Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Reexamination Certificate
1996-01-22
2002-04-16
Gorr, Rachel (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
C528S048000, C524S773000, C524S875000, C524S591000, C524S071000, C428S425800
Reexamination Certificate
active
06372875
ABSTRACT:
The present invention relates to a process for the preparation of polyurethane resins, in which, in an ester as solvent,
a) at least one compound having 2 hydroxyl and/or amino groups,
b) at least one di- and/or polyisocyanate,
c) at least one compound containing 2 groups which are reactive toward isocyanate groups, these compounds containing, at least in part, at least one group capable of forming anions which is neutralized before or after incorporation of component (1 c) into the polyurethane molecule, and
d) at least one compound having at least 3 hydroxyl and/or amino groups are used to prepare a polyurethane resin, preferably using components (a), (b) and (c) to prepare an intermediate which has terminal isocyanate groups and at least some of whose free isocyanate groups are subsequently reacted with component (d).
The present invention also relates to the use of the polyurethane resins and to the use of ethoxyethyl propionate as solvent for the preparation of polyurethane resins.
The present invention consequently also relates to coating compositions containing these polyurethane resins, processes for the production of these coating compositions and processes for coating substrates using these coating compositions.
Aqueous polyurethane dispersions and processes for their preparation are known to those skilled in the art. Of particular technical importance is the so-called acetone process. In this process, generally, a prepolymer having NCO end groups is first prepared, which is subsequently dissolved in an inert solvent followed if appropriate by chain lengthening in solution to give a polyurethane of higher molecular weight. The additions made in the so-called acetone process of low-boiling solvents such as, for example, acetone are necessary inter alia in order to reduce the viscosity of the prepolymer and thus to render it capable of being handled, subsequent dispersion only then being possible.
The disadvantage of such preparation processes, taking into account the usually applicable requirement for solvent-free products, is that a technically complex distillation step has to follow the dispersion procedure in order to remove—at least predominantly—the low-boiling component such as acetone. Ketones, for example acetone, are in fact a cause of poor coatings properties in an aqueous system. In addition, ketones such as acetone are toxic to fish and are notable for relatively low flash points (acetone −18° C., methyl ethyl ketone −4.4° C. and methyl isobutyl ketone +15.5° C.). In the USA methyl ethyl ketone and methyl isobutyl ketone are on the list of hazardous air-polluting substances, so that even residual quantities in the coating should be avoided.
Replacing ketones by other solvents is carried out in a stripping procedure. This procedure involves removing the ketone as a mixture with the second solvent in vacuo. This implies an additional process step which not only complicates the process but also leads to an increase in the cost of the product. The latter not least because the acetone preferably employed cannot be recycled directly to the process, since anhydrous acetone is preferably employed. The question for the person skilled in the art is thus whether and, if so, to what extent a residual solvent content is acceptable, since it is on this that the complexity of the process depends. To fulfill the requirement for an entirely solvent-free product prepared by this process necessitates a comparatively high expenditure.
The desire is therefore for solvent substitutes which can be employed, instead of the solvents hitherto known and used, in the preparation of aqueous polyurethane dispersions. In this context the solvents used as an alternative to ketones must meet a number of requirements:
ignition temperatures >200° C.
inert toward isocyanates (including any impurities)
widely available industrially
adequate miscibility with water (may possibly be improved by cosolvents)
flash point >55° C.
suitable for coatings (ability to be evaporated)
favorable price
appropriate boiling point.
In addition to ketones, it is in principle also possible to employ ethers and esters for the synthesis of polyurethane resins. Thus DE-A-40 13 546 discloses the employment of propanediols and/or triols esterified with acetic acid as solvents for the preparation of polyurethane dispersions by processes which are known per se.
The use of these solvents described in DE-A-40 13 546, however, has the disadvantage that esters of acetic acid, such as methoxypropyl acetate, ethoxypropyl acetate and butoxypropyl acetate, are in general susceptible to hydrolysis. In addition they are not suitable for the preparation of polyurethanes at elevated temperatures of from 100 to 130° C. since even at these temperatures they undergo considerable decomposition in the reaction mixture. However, because of ever stricter environmental regulations the requirement is for polyurethane resin solutions with as high a solids content as possible but the preparation of such solutions necessitates the use of elevated temperatures in order to overcome the viscosity problems. In addition, higher temperatures are desirable because of the reduction in reaction time which is associated with them.
Also unsuitable in practice is, for example, methoxypropyl acetate, since at 50 ppm it has too low an MAC value (maximum workplace concentration). Furthermore, methoxypropyl acetate is classified in pregnancy group C. Ethoxypropyl acetate has the disadvantage that the resulting polyurethane solutions are of very high viscosity. Butoxypropyl acetate, finally, has to be ruled out because of its poor solubility in water. DE-A 41 10 520 discloses the preparation of polyurethane resins in solvents which remain in the coating material and do not have to be exchanged. Examples of suitable solvents which are specified are methoxypropyl acetate, ethoxyethyl acetate (common name for ethylene glycol monoethyl ether acetate) and N-methylpyrroline. The acetates specified, however, have the disadvantages already indicated. Moreover, ethoxyethyl acetate has the disadvantage that this solvent, because of the regulations which apply, cannot be used in the USA, for example. These acetates mentioned in DE-A 41 10 520 are therefore likewise not used in practice. Finally, N-methylpyrrolidone (NMP) has the disadvantage that it does not evaporate from the coating film, especially at low temperatures. In addition, the resins prepared in NMP have higher viscosities in comparison with the resins prepared in EEP. The color number of the resins prepared in NMP is, moreover, higher.
The object of the present invention is therefore to provide a process for the preparation of polyurethane resins which permits the cost-effective preparation of polyurethane dispersions and does not involve a stripping procedure. This process should also enable the preparation of polyurethane resin solutions having a high solids content. Furthermore, the polyurethane dispersions prepared by this process, when employed in aqueous coating compositions, should lead to coatings whose mechanical properties conform as closely as possible to the properties of coatings obtained using polyurethane dispersions which have been prepared conventionally.
This object is surprisingly achieved by a process of the type mentioned at the outset, which is characterized in that the polyurethane resin is prepared in ethoxyethyl propionate (EEP) as solvent.
The present invention also relates to the use of the polyurethane resins and to the use of ethoxyethyl propionate as solvent for the preparation of polyurethane resins. In addition, the invention also relates to coating compositions containing these polyurethane resins, to processes for the production of these coating compositions and to methods of coating substrates using these coating compositions, and to substrates coated with these coating compositions.
It is surprising and was not foreseeable that the use, specifically, of ethoxyethyl propionate as solvent for the preparation of polyurethane dispersions in processes kn
Grabbe Michael
Mayer Bernd
Meisenburg Uwe
Rink Heinz Peter
BASF Coatings AG
Gorr Rachel
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