Coating composition

Details Coating composition Coating composition

1998-07-30

2000-01-11

Dudash, Diana

Coating processes

With post-treatment of coating or coating material

Heating or drying

4274071, 523201, 523406, 524458, 524501, 524522, 524523, 525902, 525309, 526273, B05D 136, B05D 716, C08F 2400, C08F26504

Patent

active

060133244

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an aqueous coating composition and to a process for preparing the composition. It also relates to a coating process using the coating composition and to a substrate coated by the coating process.
The painting of motor vehicles broadly involves three stages. The first stage is called pre-treatment in which the bare metal is treated with various chemicals to improve corrosion resistance and the adhesion of subsequently applied paint coatings. In the second stage, one or more coats of primer and/or surfacer are applied to even out small surface irregularities and provide a foundation for subsequent layers. In the final stage, a topcoat is applied. The topcoat gives the vehicle its final appearance and also must resist the elements and physical damage.
The topcoat on modern motor vehicles, particularly cars, is often applied using what is known as a basecoat-clearcoat process. This involves applying two different layers of paint. The first layer is a pigmented composition called a basecoat which gives the vehicle its colour as well as any other pigment effects such as a metallic or pearlescent effect. The second and final layer is an unpigmented composition called a clearcoat which provides the final glossy appearance.
European Patent EP-B-0 038 127 discloses and claims an entirely revolutionary waterborne basecoat which can be used in the original manufacture of motor vehicles. The Patent discloses a pseudoplastic or thixotropic composition which comprises an aqueous dispersion of acrylic polymer in water. The dispersed polymer particles are made from various ethylenic monomers and are internally crosslinked. The combination of the internal crosslinking and the rheological properties make these basecoats particularly good for use with metallic pigments. These basecoats give particularly good metal flake orientation and hence good metallic "flip" effect, and are very resistant to disturbance by the subsequently applied clearcoat.
European Patent EP-B-0 300 612 discloses waterborne basecoats comprising similar dispersions which have a crosslinked core and a non-crosslinked stabilising mantle having salt forming groups. These dispersions are used in combination with another different type of dispersion so as to give basecoats which can be used for the refinishing of motor vehicles.
We have now found that by including in part of the core a minimum proportion of monomers having a low water solubility, we can produce waterborne basecoats which, when overcoated with a clearcoat, will form topcoats having improved resistance to humidity and which have very good "flip" appearance.
Our new addition polymer microparticles are made by polymerising ethylenically unsaturated monomers and consist of a crosslinked core from which there extends a non-crosslinked stabilising mantle containing acid groups.
According to the present invention there is provided a sprayable aqueous coating composition for motor vehicles comprising an aqueous dispersion of addition polymer microparticles having a crosslinked core and a non-crosslinked stabilising mantle containing acid groups, characterised in that the crosslinked core comprises; least 10% by weight of monomers having a solubility in water of less than 0.1% by weight, 10:90 to 90:10, preferably 25:75 to 75:25, and the mantle has a Tg of -50.degree. C. to +50.degree. C.
The outer core preferably surrounds the inner core. The inner and outer core can have the same monomer composition in which case the crosslinked core is homogeneous. Preferably the inner and outer core have a different monomer composition.
The outer core preferably has a Tg of at least 5.degree. C. greater than that of the inner core, more preferably at least 10.degree. C. greater, still more preferably at least 20.degree. C. greater and most preferably at least 40.degree. C. greater.
The outer core preferably has a Tg of greater than 10.degree. C. The inner core preferably has a Tg of less than -10.degree. C., more preferably less than -25.degree. C.
The Tg of each portion of the micropartic

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