio of uretdione to 
3:2 
isocyanurate groups approximately 
(as determined by hot titration 
with dibutylamine solution 
and by IR spectrum). 
______________________________________ 
Example 2 (COMISON EXAMPLE) 
1200 g of HDI having the same initial CO.sub.2 content as in Example 1 were 
used in a corresponding reactor. The reactor space above the liquid phase 
was filled with dry nitrogen, but CO.sub.2 dissolved in the HDI was not 
removed by blowing out with nitrogen. 
The liquid was then heated to 60.degree. C., followed by the addition of 3 
g (0.014 mol or 0.25%, based on HDI) of tributyl phosphine. The reaction 
was initiated with thorough stirring. After 2 hours, the NCO content had 
only fallen to 47.3%, after 8 hours to 43.4% and after 13 hours to 40.2%. 
The reaction was then terminated by the addition of 2.8 g of trimethyl 
silyloxysulfonyl isocyanate. Further processing was carried out as in 
Example 1. 
402 g of a viscous liquid having the following properties were obtained: 
______________________________________ 
viscosity 120 mPa.s/23.degree. C. 
color value (HAZEN), DIN 53,409 
170 
NCO content 22.0% 
free HDI content 0.2% 
______________________________________ 
Result of the comparison: 
The process according to the invention takes place several times more 
quickly with less catalyst, less deactivator and a high yield. The end 
product had a better color value. 
When Comparison Example 2 was repeated using the quantity of catalyst from 
Example 1, the reaction took even longer. After reaching 42.0%, there was 
very little change in the NCO content. 
Example 3 
Example 6 of DE-OS 3,432,081 (Example 6 of U.S. Pat. No. 4,614,785) was 
repeated and modified in accordance with the present invention. 1 g of 
freshly distilled hexamethyl phosphorous acid triamide was added to 400 g 
of HDI freed from carbon dioxide as in Example 1 (in Example 6 of DE-OS 
3,432,081, 4 g catalyst were added to 400 g HDI). After 45 minutes at 
60.degree. C., the NCO content measured 38.8% (comparison 40.0%). Working 
up (distillation, etc.) provided 190 g of a light yellow colored 
polyisocyanate (comparison 152 g). 
The comparison shows that the process according to the invention gives a 
better yield with less catalyst. 
Examples 4 to 8 
These examples illustrate the range of variation of the process according 
to the invention in regard to the quantity of catalyst and the 
temperature. Carbon dioxide was removed to a residual content of 2 to 10 
ppm by blowing out with nitrogen at 40.degree. to 60.degree. C. 
The results are set out in Table 1 which sets forth the quantity of 
catalyst, the reaction temperature and reaction time and also the NCO 
content on termination of the reaction. Table 2 shows the properties of 
the end product. 
TABLE 1 
__________________________________________________________________________ 
CO.sub.2 content after 
Tributyl 
Reaction 
blowing out with 
phosphine 
temper- 
NCO content on 
Reaction 
Example 
HDI (g) 
nitrogen catalyst 
ature (.degree.C.) 
deactivation 
time 
__________________________________________________________________________ 
4 1600 2 ppm 4 g (0.25%) 
23 37.5% 9 h 
5 1600 7 ppm 2 g (0.125%) 
35 38.8% 5 h 
6 1600 10 ppm 2 g (0.125%) 
40 41.0% 1.5 h 
7 1600 2 ppm 8 g (0.5%) 
50 22.1% 6 h 
8 1600 4 ppm 1 g (0.06%) 
40 40.2% 10 h 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Quantity of NCO Viscosity/ 
Free 
product after 
content 23.degree. C. 
HDI content 
Example 
removal of HDI 
(%) (mPa.s) (%) 
______________________________________ 
4 698 g 21.1 160 0.4 
5 625 g 21.8 140 0.4 
6 550 g 22.6 130 0.2 
7 1240 g 15.5 15000 0.1 
8 600 g 21.8 140 0.05 
______________________________________ 
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.