Hot melt adhesives comprising low free monomer, low oligomer isocyanate prepolymers

A method for adhesively joining or sealing two substrates which comprises:

• (1) applying onto a substrate a solvent-free, moisture curable urethane hot melt adhesive composition comprising the polyisocyanate prepolymer reaction product of (a) a polyether or polyester polyol containing X OH groups (where X is 2 or 3), and (b) a diisocyanate, the adhesive composition being applied at 50° to 150° C.,
• (2) contacting the melt extruded adhesive disposed on the substrate with a second substrate such that a green bond is formed, and
• (3) exposing the adhesive bond to ambient moisture causing a crosslinking reaction within the adhesive, thereby forming a moisture-cured bond, characterized in that the polyisocyanate prepolymer reaction product consists essentially of at least 90 wt % of a prepolymer having a diisocyanate to polyether or polyester polyol stoichiometry of X:1 and prepared by reacting the diisocyanate and polyol in an NCO to OH equivalents ratio of 6 to 20:1, the prepolymer reaction product containing less than 2 wt % unreacted diisocyanate monomer and the prepolymer having free NCO ranging from 0.2 to 8 wt %.

A method for adhesively joining or sealing two substrates which comprises:

• (1) applying onto a substrate a solvent-free, moisture curable urethane hot melt adhesive composition comprising the polyisocyanate prepolymer reaction product of (a) a polyether or polyester triol and (b) toluene diisocyanate, the adhesive composition being applied at 50° to 150° C.,
• (2) contacting the melt extruded adhesive disposed on the substrate with a second substrate such that a green bond is formed, and
• (3) exposing the adhesive bond to ambient moisture causing a crosslinking reaction within the adhesive, thereby forming a moisture-cured bond, characterized in that the polyisocyanate prepolymer reaction product comprises at least 95 wt % of a prepolymer having a toluene diisocyanate to polyether or polyester triol stoichiometry of 3:1, the prepolymer reaction product containing less than 0.1 wt % unreacted toluene diisocyanate monomer and the prepolymer having free NCO ranging from 0.5 to 3 wt %.