Note: Descriptions are shown in the official language in which they were submitted.
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Stabilisation of organic polJrisoc anates
The present invention relates to the use of 3,5-di-tert.-butyl-4-
hydroxyphenylpropionic
esters for stabilising polyisocyanates.
The organic isocyanates have attained great importance in the production of
polyurethane plastics. Thus, for example, the organic polyisocyanates together
with
polyols (polyether polyols and polyester polyols) are used for the production
of foams,
fibres, films, elastomers and coatings.
However, the organic polyisocyanates have a tendency to discolour during
storage,
even at low temperatures. This property is particularly marked when storage at
elevated temperatures is necessary, for example, for a homogeneous reaction
when the
solid polyisocyanates together with co-reactants, such as polyether polyols,
polyester
1 S polyols or glycols, are to be converted to urethanes. In the production of
prepolymers,
too, the NCO-OH reaction has to be carried out at elevated temperatures. In
the course
of this it is observed that the isocyanates discolour very rapidly if no
stabilisation has
been carned out.
The addition of various stabilisers to organic polyisocyanates in order to
lessen the
tendency of the organic polyisocyanates to discolour has already been
proposed.
Known stabilisers are, for example, sterically hindered phenols,
dilakyldiphenyl
amines, phenothiazines, phosphites or mixtures of representatives from these
classes of
substances (cf., for example, US 3 71 S 301, US 4 064 157, DT-OS 1 668 275, DT-
AS
1 618 845).
2,6-di-tert.-butyl-4-methylphenol (BHT), alone or in combination with other
compounds of the classes of stabilisers listed above, is most frequently used
for
stabilising organic polyisocyanates.
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The disadvantage of BHT is the relatively high volatility and tendency to
migrate into
polyurethanes covering substrates as well as the resulting strong yellowing of
the
substrates in an atmosphere containing NOX. Stabilisers without these
disadvantages
are therefore of interest and it was the object of the present invention to
provide such
materials.
The present invention provides the use of 3,5-di-tert.-butyl-4-
hydroxyphenylpropionic
esters as stabilisers for organic polyisocyanates, in particular for aromatic
polyisocyanates.
The most suitable esters of the above-mentioned phenylpropionic acid are: the
methyl,
ethyl, propyl and tert. butyl ester, in particular the methyl ester (BHPME).
Reactions with polyisocyanates demand a balanced reactivity, which should not
be
1 S influenced either by traces of other compounds such as, for example,
antioxidants.
Surprisingly, it has been found that even the smallest amounts of 3,5-di-tert.-
butyl-4-
hydroxyphenylpropionic esters, of from 0.001 to 0.1 wt.%, preferably from
0.001 to
0.003 wt.%, based on polyisocyanate, are effective.
BHPME, which is produced, for example, by base-catalysed addition of methyl
acrylate to 2,6-di-tert.-butylphenol and is available commercially, is
suitable for all
commonly used polyisocyanates, including aliphatic, aromatic and
cycloaliphatic
polyisocyanates. Examples which may be mentioned are: ethylene diisocyanate,
tetramethylene diisocyanate, hexamethylene diisocyanate, 4,4'-
methylenebis(cyclohexyl diisocyanate), m-phenylene diisocyanate, p-phenylene
diisocyanate, tolylene 2,4-diisocyanate, tolylene 2,6-diisocyanate, 4,4'-
methylenebis(phenyl isocyanate), 2,3'-methylenebis(phenyl isocyanate), 2,3-
methylenebis(phenyl isocyanate), tolylene 2,4,6-triisocyanate.
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The phenylpropionic esters according to the invention may also be used in
combination with known isocyanate stabilisers.
The organic polyisocyanates stabilised in this way exhibit a very greatly
diminished
S tendency to discolour when stored at elevated temperatures and can be used
with
advantage for the production of polyurethanes. The polyurethanes are, for
their part,
used for the production of foams, films, coatings or elastomers.
The subject matter of the present invention is explained in more detail with
the aid of
the following Examples (% data in each case denote wt.% and all temperature
data
denote °C).
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Example 1
Isocyanate: Stabiliser Colour index (ALPHA)'
after 6, 15 and 21 days
at SO°C
Desmodur T 80~ 10 - 50 -
1000
(Toluene diisocyanate; 2,4 80% 2,6 20%)
30 ppm BHT - 5 - 250
100 ppm BHT - - 150
30 ppm BHPME - - 50
10 ppm BHPME - - 100
From this list it follows that addition of 10 ppm of BHPME exhibits a better
stabilising
1 S action than does addition of 100 ppm of BHT.
Colour index in accordance with DIN 53 409 (July'67) or ISO (July 1988)
Example 2
Desmodur 441VI~ (diphenylmethane diisocyanate)
Sample A 44 M~ S00 g + 50 mg BHT
Sample B 44 M~ S00 g + SO mg BHPME
Sample C 44 M~ without additive
The samples were subjected to a W irradiation test in order to investigate the
yellowing behaviour. After exposure to light for 70 hours, colour indices of
these
samples were measured by the APHA method.
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Colour index (APHA)
Sample A 100
Sample B 40
Sample C 250
Before irradiation 0 - 5
Example 3
Isocyanate Stabiliser Colour index pity (~)
(APHA)
Desmodur T 80~ 100 ppm BHT 5 - 10 99.9%
30 ppm BHPME 5 - 10 99.9%
1 S The isocyanate was heated to 60°C and cooled again S times.
