Prepolymer heating alternatives

Special rules apply to the world of hydrophilic polyurethanes. These alternate rules are based on the fact that hydrophilic polyurethanes can and should be processed in water. Rather than emulsifying a prepolymer with a polyol, as would be done with a hydrophobic polyurethane, hydrophilics are mixed with water. While the properties of the foam are governed loosely by the guidelines described above, one has more flexibility and control of the formulation and process by which the polyurethane is made. For example, the water can serve as a heat sink to closely control the temperature of the foam the water controls the rate of reaction. 

Adhesives wet, flow, and set to a solid during bond formation. The transformation from liquid adhesive to solid bond can be achieved in a number of ways. Where the adhesive is a polymer, the initial starting material is a liquid monomer or prepolymer that, under the conditions of bonding with heat, pressure, and/or catalyst, polymerizes to the solid polymer in the glue line. It is also usual to apply solutions of preformed polymers in suitable solvents to the faces of adherends, and allow bond formation to take place with evaporation of solvent. Alternatively, polymers that can be melted or softened to flow at elevated temperatures can be applied as hot melt adhesives to form the bond on cooling. With porous adherends like wood, penetration of the pores by liquid or molten adhesives is an important factor in bond formation. 

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