Flame retardant polyurethane foams containing biuret linkages
Polyurethane foams are made using a biuret-modified MDI and/or PMDI as the isocyanate component. The isocyanate is characterized by having a high content of biuret-containing oligomers and, preferably, a low proportion (if any) of biuret-urea oligomers. Flexible, resilient foams made using this biuret-modified isocyanate perform well on flammability tests such as the UNI 9175 CS. E. RF4 test, even without added flame retardants.
1. A process for preparing a flexible polyurethane foam, comprising
A. forming a reaction mixture including water, an alkali metal carboxylate salt, at least one polyisocyanate and at least one catalyst and
B. subjecting the reaction mixture to conditions sufficient to cause the reaction mixture to expand and cure to form a polyurethane foam having a density of from 1.8 to 4 pounds per cubic foot and which exhibits a value of at least 40% on the ASTM D 3574 ball rebound test, wherein
the polyisocyanate is a diphenylmethane diisocyanate and/or polymethylene polyphenylisocyanate, which is modified to contain at least 20% by weight of biuret-containing oligomers that have three or more isocyanate groups and the reaction mixture further contains (i) a propylene oxide homopolymer which is end-capped with polyoxyethylene blocks and has an ethylene oxide content from 8 to 30% by weight, an equivalent weight from 1200 to 2000, an average hydroxyl functionality of from 3.6 to 4.5, and a primary hydroxyl content from 75 to 100%, (ii) a propylene oxide homopolymer which is end-capped with polyoxyethylene blocks and has an ethylene oxide content from 8 to 30% by weight, an equivalent weight from 1200 to 12000, a average hydroxyl functionality of from 2.5 to 3.5, and primary hydroxyl content from 75 to 100% and (iii) a random copolymer of from 70-95% of propylene oxide and from 5 to 30% of ethylene oxide which has from 0-70% primary hydroxyl groups, an equivalent weight of from 1000 to 2000; and an average hydroxyl functionality from 2.5 to 4.5.
2. The process of claim 1 wherein the reaction mixture is devoid of a halogenated or phosphorus-containing flame retardant.
3. The process of claim 1 , wherein the polyurethane foam exhibits, as naked foam, at least a class 3 IM rating on the UNI 9175 C.S.E. FR4 test.
4. The process of claim 3 , wherein the polyurethane foam exhibits, as naked foam, a class 1 IM rating on the UNI 9175 C.S.E. FR4 test.
5. The process of claim 1 wherein the polyisocyanate is produced in a process that includes reacting 2,4′-MDI, 4,4′-MDI, a mixture of 2,4′- and 4,4′-MDI, or a polymethylene polyphenylisocyanate, with water.