Low free mdi prepolymers for rotational casting

Abstract

Rotational cast polyurethane composition prepared from a prepolymer composition comprising: a) an isocyanate-terminated polyurethane prepolymer; and b) a curative agent comprising i) a polyol; ii) an aromatic diamine; iii) a thixotropic aliphatic amine; and iv) a thixotropic colloidal additive, wherein the prepolymer comprises a product produce by the reaction of a polyol with an organic diisocyanate monomer comprising 4,4′-diisocyanato diphenylmethane (MDI), and which prepolymer comprises less than 1.0% by weight of free MDI monomer, based on the total weight of the prepolymer, exhibits a range of enhanced physical properties compared to those obtained from prepolymers comprising a higher level of free MDI monomer.

What is claimed is: 
     
         1 . A rotational casting method for producing a polyurethane polymer coating on a cylindrical substrate, the method comprising:
 rotating the cylindrical substrate about an axis at a selected rotational speed, applying a polyurethane prepolymer composition to a surface of the rotating substrate by ejecting the polyurethane prepolymer composition through a die at a selected flow rate, effecting relative linear movement between the rotating substrate and the die in a direction parallel to the axis of rotation at a selected relative linear speed, and synchronizing the reaction mixture flow rate, the relative linear speed and the rotational speed in such a way that successive convolutions of the outlet streams of the polymeric reaction mixture overlap and meld together seamlessly;   wherein the polyurethane prepolymer composition comprises:   a) an isocyanate-terminated polyurethane prepolymer; prepared by reacting 4,4′-diisocyanato diphenylmethane and a polytetramethylene ether glycol, and which prepolymer comprises less than 1.0% by weight of free 4,4′-diisocyanato diphenylmethane monomer, based on the total weight of the prepolymer, and   b) a curative comprising i) polytetramethylene ether glycol, ii) diamine diethyl toluene diamine and/or dimethylthio-toluene diamine, and iii) about 0.1 wt % to about 1.5 wt %, based on the total weight of the curative, of a thixotropic aliphatic amine selected from the group consisting of ethylene diamine, 1,6-hexanediamine, 1,12-dodecane diamine, 1,4-cyclohexane diamine, and diethylene triamine,   and wherein the total active hydrogen content of the curative agent is equal to about 80-115% of the total isocyanate content of the isocyanate-terminated polyurethane prepolymer to provide the polyurethane polymer coating as a rotationally cast polyurethane layer.   
     
     
         2 . The method according to  claim 1  wherein the polyurethane prepolymer composition is ejected through a die dividing an inlet stream of the polymeric reaction mixture into plural outlet streams. 
     
     
         3 . The method according to  claim 1  wherein the prepolymer comprises less than 0.7% by weight of free 4,4′-diisocyanato diphenylmethane monomer. 
     
     
         4 . The method according to  claim 1  wherein the prepolymer comprises less than 0.5% by weight of free 4,4′-diisocyanato diphenylmethane monomer. 
     
     
         5 . The method according to  claim 1  wherein the prepolymer comprises less than 0.3% by weight of free 4,4′-diisocyanato diphenylmethane monomer. 
     
     
         6 . The method according to  claim 1  wherein the curative further comprises iv) from about 1.0 wt % to about 10 wt %, based on the total weight of the curative, of a thixotropic colloidal additive selected from the group consisting of fumed silica, clay, bentonite, and talc. 
     
     
         7 . The method according to  claim 2  wherein the curative further comprises iv) from about 1.0 wt % to about 10 wt %, based on the total weight of the curative, of a thixotropic colloidal additive selected from the group consisting of fumed silica, clay, bentonite, and talc. 
     
     
         8 . The method according to  claim 1  wherein rotationally cast polyurethane layer has a hardness in the range of 40 to 70 Shore A. 
     
     
         9 . The method according to  claim 1  wherein rotationally cast polyurethane layer has a hardness in the range of 70 to 80 Shore A. 
     
     
         10 . The method according to  claim 1  wherein the substrate is a paper mill roll. 
     
     
         11 . The method according to  claim 2  wherein rotationally cast polyurethane layer has a hardness in the range of 40 to 70 Shore A. 
     
     
         12 . The method according to  claim 2  wherein rotationally cast polyurethane layer has a hardness in the range of 70 to 80 Shore A. 
     
     
         13 . The method according to  claim 2  wherein the substrate is a paper mill roll. 
     
     
         14 . The method according to  claim 6  wherein rotationally cast polyurethane layer has a hardness in the range of 40 to 70 Shore A. 
     
     
         15 . The method according to  claim 6  wherein rotationally cast polyurethane layer has a hardness in the range of 70 to 80 Shore A. 
     
     
         16 . The method according to  claim 6  wherein the substrate is a paper mill roll. 
     
     
         17 . The method according to  claim 7  wherein rotationally cast polyurethane layer has a hardness in the range of 40 to 70 Shore A. 
     
     
         18 . The method according to  claim 7  wherein rotationally cast polyurethane layer has a hardness in the range of 70 to 80 Shore A. 
     
     
         19 . The method according to  claim 7  wherein the substrate is a paper mill roll.