What is Hot Melt Polyurethane Adhesive
Hot melt polyurethane adhesives are manufactured by reacting mixtures of polyols with an excess of diisocyanate. The reaction of an isocyanate group with an alcohol group produces the urethane group. The fact that there are excess diisocyanates present means that the molecular weight of the resulting product is not too high and that the hot melt polyurethane product will have a controlled melt viscosity.
These adhesives are then transferred from the reactor into the container package as a melt. The container is sealed to prevent exposure to moist air while the adhesive solidifies on cooling. Typical containers are drums (400 lb), pails (40 lb), slugs (4.4 lb) and cartridges (0.6 lb).
The adhesive is melted using a specifically designed premelter and then applied to the article to be bonded. During the next several hours or days after application, the reaction takes place. This reaction of isocyanate with water forms the highly stable polyurea structure that gives the cured hot melt polyurethane its temperature and environmental performance. The polyurea structure effectively crosslinks the adhesive to prevent its remelting or dissolving as can occur with standard thermoplastic hot melt adhesives.
Specific properties required for varying applications are built into the adhesive by the use of a variety of polyols, isocyanates and additives. Polyols are used to vary the open time, set time and is able to wet effectively various surfaces to produce effective bonds. Some typical polyols used are:
1. Polyesters, which can be crystalline or amorphous. The crystalline polyols can be used to give short set times and rapid green-strength development. Amorphous polyesters can improve adhesion to specific substrates and increase open time, if required.
2. Polyethers, which are low-Tg, amorphous liquids. These help lengthen open time, reduce viscosity and provide good low-temperature flexibility.
3. Vinyl-polymerized polyols, which are typically high-molecular-weight, glassy solids. These materials can assist in building green strength and tack while maintaining extended open times.
A mixture of polyols to give the desired performance is reacted with an excess of a diisocyanate. Methylene diphenylisocyanate is used for most hot melt polyurethane adhesives. It has a highly reactive isocyanate group and has a relatively low vapor pressure. Since polyurethanes made with methylene diphenylisocyanate tend to yellow when exposed to sunlight, hydrogenated methylene diphenylisocyanate is used to resist yellowing.
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