Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1998-10-02
2001-01-16
Gorgos, Kathryn (Department: 1744)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S499000, C204S489000, C502S151000, C502S152000
Reexamination Certificate
active
06174422
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to acid-free catalyst pastes, to a process for producing them and to their use in processes for cathodic electro-dip coating (CEC).
BACKGROUND OF THE INVENTION
It is generally customary to employ metal catalysts for the crosslinking of CEC coating layers in order to reduce the stoving temperature. In this connection, organotin catalysts have generally been used. The use of dialkyltin oxide is desirable, and the use of dibutyltin oxide (DBTO) is particularly desirable due to its ready availability and favourable price. However, in order that it can successfully be used in CEC coatings, DBTO requires conditioning. This conditioning may consist of a separate synthesis step, i.e. in the chemical conversion of the DBTO into a dibutyltin dicarboxylate for example.
Thus CEC coating media which crosslink by means of blocked isocyanates are known from EP-A-0 509 437. These contain dialkyltin dicarboxylates derived from aromatic carboxylic acids as crosslinking catalysts, in addition to bismuth or zirconium compounds as additional catalysts. Bismuth hydroxide, trioxide, nitrate, benzoate, citrate, oxycarbonate and silicate can be used as the bismuth compounds.
DE-A-39 40 781 describes a process for producing catalyst pastes containing dibutyltin oxide by the preliminary dispersion of DBTO with organic solvent and 0.05 to 5 moles, per mole of DBTO, of an organic acid which is customary for the neutralisation of cathodically depositable electro-dip coatings, at a water content of up to 5% by weight with respect to the sum of DBTO, solvent, acid and water, and the subsequent dispersion and comminution of the mixture obtained with grinding binder vehicles and water. Preliminary dispersion is preferably effected within a temperature range from 50 to 80° C. During the production of CEC coating media, the catalyst pastes drag in organic acid. However, low acid contents are wanted in CEC coating media, since they promote good wrap-around behaviour and low current consumption during cathodic lacquer deposition.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a catalyst paste containing tin which is introduced as dialkyltin oxide, particularly as DBTO. Production of the catalyst paste should be simple, and coating media produced using the catalyst paste should have a reduced content of acid which is dragged in by the paste.
It has surprisingly been shown that this object is achieved by the acid-free, aqueous catalyst paste with a solids content of 30 to 70% by weight and containing dialkyltin oxide and a water-thinnable cationic binder vehicle, to which the present invention relates, which is obtainable by
a) dispersing dialkyltin oxide in an acid-free aqueous dispersion or solution of one or more hydroxycarboxylic acid salts of bismuth(III) in a weight ratio of tin to bismuth, calculated as the metal, from 1:3 to 3:1, and
b) comminuting the preparation obtained with one or more cationic binder vehicles in a weight ratio of dialkyltin oxide plus hydroxycarboxylic acid salt of bismuth(III) to cationic binder vehicle from 0.5:1 to 6:1.
The present invention also relates to the process for producing the catalyst paste, which is characterised by the above process steps a) and b).
The process according to the invention is particularly suitable for producing catalyst pastes which contain DBTO and/or dioctyltin oxide (DOTO) as the dialkyltin oxide, for example.
The solutions used for producing the catalyst paste according to the invention, which include colloidal solutions or dispersions, are hereinafter called “aqueous preparations”. Examples of aqueous preparations of hydroxycarboxylic acid salts of bismuth (III) which can be employed include those which can be produced by the reaction of bismuth(III) oxide and/or hydroxide with a hydroxycarboxylic acid in a molar ratio of 1.0 to 3.0 carboxyl groups per mole of bismuth in the presence of water. The preferred hydroxycarboxylic acids for reaction with bismuth(III) oxide and/or hydroxide are aliphatic hydroxycarboxylic acids; aliphatic hydroxymonocarboxylic acids containing 3 to 8 C atoms are particularly preferred, especially lactic acid and dimethylolpropionic acid. One or more hydroxycarboxylic acids can be used in admixture. Aqueous preparations of hydroxycarboxylic acid salts of bismuth(III) which have a bismuth content between 2 and 25% by weight, calculated as the metal, are preferably used, particularly aqueous preparations of bismuth(III) dimethylolpropionate and/or lactate which have a bismuth content between 2 and 25% by weight.
The dialkyltin oxide, e.g. DBTO, is dispersed in the presence of the aqueous preparation of the hydroxycarboxylic acid salt of bismuth(III). In this respect, the weight ratio of tin to bismuth is between 1:3 and 3:1, calculated as the metal. The dispersion operation in process step a) can additionally and preferably be effected in the presence of a partial amount of the cationic binder vehicle. If a binder vehicle containing a basic group is used as the cationic binder vehicle, the basic groups of which are neutralised by acid, it is preferable to employ the partial amount used in process step a) in un-neutralised form. For example, up to 70% by weight of the amount of binder vehicle from process step b) which is used can be used in conjunction in process step a).
Customary binder vehicles which are suitable as CEC paste resins are preferably used for this purpose as the cationic binder vehicles, which are hereinafter called “grinding resins” for the sake of simplicity. However, these may also be other binder vehicles, such as those which are used in cathodic electro-dip lacquers for the formation of coatings; these are described later as examples of binder vehicles for CEC coating media. They are for the most part characterised in that they exhibit good solubility in water and also have good wettability
They are resins which contain basic groups and which are dispersible in water after neutralisation with an acid. Examples of preferred paste binder vehicles such as these are described in EP-A-0 183 025 or in EP-A-0 469 497, for example.
The dispersion operation in process step a) is effected, with or without the addition of a grinding resin, so that a suitable viscosity of the mixture is obtained, i.e. so that a free-flowing liquid material is produced which can be stirred in an agitator device such as a dissolver. A suitable viscosity can be obtained by the amount of added water.
In general, times of dispersion of 0.5 to 5 hours are sufficient, for example. During the dispersion operation, the temperature is preferably no higher than 40° C., most preferably no higher than 30° C. The lower limit is preferably 20° C., but may also be below this.
The dispersion obtained in process step a) is further dispersed and is optionally comminuted in process step b), after the addition of grinding resin, for example the residual amount of at least 30% by weight of the grinding resin, of an amount of acid sufficient to ensure water-thinnability for the grinding resin as a whole, and of water. In the course of this procedure, the acid is added, as a maximum, in the stoichiometric amount for neutralising all the neutralisable basic groups present in the grinding binder vehicle, and is preferably added as a deficit, e.g. as 40 to 95%, most preferably 40 to 80%, of the maximum stoichiometric amount. The addition of acid is not necessary if the grinding resin only contains cationic groups and no basic groups.
Organic acids such as formic acid, acetic acid, lactic acid and dimethylolpropionic acid are particularly suitable as acids for neutralising the grinding resin.
It is advantageous if the grinding resin, acid and water are added to the dispersion, with stirring, directly after the dispersion operation of process step a), without an extended holding period. For example, the grinding resin may be added first, followed by the acid and water, or a mixture of grinding resin, which is neutralised with acid, and water may be added. In particular, water is only used
B{umlaut over (u)}ttner Gabriele
Honig Helmut
Klein Klausjorg
Gorgos Kathryn
Herberts GmbH
Maisano J.
Pillsbury Madison & Sutro LLP
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