Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-02-20
2004-10-05
Gorr, Rachel (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S130000, C252S182200, C560S331000
Reexamination Certificate
active
06800667
ABSTRACT:
The present invention relates to mixtures comprising
(i) at least one isocyanate and
(ii) at least one organic and/or inorganic acid anhydride, preferably at least one carboxylic anhydride, where (ii) is preferably present in an amount of from 0.01 to 20% by weight, based on the weight of the mixture.
Furthermore, the invention relates to processes for producing polyisocyanate polyaddition products by reacting isocyanates with compounds which are reactive toward isocyanates in the presence or absence of catalysts, blowing agents, additives and/or auxiliaries, to polyisocyanate polyaddition products which can be produced in this way and to the use of the acid anhydrides according to the present invention in polyisocyanate polyaddition products.
The production of polyisocyanate polyaddition products by reacting polyisocyanates with compounds which are reactive toward isocyanates, catalysts which accelerate the reaction of the compounds which are reactive toward isocyanates with isocyanates and, if desired, blowing agents, additives and/or auxiliaries is generally known.
Like other plastics, polyisocyanate polyaddition products are subject to aging processes which, over the course of time, generally lead to impairment of the use properties. Significant aging influences are, for example, hydrolysis, photooxidation and thermal oxidation which lead to the rupture of bonds in the polymer chains. in the case of polyisocyanate polyaddition products, for example, polyurethane, the action of moisture and even more so the combination of moisture and heat results in hydrolytic cleavage of the urethane and urea bonds.
This cleavage not only shows up in a significant deterioration in the use properties but also leads to formation of primary aromatic amines such as toluenediamine (TDA) and diaminodiphenylmethane (MDA) or primary aliphatic amines such as hexamethylenediamine or isophoronediamine.
As has been determined in experiments, amine formation is influenced by a series of parameters. In particular, high temperatures above 80° C. combined with high atmospheric humidity lead to hydrolytic cleavage of the urethane and urea bonds. Such conditions are important for some specific applications of flexible PUR foams.
DE-A 42 32 420 discloses the use of &agr;,&bgr;-unsaturated ester carboxylates, which have been used as catalysts in addition to amines, for producing polyurethane foams which have improved compressive strength and elongation at break. The olefinic double bonds of the ester carboxylates are said to remove amines by addition onto the double bond. U.S. Pat. No. 4,255,526 describes the use of amino acids in the production of polyurethane foams to increase the stability toward moisture and heat.
Disadvantages of these known teachings is that the materials used are relatively expensive and, in addition, according to the prior art are not added until the production of the polyisocyanate polyaddition products. It has hitherto not been possible to develop hydrolysis stabilizers which do not develop their activity until after the processing stage. In the majority of cases, the materials used hitherto enter directly into the course of the reaction in the processing of isocyanates and compounds which are reactive toward isocyanates, alter the reactivity of these components toward one another and make further system modification unavoidable. The catalyst-acid complexes described in DE-A 23 50 684 are likewise added to the starting components in the production of polyurethanes. An addition of a hydrolysis stabilizer which does not influence the reactivity of the system during the processing stage and is added via the isocyanate component is not known.
EP-A 711 799 describes the production of polyurethane moldings having a cellular core and a compacted surface zone which are produced in the presence of polymeric carboxylic acids or their derivatives, with the polymers being added to the component which is reactive toward isocyanates. The object of that document was to replace chlorofluorocarbons as blowing agents and to produce moldings having an improved skin. The problem of the aging processes in polyurethanes is not addressed in this document.
In the production of polyurethane systems, use is frequently made of catalysts, for example organic amines, which, in the production of polyurethane foams, preferably accelerate both the blowing reaction, i.e. the reaction of the isocyanate groups with, for example, water to form carbon dioxide and also the crosslinking reaction between alcoholic hydroxyl groups and isocyanates to form urethane groups. To improve the flow and curing of reaction mixtures, it can be advantageous, particularly in the production of foamed polyurethanes, to use the amines in a form which is blocked by salt formation with an organic acid, customarily formic, acetic or ethylhexanoic acid. During the polyisocyanate polyaddition reaction, the catalysts which are blocked in a thermally reversible way decompose under the action of the heat of reaction, the catalytically active amine is set free and the crosslinking—or foaming reaction commences at an increased rate only after sufficient cream and rise times of the reaction mixture. Catalysts of this type are described in DE-A 28 12 256.
A disadvantage of this use of delayed-action catalysts is that these catalysts are used in an equimolar ratio of basic catalyst to blocking acid and, after catalysis has taken place, the catalyst is present in unblocked form in the polyisocyanate polyaddition product. It should also be noted that the catalysts are usually blocked by means of volatile acids and the latter vaporize from the system as a result of the high temperatures during the processing stage and are no longer available for blocking the catalyst. Furthermore, in the majority of cases it is not possible to use only blocked catalysts because otherwise the reaction becomes too slow, so that the total amount of catalyst remaining in the system is never blocked and very large proportions of free catalyst can catalyze the cleavage of urethane groups.
It is an object of the present invention to develop a mixture which, in the polyisocyanate polyaddition reaction, leads to products having an improved stability to aging processes, in particular to hydrolysis. A further object is to develop a stabilizer which makes it possible to suppress hydrolysis in polyether urethanes and thus also prevent aromatic amines from being liberated.
We have found that this object is achieved by the mixtures described at the outset, which can be advantageously used as components in the production of polyisocyanate polyaddition products.
It was surprisingly found that an amine catalyst present in the production of polyisocyanate polyaddition products not only catalyzes the polyaddition reaction, i.e. accelerates the formation of urethane groups, but also, after the polyaddition reaction is complete, catalyzes the cleavage of the urethane bonds to an increased degree. This applies particularly when the polyisocyanate products are stored under moist and warm conditions and is made worse by the fact that the catalyst, after the production of the polyisocyanate polyaddition products, the catalyst is present in unblocked and therefore active form and catalyzes the redissociation. The cleavage of the urethane bond leads not only to impairment of the properties of the polyisocyanate polyaddition products but can also lead to the formation of amines which are undesirable.
As a result of the use according to the present invention of (ii) at least one acid anhydride, the anhydrides in the polyisocyanate polyaddition products are hydrolyzed to the acids especially under moist and warm conditions. These acids formed after hydrolysis block any amine catalysts present in the products, for example by protonation or reaction, and thus effectively prevent redissociation of the urethane and/or urea bonds under the abovementioned conditions. In addition, any free amino groups formed by undesired cleavage of urethane and/or urea bonds are bound by reaction with the acids anhydride
Arlt Andreas
Kreyenschmidt Martin
Lorenz Reinhard
Treuling Ulrich
BASF Corporation
Borrego Fernando A.
Gorr Rachel
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