Aqueous 2-component PU systems

Details Aqueous 2-component PU systems Aqueous 2-component PU systems

2001-08-30

2003-06-24

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, C524S507000, C524S590000, C524S591000, C524S839000, C524S840000, C525S123000, C525S127000, C525S455000

Reexamination Certificate

active

06583215

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to aqueous 2-component polyurethane (PU) coating compositions containing specially modified vinyl polymer polyol dispersions and polyisocyanates, a process for the preparation of such aqueous 2-component PU systems with improved stability in respect of aqueous chemicals, and their use as 2-component PU paints for painting any type of substrate, e.g. wood, metal, plastic.
BACKGROUND OF THE INVENTION
EP-A 0 358 979, EP-A 0 496 210 and EP-A 0 557 844 and other patent literature describe the preparation of aqueous 2-component PU dispersions in which both so-called secondary dispersions and so-called primary dispersions as polyol components are used with suitable polyisocyanates. The term “secondary dispersions” refers to such aqueous dispersions which are first polymerized in the homogeneous organic medium and then redispersed in the aqueous medium with neutralization, generally without the addition of external emulsifiers.
The term “primary dispersions” refers to polyol dispersions that are prepared directly in the aqueous phase by the method of emulsion polymerization. They generally contain external emulsifiers for electrostatic or steric stabilization.
Because of their relatively low molecular weights, M
n
generally below 5000 g/mol and M
w
generally below 30,000 g/mol, and by reason of their balanced hydrophilic/lipophilic character, the secondary dispersions are extremely suitable for effecting a stable emulsification of both hydrophilic and hydrophobic non-self dispersing polyisocyanates in an aqueous environment and at the same time for acting as reactive components (cf. EP-A 0 358 979). The relatively low molecular weights of the secondary polyol dispersions means that such aqueous 2-component PU systems are generally not sufficiently suitable, however, in terms of their physical surface drying on wood as a substrate. The drying times (touch and dust drying) are too long and are unsuitable for industrial painting.
Examples containing primary dispersions as polyol components are generally more suitable as physically quick drying aqueous 2-component PU systems. These generally display molecular weight values of M
n
significantly higher than 5000 g/mol and M
w
values of generally above 30,000 g/mol. These primary dispersions can generally only be combined with (partially) hydrophilically modified polyisocyanates; a relatively uncomplicated dispersion can be performed with reasonably simple mixing equipment or by hand with a glass rod (as described in EP-A 0 557 844). If, for example, polyisocyanates hydrophilcally modified with polyether groups are then used as crosslinking agents and such coatings are applied to wood as a substrate, then paint films are obtained after drying which retain lasting discoloration under the action of color-imparting substances such as e.g. red wine, coffee, tea, mustard, etc., which on furniture in particular can lead to permanent staining. Such coating systems also display slight haze. If, alternatively, polyisocyanates containing external or internal anionic groups are used, the stain resistance improves but the level of resistance is still not satisfactory. Moreover, the mutual compatibility of the components in such systems deteriorates, and the films exhibit increased haze.
SUMMARY OF THE INVENTION
It was an object of the present invention to provide polyol dispersions as components for 2-component PU systems which form stain-resistant, haze-free coating films with polyisocyanate components containing anionic groups.
This object could be achieved by combining aqueous polyol dispersions, preferably of the primary dispersion type produced in the presence of special non-ionic surfactants, with polyisocyanates displaying both a hydrophobic character and a non-ionically or anionically modified hydrophilic character. This fact was all the more surprising because, as has already been mentioned above, polyol dispersions without such special non-ionic surfactants in combination with non-ionically modified hydrophilic polyisocyanates lead to paints with unsatisfactory stability values in respect of aqueous color-imparting liquids, and further addition of non-ionic surfactants to the 2-component PU system would have been expected to result in a further marked deterioration in resistance values.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to an aqueous 2-component polyurethane (PU) coating composition containing
a) a vinyl polymer polyol dispersion, having a hydroxyl value between 10 and 264 mg KOH/g solid resin, an acid value (calculated from the sum of neutralized and non-neutralized acid groups) between 0 and 55 mg KOH/g solid resin, a number-average molecular weight M
n
of at least 5000 g/mol and a weight average molecular weight M
w
of at least 30,000 g/mol, a glass transition temperature of at least 20° C. and an average particle diameter of no greater than 300 nm, which is prepared in the presence of
a1) 0.1 to 10.0 wt. %, relative to the sum of the solids contents of polymer polyol and polyisocyanate, of a non-ionic polyether surfactant,
and
b) a polyisocyanate having a viscosity at 23° C. of no more than 12,000 mPa*s and either a non-hydrophilic or a non-ionically or anionically hydrophilic character, and
whereby
the equivalent ratio of isocyanate groups of component b) to hydroxyl groups of component a) is 0.2:1 to 5:1.
The invention also relates to a process for the preparation of such aqueous 2-component PU systems.
Component a) is preferably produced in the presence of
a1) 1.0 to 8.0 wt. % of the polyether-type non-ionic surfactant. Block polymers of ethylene oxide and propylene oxide according to formula (I)
 and/or block and random ethylene oxide/propylene oxide copolymers based on fatty alcohols (II), and/or block polymers of the type according to formula (III), which can be obtained by polycondensation of propylene oxide and ethylene oxide on ethylene diamine,
 and/or adducts of polyethylene oxide on fatty alcohols (IV) and/or polyether/polyester block polymers (V), are preferably produced or used as surfactant components a1).
Surfactant classes (I), (II), (III) and (IV) are particularly preferred as surfactant components a1). These non-ionic surfactants generally display a polyethylene oxide content of 5 to 80 wt. %, molecular weights up to 10,000 g/mol and hydroxyl contents of between 0.3 and 8.0 wt. %.
Component a) is produced in such a way that the non-ionic surfactant a1) is added either before or during polymerization. The introduction of a1) into the polymer batch prior to the production of a) is preferred.
The polyisocyanate component b) can display either
b1) a non-hydrophilic character or
b2) a non-ionically modified hydrophilic character and/or alternatively
b3) an anionically modified hydrophilic character.
If a non-hydrophilic polyisocyanate component b1) that is insoluble or non-dispersible in water is used in the aqueous 2-component PU system, a good dispersing effect can only be achieved using a highly efficient dispersing unit, e.g. a jet dispersing unit according to EP-A 0 685 544. Only in this way can the maximum possible chemical resistance values of the 2-component PU system be achieved after application of the film.
If, however, the polyisocyanate component b) is to be dispersed with the polyol component effectively and in a fine-particle manner using simpler dispersing units, such as e.g. an agitator, or using a 2-component plant with pre-atomising nozzle or possibly even manually, a non-ionic or an anionic hydrophilization of b) (as component b2) or b3)) is absolutely essential.
The non-ionic hydrophilisation of the polyisocyanate component b) is generally performed by modifying an unmodified hydrophobic polyisocyanate with a polyether monoalcohol, e.g. according to EP-A 0 540 985 and EP-A 0 959 087.
The anionic hydrophilization of component b) is preferably performed by adding 0.2 to 5.0 wt. % (relative to the sum of the solids contents of polymer polyol and polyisocyanate) of surfactants containing sulfate, sulfonate or phosphate g

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