Preparation and use of biuret-containing polyisocyanates as cross-linking agents for coatings
Biuret group-containing polyisocyanates are produced having an isocyanate functionality of at least 4 are prepared by reacting polyisocyanates having a functionality of at least 2.8 with water as the biuretizing agent. Most preferably water is used as the sole biuretizing agent. The preparation is useful as a curing agent in crosslinkable coating compositions. The coating composition can be used as an automotive clearcoat over a conventional pigmented basecoat, or as a basecoat or monocoat or even as a primer or sealer when a suitable amount of pigment is incorporated therein.
1 . A crosslinkable coating composition containing a film-forming binder and an optional liquid carrier,
wherein the binder contains
a) an oligomer or polymer or dispersed gelled polymer having functional groups capable of reacting with isocyanate groups on component (b); and
b) a blocked or unblocked biuret group-containing polyisocyanate curing agent having a functionality of at least 4 and a number average molecular weight of about 500 to 3,000 prepared by reacting a polyisocyanate adduct which
i) is prepared from an aliphatic, cycloaliphatic, or aromatic diisocyanate;
ii) has an average isocyanate functionality of at least 2.8; and
iii) contains either isocyanurate or iminooxadiazine dione groups or mixtures thereof, provided that a total of at least 50 mole percent, based on the total moles of isocyanate adduct groups present in the polyisocyanate adduct, of said isocyanurate and iminooxadiazine dione groups are present,
with 0.01 to 0.15 moles of water, or of a mixture of water and other biuretizing agents wherein the other biuretizing agents are present in an amount of up to 50 mole percent based on the total moles of biuretizing agent for each equivalent of isocyanate in the polyisocyanate adducts, and wherein said other biuretizing agent is other than tertiary alcohols and wherein the reaction is conducted at a temperature of 50 to 180° C. to incorporate biuret groups into the polyisocyanate adduct.
2 . The coating composition of claim 1 wherein i) is an aliphatic diisocyanate.
3 . The coating composition of claim 2 wherein i) is 1,6-hexamethylene diisocyanate.
4 . The coating composition of claim 1 wherein iminooxadiazine dione groups are present in admixture with the isocyanurate groups in an amount of at least 10 mole percent, based on the total moles of iminooxadiazine dione and isocyanurate groups.
5 . The coating composition of claim 1 wherein the biuret group-containing polyisocyanate has an average isocyanate functionality in the range of 4-10.
6 . The coating composition of claim 1 wherein the biuret group-containing polyisocyanate has a number average molecular weight of 500 to 3000.
7 . The coating composition of claim 1 wherein the coating is a liquid solvent borne coating.
8 . The coating composition of claim 1 wherein the coating is a liquid water borne coating.
9 . The coating composition of claim 1 wherein the coating is a powder coating.
10 . The coating composition of claim 1 wherein said composition is suitable for the production of the base coat or the clear coat or undercoat in a clear coat/color coat finish for automobiles and trucks.
11 . A substrate coated with a dried cured layer of the coating composition of claim 1 .
12 . The substrate of claim 11 wherein the substrate is an automotive substrate comprises multiple coating layers that are dried and cured, and wherein at least one of the dried, cured coating layers is the coating composition of claim 1 .
13 . The coating composition of claim l wherein water is used as the sole biuretizing agent to form the biuret group-containing polyisocyanate curing agent.
14 . The coating composition of claim 1 wherein i) is isophorone diisocyanate.
15 . The coating composition of claim 1 wherein i) is a mixture of 1,6-hexamethylene diisocyanate and isophorone diisocyanate.