Special Applications of hot melt polyurethane
There has been a recent new requirement on construction panels and steel doors to pass a UL 10C fire rating. There is a gas burner inside a brick wall with openings where the panels or doors are placed. The gas flame at a positive pressure is directed at the test pieces. The hot melt polyurethane is that there are no flames issuing from the outside of the test panel for a specified time.
The panels are typically a laminated construction with steel outer skins laminated to 1.5-inch-thick cores. The cores can be honeycomb paper, expanded polystyrene (EPS) foam, polyurethane foam or mineral board. The adhesive is coated on the core with a double-sided roll coater. This adhesive-coated core is then bonded to the steel skins. The edges are typically closed with steel rails, which have the appropriate attachments for hinges, locks, etc.
After assembly, the parts are allowed to cure for a few days and then fire tested. Panels made with earlier versions of the hot melt polyurethane adhesive did not pass the UL 10C fire test. Flames were observed on the outside of the test panels from the burning adhesive. However, new hot melt polyurethane adhesives using fire-retardant materials have been recently developed that allow panels to pass this test.
Recreational vehicle side walls are a major application for hot melt polyurethane adhesives as well. The requirements for this application are very demanding. Recreational vehicles are exposed to a range of environments from cold winters at -40°F to the heat of summers with the sun beating down on dark sections of side walls to produce 160°F.
The typical construction of the recreational vehicle side wall is a multilayer lamination using hot melt polyurethane adhesives. The layers are a sheet of fiberglass-reinforced plastic, two layers of lauan plywood, a sheet of EPS foam, and an interior-finish board often with decorative paper.
The dimensions of the side walls are as much as 40-feet long by 9-feet high. The ambient temperature during the side-wall manufacture can vary from 60°F to 105°F from winter to summer. The large dimensions of the recreational vehicle means that long open times, up to three to eight minutes, can be required for the lamination. Also, excellent creep resistance of the adhesive is required to maintain good bonds between stiff and somewhat warped lauan panels.
A significant issue with adhesives for recreational vehicle side walls as they are being used on vehicles is the potential for exposure to high temperatures during summer months. Dark areas of the side walls can easily reach temperatures of 180°F or higher when exposed to direct sunlight. Hot melt polyurethane adhesives are fully reactive, containing no blended thermoplastic material. This gives them substantially better high-temperature performance as compared to other hot melt products, which incorporate thermoplastic materials.
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