In order to apply the projected polyurethane correctly, we should take a series of factors into account, ranging from atmospheric conditions and machine adjustments to the types and forms of applications.
Surfaces must be clean, dry and free of dust and grease to ensure the good adhesion of polyurethane foam to the substrate; if the substrate is metallic, the surfaces must also be free of rust. To ensure good adhesion on metallic substrates, it is recommended to use a suitable primer. The recommended minimum temperature of the substrate during spraying is 5ºC.
The coating thickness is perfectly controllable and can be modified by varying the application speed and/or mixing chamber of the gun. It should be noted that the lower the number of layers applied for the same thickness is, the higher the foam performance will be. However, it is not advisable to apply very large thicknesses, on the one hand, in order to avoid the formation of bags and, on the other hand, in order to avoid problems due to high exothermic reactions.
The rigid polyurethane foam applied in situ by projection has an excellent behaviour against atmospheric agents (water, extreme temperatures, wind, ...). It is only attacked by long-term exposure to ultraviolet radiation, contained, for example, in direct sunlight.
This attack causes the destruction of the polyurethane surface, resulting in a dusty appearance on the surface of the foam. This attack produces a decrease in thickness at a rate of 1 or 2 millimeters per year, depending on the combined effect of rain and wind, being lower during the first year thanks to the presence of the external skin of the foam, a layer of high density polyurethane.
To avoid this action, exterior applications must be coated with specific materials such as polyurethane elastomers, polyurethanes, paints or any other specific coating for exterior use. These protections, like any other material, must be monitored for their conservation and maintenance. In the case of partially degraded foams, sanitizing must be carried out by means of a barbed brush or water under pressure and the subsequent application of a new layer of polyurethane foam and the suitable surface protection.
Prevention of cracking by expansion joints
In the polyurethane foam that is directly applied on expansion joints, cracks may occur due to the movement of the substrate by expansion or contraction. This effect is especially important in decks or terraces where cracking can break the waterproofing. It is easily avoidable if, at the time of the application, the proper precautions are taken by treating the joint properly.
Although polyurethane foam does admit some permanent deformation, the stresses generated by the effects of expansion-contraction on very wide joints, from 2 to 4 cm, cannot be absorbed by the foam. The way in which these stresses can be absorbed is to distribute them, minimizing their effects, by installing an elastic separating membrane, for example, made of synthetic rubber with a width of 30 cm, and applying polyurethane on top of it.