Water dilutable binders for soft feel coating materials

Details Water dilutable binders for soft feel coating materials Water dilutable binders for soft feel coating materials

2000-09-22

2002-07-02

Cain, Edward J. (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...

C524S590000, C524S591000

Reexamination Certificate

active

06414079

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to aqueous binders which can be formulated into coating compositions for any substrates, especially rigid substrates, yielding soft-feel coatings thereon.
2. Descirpition of the Prior Art
In the coating of hard substrates, such as plastics, there is frequently a desire for the coating to achieve a soft feel, where the coated surface, although feeling soft to the touch (like skin or leather), is unimpaired in terms of its mechanical and chemical resistance in comparison to the conventional coatings.
The so-called soft feel coating materials achieve this objective. The substrates coated with them have König pendulum hardnesses of less than 20 seconds for coating films approximately 50 &mgr;m in thickness. The surfaces are scratch resistant (scratch test with the fingernail) where scratching leaves a trace, it can be removed again simply by wiping with the fingertip. Without sticking or being tacky, the coated surfaces are slip resistant; for example, a glass or metal block does not slip on a surface coated with a soft feel coating material but instead tips when the angle of inclination is gradually increased. If a coating film is detached from a glass surface coated with it, and its extension is measured, a reversible extension of up to 500% is found on measurement at room temperature with a stretching rate of 0.6 cm/s.
Binders for these soft feel coating materials preferably have segments of polymers having a low glass transition temperature; the crosslinking density must still be high enough for there to be very little or no reduction at all in the mechanical and chemical resistance properties but low enough to allow the effect of the low glass transition temperature to be manifested macroscopically as well.
Two-component, or two-pack binders have been disclosed for soft feel coating materials that are based on hydroxyl-containing polyester-urethanes, which are to be cured preferably with isocyanates but also, for example, with acid anhydrides or amino resins.
It has proven impossible to date to provide practicable one-component, or one-pack binders for soft feel coating materials. Although crosslinking with amino resins may be effected at elevated temperature in the manner of a one-component system, the soft surface of the soft feel coating is adversely affected by the high temperatures required. Especially when coating plastics, the crosslinking temperature is limited by the softening temperature of the polymer substrate.
The known, polyisocyanate-based systems which cure at room temperature share the typical problems of isocyanate-cured systems: the limited pot life and limited acceptability from the standpoint of occupational hygiene. A further important disadvantage of isocyanate curing agents in conjunction with soft feel coating materials is the “aftercuring” which is observed. The polyfunctional isocyanates used for the crosslinking are usually employed in excess. They are able to react with water, or even with the atmospheric moisture, and do so with decarboxylation and formation of the corresponding amines, which in turn form ureas with isocyanates that have not yet reacted. This reaction occurs during the drying of the coating material and adversely affects the feel of the coating film, which is still soft directly following application and initial crosslinking.
There was therefore a need to develop a binder for soft feel coating materials which durably retains this soft feel. A further object was to provide a one-component binder which cures even at moderate temperatures (above 70° C., in particular from about 80 to about 150° C.) and yet imposes very little restriction on the pot life.
This object has been achieved by the provision of a reactive combination comprising a hydroxyl-containing polyester-urethane and a melamine resin.
SUMMARY OF THE INVENTION
The invention accordingly provides a water dilutable binder for soft feel coating materials, comprising an aqueous dispersion of a hydroxyl-containing polyester-urethane A, obtainable by reacting a polyester A1 with hydroxyl groups as functional groups, said polyester A1 having been synthesised from linear, branched or cyclic aliphatic or aromatic difunctional or polyfunctional carboxylic acids A11 and aliphatic linear or branched difunctional or polyfunctional alcohols A12 as reactants, with a polyfunctional isocyanate A2, a compound A3 having at least one hydroxyl, mercapto or amino group which is reactive toward isocyanates and having at least one acid group which reacts only to a minor extent if at all with isocyanates, and also, optionally, with a drying or, preferably, nondrying oil A4, an aqueous solution or dispersion of a triazine resin B which for each molecule derived from a triazine has between 2. and 6 groups selected from methylene groups, methylene ether groups, methylol groups and alkoxymethyl groups, wherein the glass transition temperature of the polyester-urethane A is between −70 and −20° C., preferably from −60 to −25° C., and in particular from −50 to −30° C., and wherein the hydroxyl number of the polyester-urethane A is from 10 to 120 mg/g, preferably from about 20 to about 100, and in particular from 25 to 95 mg/g.
The term “minor extent” as used herein means that, under otherwise identical reaction conditions, the reaction rate of the isocyanate-reactive hydroxyl, mercapto or amino groups is at least 5 times as great as that of the acid groups with respect to the same compound containing isocyanate groups.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The polyester A1 is prepared preferably in two stages in a known manner by condensing the entirety of the carboxylic acid component A11 with at least a portion A121 of the polyol component A12, the average functionality of A121 in the first stage being between 1.8 and 2.3, preferably from 1.9 to 2.1.
As component A11, it is preferred to use dicarboxylic acids having 4 to 40 carbon atoms, particular preference being given to succinic acid, adipic acid, hexahydrophthalic acid, cyclohexanedicarboxylic acid, and a mixture of branched aliphatic dicarboxylic acids obtainable by dimerising unsaturated fatty acids. Up to 10%, preferably up to 5%, of the mass of A11 may comprise one or more higher carboxylic acids having three or more carboxylic acid groups per molecule, such as tricarboxylic and tetracarboxylic acids.
As component A121, it is preferred to use difunctional alcohols having 2 to 20 carbon atoms. Glycol, 1,2- and 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, and cyclohexanedimethanol are especially suitable, as are the ether alcohols, diethylene glycol, triethylene glycol and higher oxyethylene diol oligomers up to a degree of polymerisation of approximately 10. It is also possible to use oligomeric propylene glycols; the mass fraction of the ether alcohols in component A121 is preferably up to 30% of the mass of component A121. In the context of the invention it is also possible to use alcohols having a functionality of three or more, and monofunctional alcohols, in each case individually or together, as constituents of component A121. In that case, the amount of these alcohols of higher or lower functionality must be chosen such that the average functionality satisfies the above condition.
Following the condensation of the preferably difunctional reactants of the first stage, the resulting polyester, which typically has an acid number of less than 10 mg/g, preferably less than 5 mg/g, is transesterified with a trifunctional or higher polyfunctional aliphatic alcohol or with a mixture of two or more aliphatic linear or branched alcohols having an average functionality of from 2.2 to 4.5, preferably from 2.5 to 4.0, and in particular from 2.8 to 3.8, referred to collectively below as A122. The component A122 preferably comprises trifunctional alcohols such as glycerol, trimethylolpropane and trimethylolethane, tetrafunctional alcohols such as pentaerythritol, ditrimethylolethane and ditrimethylolpropane, which in accordance wi

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