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...
Reexamination Certificate
1999-12-03
2002-10-29
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, C524S839000, C524S840000
Reexamination Certificate
active
06472465
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a coating composition and to its use, especially for painting bodies in the automotive industry.
2. Discussion of the Prior Art
A major problem associated with the painting of car bodies is the stone-chip resistance of the overall paint system and also the UV protection of the electrophoretically applied coat (CED coat).
A conventional car paint finish in accordance with the so-called basecoat/clearcoat process with sufficient stone-chip resistance and good UV protection consists of a total of four different coats (four-coat structure). These four coats are applied in succession in separate coating units. The first coat, situated directly on the metal panel of the car, is an electrophoretically applied coat (electrocoat, CED coat) which is applied by electrodeposition coating primarily cathodic deposition coating (CED)—for the purpose of corrosion protection and is subsequently stoved.
The second coat, situated on the electrocoat and having a thickness of from about 30 to 40 &mgr;m, is a so-called surfacer coat which firstly offers protection against mechanical attack (anti-stone-chip function) and secondly evens out the rough surface of the unfinished body for the subsequent topcoat, levels minor unevennesses and protects the electrophoretically deposited coat (CED coat) against natural UV radiation. This coat is produced in the majority of cases by application of stoving enamel, using electrostatic high-speed rotary bells, for example, with a subsequent stoving operation at temperatures of more than 130° C.
The third coat, situated on the surfacer coat, is the basecoat, which by means of appropriate pigments gives the body the desired colour. The basecoat is applied in the conventional spraying process. Depending on shade, the thickness of this conventional basecoat is from about 12 to 25 &mgr;m. This coat is usually applied in two process steps: for example, in a first step by application using electrostatic high-speed rotary bells, followed by a second application by means of pneumatic atomization. This coat is subsequently subjected to intermediate drying using infrared radiators and/or by means of hot-air convection.
The fourth and topmost coat, situated on the basecoat, is the clearcoat, which is usually applied in one application by means of electrostatic high-speed rotary bells. It gives the body the desired gloss and protects the basecoat against environmental influences (UV radiation, salt water, etc). Subsequently, the basecoat and the clearcoat are stoved together.
It is a concern of the automotive industry to provide motor vehicle paint systems whose overall level of properties is comparable with that of the prior art but which exhibit a reduced thickness of the overall paint structure, which minimize the effort required to produce the overall paint system, and which can be carried out with as few stoving steps as possible.
In this context, the aim is for a paint system consisting of three different coats (three-coat structure) in which the surfacer coat of the conventional four-coat structure is omitted and the basecoat is applied directly to the electrophoretically deposited first coat. This means that the basecoat, previously accorded primarily decorative properties, is now required to take over the functional properties of the previous surfacer coat as well.
A key concern of the automotive industry in respect of this three-coat structure is to increase the thickness of the basecoat in order to protect the CED coat more effectively against UV radiation. Although the topcoat usually includes a UV absorber, this absorber serves primarily to protect the clearcoat against free-radical degradation and to protect the basecoat, especially the organic pigments of the basecoat. This UV absorber does not offer sufficient protection for the CED coat.
The problem with simply increasing the thickness of the basecoat is that there is a drastic deterioration in the boil and run resistance. By boil resistance of a coating material is meant the fact that the applied coat does not form bubbles during the stoving operation; run resistance means that the applied coat shows no sagging during the stoving operation.
The attempts to date to apply conventional basecoat directly to the CED coat resulted in drastically impaired properties of the resultant coat structure, and in particular resulted in inadequate behaviour with respect to stone chipping.
From EP-A-0 265 363 it is known to provide a stoved, cataphoretically applied primer (CED coat) with a conventional two-coat basecoat/clearcoat finish applied by the wet-on-wet process. The disadvantage is that stone-chip damage on motor vehicles painted by this process is evident to a particularly marked extent, since topcoat delamination due to stone chipping exposes large areas of the primer coat.
From DE 195 12 017 it is known to apply a conventional basecoat wet-on-wet directly to the CED coat and to stove the two coats together. Over this stoved coat, a further conventional, waterborne base-coat is applied wet-on-wet with a topcoat, and stoved. The disadvantage associated with this process is that it requires the fundamental reorganization, and rebuilding at considerable expense, of the existing coating lines of the car manufacturers.
Furthermore, there are efforts to provide a specific basecoat which exhibits both the required decorative properties and functional properties.
For instance, DE 195 29 394 discloses the use of a specific aqueous basecoat, containing free isocyanate groups, which is applied directly to the stoved CED coat. However, this basecoat has the disadvantage that it is not stable on storage and can be processed only using a two-component unit.
Furthermore, DE 195 04 947 discloses conditioning a basecoat by means of a compensation coating material in such a way as to give the desired properties. However, it is not possible to achieve a coat thickness sufficiently high for adequate UV protection.
SUMMARY AND DESCRIPTION OF THE INVENTION
The object of the present invention is to provide a coating composition which can be used for automotive OEM finishing and which when so used imparts both the decorative properties of the conventional basecoat and the functional properties of the conventional surfacer coat in the conventional four-coat structure.
Furthermore, it must be possible to apply this coating composition in a thickness such as on the one hand to ensure effective UV protection of the first CED coat without on the other hand the occurrence at such coat thicknesses of the known problems in respect of boil and run resistance, as compared with conventional basecoats of the prior art.
This object is achieved in accordance with the invention by a coating composition obtainable by mixing
a) a composition A, this composition comprising a dispersion of a polyurethane having an acid number of more than 20 and comprising as its soft segment at least one polytetrahydrofuran segment having a number-average molecular weight of between 650 and 5000 and as its hard segment at least one segment derived from a diisocyanate; with
b) a composition B, this composition comprising a dispersion of a polyurethane having an acid number of less than 20 and comprising as its soft segment at least one polytetrahydrofuran segment having a number-average molecular weight of between 800 and 5000 and as its hard segment at least one segment derived from a diisocyanate.
The acid number is established by adding an appropriate amount of a compound which contains at least two groups which react with isocyanate groups (isocyanate-reactive groups) and at least one group which is capable of forming anions (anion-forming group) to the polyurethane dispersion that is to be adjusted. Examples of a suitable compound which contains at least two isocyanate-reactive groups and at least one anion-forming group are dihydroxypropionic acid, dimethylolpropionic acid, dihydroxysuccinic acid or dihydroxybenzoic acid. Examples of amino-containing compounds are &agr;,&dgr;-di
Hille Hans-Dieter
Müller Horst
Bollig & Kemper
Cohen & Pontani, Lieberman & Pavane
Niland Patrick D.
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