Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-05-06
2004-08-31
Niland, Patrick D. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C427S372200, C427S385500, C428S423100, C524S590000, C524S591000, C524S839000, C524S840000
Reexamination Certificate
active
06784242
ABSTRACT:
This invention relates to a polyurethane binder and to processes for producing a low-viscosity polyurethane binder containing isocyanate groups which, despite its low viscosity, only has a low content of readily volatile residual monomers and essentially forms no “migrates”. The invention also relates to the use of a low-viscosity polyurethane binder containing isocyanate groups (NCO groups) in the production of adhesives, more particularly one-component and two-component adhesives, for example for bonding web-form materials of paper, plastic or aluminium or mixtures of two or more thereof, coatings, more particularly lacquers, emulsion paints and casting resins as well as moldings.
Isocyanate-terminated polyurethane prepolymers have been known for some time. They may readily be chain-extended or crosslinked with suitable hardeners, usually polyhydric alcohols, to form high molecular weight materials. Polyurethane prepolymers have acquired significance in many fields of application, including for example the production of adhesives, coatings, casting resins and moldings.
In order to obtain isocyanate-terminated polyurethane prepolymers, it is standard practice to react polyhydric alcohols with an excess of polyisocyanates, generally at least predominantly diisocyanates. Molecular weight can be controlled at least approximately through the ratio of OH groups to isocyanate groups. Whereas a ratio of OH groups to isocyanate groups of, or approaching, 1:1 leads to generally high molecular weights, a statistical average of one diisocyanate molecule—where diisocyanates are used—is attached to each OH group where the OH: isocyanate group ratio is about 2:1, so that ideally no oligomerization or chain extension occurs in the course of the reaction.
In practice, however, chain-extending reactions are impossible to suppress with the result that, on completion of the reaction, a certain quantity of the component used in excess is left over irrespective of the reaction time. If diisocyanate, for example, is used as the excess component, a generally considerable proportion of this component remains behind in the reaction mixture for the reasons explained above.
The presence of such components is particularly problematical when they consist of readily volatile diisocyanates. The vapors of these diisocyanates are often harmful to the skin and the application of products with a high content of such readily volatile diisocyanates requires elaborate measures on the part of the user to protect the people involved in processing the product, more particularly elaborate measures for keeping the surrounding air clean to breathe.
Since protective measures and cleaning measures generally involve considerable expense, there is a need on the part of the user for products which have a low percentage content of readily volatile diisocyanates depending on the isocyanate used.
In the context of the present invention, “readily volatile” substances are understood to be substances which have a vapor pressure at around 30° C. of more than about 0.0007 mmHg or a boiling point of less than about 190° C. (70 mPa).
If low-volatility diisocyanates, more particularly the widely used bicyclic diisocyanates, for example diphenyl methane diisocyanates, are used instead of the readily volatile diisocyanates, polyurethane binders with a viscosity normally outside the range suitable for simple processing methods are generally obtained. In cases such as these, the viscosity of the polyurethane prepolymers can be reduced by adding suitable solvents although this is not consistent with the absence of solvents normally demanded. Another way of reducing viscosity without solvents is to add an excess of monomeric polyisocyanates which are incorporated in the coating or bond (reactive diluent) in the course of a subsequent curing/hardening process (after the addition of a hardener or by curing under the influence of moisture).
Whereas the viscosity of the polyurethane prepolymers can actually be reduced in this way, the generally incomplete reaction of the reactive diluent often leads to the presence in the bond or coating of free monomeric polyisocyanates which are capable of “migrating”, for example within the coating or bond or, in some cases, even into the coated or bonded materials themselves. Corresponding constituents of a coating or bond are often referred to among experts as “migrates”. By contact with moisture, the isocyanate groups of the migrates are continuously reacted to form amino groups. The aromatic amines normally formed in this way are suspected of having a carcinogenic effect.
Migrates are often not tolerable, above all in the packaging field, because any migration of the migrates through the packaging material would result in contamination of the packaged product and the consumer would inevitably come into contact with the migrates when using the product.
Accordingly, the migrates in question are undesirable above all in the packaging field, especially in the packaging of foods.
In order to avoid the disadvantages described above, EP-A 0 118 065 proposes producing polyurethane prepolymers by a two-stage process. In the first stage of this process, a monocyclic diisocyanate is reacted with a polyhydric alcohol in an OH group: isocyanate group ratio of <1 and, in the second step, a bicyclic diisocyanate is reacted with polyhydric alcohols in an OH group: isocyanate group ratio of <1 in the presence of the prepolymer prepared in the first step. A ratio of OH groups to isocyanate groups of 0.65 to 0.8:1 and preferably 0.7 to 0.75:1 is proposed for the second stage. The prepolymers obtainable in this way still have viscosities of 2500 mPas, 7150 mPas and 9260 mPas at high temperatures (75° C. and 90° C.).
DE-A 34 01 129 relates to a process for the production of mixed polyurethane prepolymers in which polyhydric alcohols are first reacted with the faster reacting isocyanate group of an asymmetrical diisocyanate, the more slowly reacting group being left intact, after which the reaction products are combined with a symmetrical diisocyanate of which the equally reactive isocyanate groups react more quickly than the slowly reacting groups of the first polyfunctional isocyanate compound mentioned. The described polyurethane prepolymers have high viscosities and hence high processing temperatures so that they can only be used under conditions which allow high processing temperatures.
EP-A 0 019 120 relates to a two-stage process for the production of elastic weather-resistant sheet-form materials. In the first stage of this process, toluene diisocyanate (TDI) is reacted with at least equimolar quantities of a polyol and the reaction product obtained is subsequently reacted with diphenyl methane diisocyanate (MDI) and a polyol. The polyurethane binders obtainable in this way are said to be capable of curing with water or with atmospheric moisture. Although the described process does give products with a relatively low viscosity, the content of free readily volatile diisocyanate (in the present case TDI) is still high (0.7% by weight) and can only be reduced when time-consuming and energy-intensive methods, for example thin-layer distillation, are used to remove excess readily volatile diisocyanate.
In many cases, laminated films are used in applications involving elevated temperatures, for example in the preparation of foods. Unfortunately, laminated films produced with conventional adhesives often undergo delamination whenever they are exposed to the temperatures normally prevailing in the preparation of foods.
Accordingly, the problem addressed by the present invention was to provide a polyurethane binder which would have a low viscosity and a low residual content of less than about 1% by weight of readily volatile diisocyanates. In the case of toluene diisocyanate (TDI), the residual content of readily volatile isocyanate should be less than about 0.1% by weight.
Another problem addressed by the present invention was to provide a polyurethane binder which would enable processing to be carried out at low tempe
Bauer Andreas
Henke Guenter
Huebener Achim
Harper Stephen D.
Henkel Kommanditgesellschaft auf Aktien
Niland Patrick D.
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