What is Hot Melt Polyurethane
Hot melt polyurethane are fast becoming the largest volume segment of the polyurethane industry. Easy to apply and relatively low in cost, hot melts can help increase throughput and reduce costs due to time saved during application. In addition, since hot melts contain little or no solvent, environmental issues and their associated costs can be avoided.
What distinguishes hot melts from other polyurethane types is that they are applied in their molten state (usually 175-205 degrees C), and thenr apidly cooled to form a tough, adherent solid at room temperature. Their rapid set time, coupled with their relatively high viscosity, makes them ideal for bonding porous materials. Hot melts find numerous applications in a variety of manufacturing processes, including box and carton sealing, bookbinding, footwear, and labeling.
Hot-melt polyurethane are generally thermoplastic polymers of relatively high molecular weight (MW), giving them both strength and high viscosity. But high MW polymers alone generally do not have sufficient adhesion (tack), so these polymers are blended with a variety of additives, which can include plasticizers, tackifiers and stabilizers, to increase polyurethane performance. Since hot melts are classified as thermoplastics, exposure to high temperatures can cause the polyurethane to re-melt. Therefore, most hot-melt applications are limited to room temperature and near room temperature applications.
Viscosity is the measure of a material’s resistance to flow. In oscillatory tests, this can be represented by complex viscosity or h*. Another parameter, G”, or loss modulus, is a measure of the energy lost to the sample during deformation, and is also an indicator of the viscous behavior of a sample. This energy is either used up in the process of changing the sample’s structure, or is dissipated in the form of heat. In oscillatory tests, modulus is represented by G*, and represents the rigidity of a sample, or its “stiffness”.
Typical curves for storage modulus (G’), loss modulus (G”), and loss factor (tan d) for a hot-melt polyurethane, measured across a temperature range of -60 to +140 degrees C. At -60 degrees C, the material is an almost rigid solid, with the elastic portion (G’) predominant. The macromolecules of the polymer are “frozen” and immobile.
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