Weathering-stable, radiation-curable polyurethanes

Details Weathering-stable, radiation-curable polyurethanes Weathering-stable, radiation-curable polyurethanes

2001-01-03

2002-09-03

Berman, Susan W. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C522S085000, C522S086000, C522S090000, C522S096000, C522S075000, C522S078000, C524S839000, C528S065000

Reexamination Certificate

active

06444721

ABSTRACT:

The invention relates to water-dispersible, radiation-curable polyurethanes possessing improved weathering stability. The invention likewise relates to dispersions comprising such polyurethanes. The invention further relates to a method of coating substrates using the polyurethane dispersions, to the use of the polyurethanes or their dispersions as coating compositions for exterior applications, and to substrates coated with the polyurethanes.
Water-dispersible, radiation-curable polyurethanes are known, for example, from U.S. Pat. No. 5,905,113, EP-A-801092 and EP-A-704469. They are generally obtainable by reacting a mixture of a chain extender, i.e., a component containing at least two OH groups or at least two NH
2
groups, at least one compound containing dispersing-active groups, i.e., ionic or ionizable groups, and containing an isocyanate-reactive group, and at least one hydroxyalkyl ester of an ethylenically unsaturated carboxylic acid, especially hydroxyethyl acrylate, with a compound containing at least two isocyanate groups. Bifunctional compounds containing two OH groups that are frequently used are hydroxyl-bearing polyesters or hydroxyl-bearing polyethers. When using hydroxyl-bearing polyethers, however, unsatisfactory weathering stabilities are obtained. Polyurethanes containing polyester structures are unstable to hydrolysis and therefore unsuited to producing weather-stable coatings. Moreover, the surface tack of the coatings obtained is high. The use of polycaprolactones leads to improved weathering stability.
Aqueous polyurethane dispersions are used predominantly to coat wood, leather and paper in the interior sector. The use of radiation-curable coatings in the exterior sector is still problematic, since aqueous dispersions in particular do not have the desired weathering and chemical stability.
U.S. Pat. No. 5,859,135 discloses aqueous dispersions of polymers containing functional crosslinkable groups and possessing lipophilic main chains, based in particular on polyesters. Owing to the high molecular weight of the disclosed lipophilic main chains, however, the crosslinking densities achieved are inadequate for good scratch resistance and chemical stability.
EP-A-26313 and EP-A-554 784 disclose aqueous polyurethane dispersions and their use as coating compositions. The presence of sulfonic acid groups and/or sulfonate groups gives these polyurethanes a high hydrophilicity, leading to inadequate wet strength of coatings produced from them.
Polyurethanes containing carboxylic acid groups as dispersing-active component have also to date been unsatisfactory in their performance properties. For instance, polyurethane dispersions in accordance with EP-A-392 352, EP-A-181 486 and EP-A-209 684 exhibit disadvantages in terms of chemical resistance and mechanical properties, such as hardness, elasticity and flexibility of coatings for example.
Water-dispersible, radiation-curable polyurethanes are also known from EP-A-704469. The polyurethanes are composed of organic polyisocyanates, polyesterpolyols, compounds containing at least one isocyanate-reactive group and at least one carboxyl or carboxylate group, compounds containing at least one isocyanate-reactive group and at least one copolymerizable unsaturated group, and further components if desired. These polyurethanes represent an improvement in terms of wet strength, chemical resistance and mechanical properties of the coatings produced with them.
There continues to be a need for further improvement, especially as regards weathering stability, chemical stability, scratch resistance, flexibility and other mechanical properties.
It is an object of the present invention to provide weathering-stable, radiation-curable, polyurethanes which give coatings possessing good weathering stability and good chemical resistance and are therefore suitable for exterior applications. Moreover, it is intended that the coatings should have high scratch resistance and good mechanical properties.
A further object of the present invention is to provide polyurethane dispersions comprising such polyurethanes and a process for preparing these polyurethane dispersions.
We have found that these objects are achieved by means of a water-dispersible, radiation-curable polyurethane composed essentially of:
a) organic, aliphatic or alicyclic polyisocyanates,
b) cycloaliphatic diols and/or cycloaliphatic diamines,
c) compounds containing at least one isocyanate-reactive group and at least one free-radically polymerizable unsaturated group,
d) compounds containing at least one isocyanate-reactive group and at least one dispersing-active group.
Preferably, the water-dispersible, radiation-curable polyurethane further comprises one or more of the following constituents:
e) compounds containing at least two isocyanate-reactive groups having a molecular weight <1000 g/mol, preferably <500 g/mol,
f) compounds other than b) to d) containing at least one isocyanate-reactive group.
The dispersions of the polyurethanes, optionally, comprise
g) from 0 to 10% by weight of one or more photochemically and/or thermally activatable initiators,
h) from 0 to 10% by weight of UV absorbers and light stabilizer compounds based on sterically hindered amines.
The polyurethanes, and their aqueous dispersions, are suitable as coating compositions for exterior applications. The coatings obtained exhibit high weathering stability and chemical resistance and no surface tack.
Component a)
Suitable polyisocyanates a) are preferably those having an NCO functionality of at least 2, in particular at least 3.
Suitable examples include linear or branched C
4
--C
14
alkylene diisocyanates, cycloaliphatic diisocyanates having in all 6 to 12 carbon atoms, polyisocyanates containing isocyanurate groups, uretdione diisocyanates, polyisocyanates containing biuret groups, polyisocyanates containing urethane and/or allophanate groups, polyisocyanates containing oxadiazinetrione groups, uretonimine-modified polyisocyanates, or mixtures thereof.
Examples of suitable diisocyanates that may be mentioned include tetramethylene diisocyanate, hexamethylene diisocyanate(1,6-diisocyanatohexane; HDI), octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, trimethylhexane diisocyanate or tetramethylhexane diisocyanate, cycloaliphatic diisocyanates such as 1,4-, 1,3- or 1,2-diisocyanatocyclohexane, 4,4′-di(isocyanatocyclohexyl)methane, 1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophorone diisocyanate; IPDI) or 2,4- or 2,6-diisocyanato-1-methylcyclohexane.
The polyisocyanates containing isocyanurate groups comprise, in particular, simple trisisocyanato isocyanurates, which constitute cyclic trimers of the diisocyanates, or mixtures with their higher homologues containing more than one isocyanurate ring. The isocyanato isocyanurates generally have an NCO content of from 10 to 30% by weight, in particular from 15 to 25% by weight, and an average NCO functionality of from 3 to 4.5.
The uretdione diisocyanates are cyclic dimerization products of diisocyanates. The uretdione diisocyanates may be used, for example, as sole component or in a mixture of other polyisocyanates, especially the isocyanates containing isocyanurate groups.
Suitable polyisocyanates containing biuret groups preferably have an NCO content of from 18 to 22% by weight and an average NCO functionality of 3 to 4.5.
Polyisocyanates containing urethane and/or allophanate groups may be obtained, for example, by reacting excess amounts of diisocyanates with simple polyhydric alcohols such as trimethylolpropane, glycerol, 1,2-dihydroxypropane or mixtures thereof. These polyisocyanates containing urethane and/or allophanate groups generally have an NCO content of from 12 to 20% by weight and an average NCO functionality of from 2.5 to 3.
Polyisocyanates containing oxadiazinetrione groups may be prepared from diisocyanate and carbon dioxide.
Said polyisocyanates may also be used with preference in a mixture.
Preferred polyisocyanates are diisocyan

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