Process for the preparation of the top coat layer of an automotive OEM multi-layer coating
A coating process wherein a precoated automotive substrate is OEM top coated with an aqueous two-component polyurethane top coating composition prepared by static mixing a non-aqueous hydrophobic polyisocyanate crosslinker component and an aqueous binder component having a binder solids content comprising at least one water-dilutable hydroxyl-functional binder and 5 to 30 wt.-% of a urethane compound A with at least two terminal urethane groups (—NHC(O)OR; R=hydrocarbyl residue of a monoalcohol) and having a number-average molar mass (Mn) of from 400 to 5,000.
1. A process for the preparation of the outer top coat layer of an automotive Original Equipment Manufacture multi-layer coating comprising the steps:
(1) providing an automotive substrate to be Original Equipment Manufacture top coated,
(2) preparing an aqueous two-component polyurethane top coating composition by static mixing a non-aqueous hydrophobic polyisocyanate crosslinker component and an aqueous binder component having a binder solids content comprising at least one water-dilutable hydroxyl-functional binder,
(3) spray-applying the aqueous two-component polyurethane top coating composition on the automotive substrate to be Original Equipment Manufacture top coated, and
(4) thermally curing the top coat layer applied in step (3),
wherein the binder solids content of the aqueous binder component comprises 5 to 30 wt.-% of at least one urethane compound A with at least two terminal urethane groups (—NHC(O)OR; R=hydrocarbyl residue of a monoalcohol) and having a number-average molar mass (Mn) of from 400 to 5,000, and
wherein the binder solids content consists of 5 to 30 wt.-% of the at least one urethane compound A and 70 to 95 wt.-% of the at least one water-dilutable hydroxyl-functional binder, wherein the sum of the wt.-% totals 100 wt.-%.
2. The process of claim 1 , wherein the aqueous two-component polyurethane top coating composition is a clear top coating composition or a pigmented top coating composition.
3. The process of claim 1 , wherein the at least one urethane compound A contains 15 to 25 wt.-% of terminal urethane groups —NHC(O)OR, calculated as terminal —NHC(O)O—.
4. The process of claim 1 wherein the hydrocarbyl residue R has up to 12 carbon atoms.
5. The process of any one of claim 1 , wherein R is cyclohexyl.
6. The process of claim 1 , wherein the at least one urethane compound A is selected from the group consisting of trimerization products of 1,6-hexane diisocyanate fully blocked with cyclohexanol and trimerization products of isophorone diisocyanate fully blocked with cyclohexanol.
7. The process of claim 1 , wherein the at least one hydrophobic polyisocyanate of the non-aqueous hydrophobic polyisocyanate crosslinker component is selected from the group consisting of trimerization products of 1,6-hexane diisocyanate and trimerization products of isophorone diisocyanate.
8. The process of claim 1 , wherein the mixing is carried out in a ratio corresponding to a 1:1 to 1:2 stoichiometry between the hydroxyl groups originating from the binder solids of the aqueous binder component and the free isocyanate groups originating from the non-aqueous hydrophobic polyisocyanate crosslinker component.
9. The process of claim 1 , wherein the static mixer used in step (2) is a Kenics mixer.
10. The process of claim 1 , wherein the thermal curing is carried out at object temperatures of 80 to 160° C.
11. The process of claim 1 being carried out in the context of an industrial automotive Original Equipment Manufacture mass production coating process.