SPRAY Application Guide
SPRAY Application Guide
To achieve the best results from these systems, good spraying practices must be followed.
The following recommendations are provided as a guide only.
All surfaces to be sprayed must be free of oil, grease, waxes, rust scale, loose dirt and water.
Some metal surfaces may require sandblasting and priming prior to foam spraying to ensure adequate adhesion.
The temperature of the substrate has a major effect on foam density and adhesion.
Certain compromises are necessary to spray in cold weather.
The section in “COLD WEATHER SPRAYING” offers more information on this topic.
If in doubt about the substrate temperature or surface conditions, a trial application should be made to check foam quality and spray performance.
Water on the surface from rain, fog condensation etc. will react chemically with the isocyanate, adversely affecting the foam and resulting properties, particularly adhesion.
If necessary, the applicator should protect the area to be sprayed with a temporary cover to keep rain.
SS-40 should not be sprayed when the relative humidity is 80% or above, as high relative humidity can adversely affect the physical properties of the foam.
Wind velocities greater than 10 miles per hour will result in high loss from overspray and may result in excessive loss of exothermal affecting foam density and thermal properties.
Openly, spraying foams under high wind condition should not be done. The employment of wind breaks could be used especially when spraying on the exterior of buildings.
The surface finish of the resultant foam will also be affected under these conditions due to overspray and material loss.
Although urethane foam spraying normally should be carried out at ambient and substrate temperatures of 70°F or above, it is practical to spray at the temperatures well below 32°F, provided that suitable precautions are taken and that the correct foam system is used.
Urethane foam formation is initiated by a chemical reaction between an isocyanate and the resin, when they meet in the mixing chamber of the spray gun.
This reaction is exothermic i.e., it gives off heat.
The heat produced by the chemical reaction causes the blowing agent to vaporize and expand the reacting chemicals into foam.
The whole process must be controlled by proper formulating, so that foam rises fully before the foam hardens into its rigid form.
If the foam hardens before the complete rise, this leads to creep.
Creep can lead to improper adhesion to the surface.
Low Temperatures Affect The Foaming Process In Two Ways.
1. Chemical reactions can be slowed down due to reduced temperature rise within the expanding mass; this could lead to dripping on the wall.
2. The reduced temperature can prevent the physical blowing agent from expanding; this could lead to reduced yield.
The temperature of the substrate has a greater influence of the heat withdrawal from the reacting chemicals than does the temperature of the air, because the heat transfer from liquid to air is as much slower than the rate of heat transfer from liquid to solid.
If the substrate temperature is too low, or it is highly conducting substance such as most metals, the heat produced by the chemical reaction may be withdrawn into the substrate so rapidly that the blowing agent will not vaporize and therefore little or no foam will be produced.
It is not a good practice to use the heated chemicals to warm up the surface.
The reversion, the non-expansion of the blowing agent, in the reacting polyurethane mass may eventually expands when the temperature of the surface increase.
This can be seen as voids the following summer on metal buildings after direct sun exposure.
Polyurethane foams such as SS-40 winter version can be sprayed at temperatures as low as 25°F. The chemical makeup of this formulation has been adjusted with catalyst to compensate for cold weather spraying.
Special precautions should be taken when spraying various substrates in cold weather.
Spraying too thin or too thick can result into reduced yield or cracking.
When the urethane foam is processed, the reaction may appear to be completed within a few minutes after application, it actually takes from 24 to 48 hours for the foam to develop its full a strength and the lower temperature, the longer process takes.
The initially applied foam is hot due to the heating of the chemicals from the equipment and heat from the chemical reactions resulting into hot gases within the cells.
In cold weather conditions, the foam cools rapidly causing the gases to contract and eventually to condense into a liquid.
This creates a full or partial vacuum in the cells resulting into considerable stress within the foam itself.
The partially cured foam without achieving its maximum properties could shrink and eventually crack during the curing process.
The spraying of foams at thicknesses about one inch per pass could assist in the reduction of the internal stresses.
Furthermore, in cold weather conditions, the adhesive bond of the foam to the substrate may not be fully developed so it cannot resist the shrinkage forces, causing the foam to pull away from the substrate.
The greater the foam thickness laid down in one pass, the more critical is the problem.
If the foam layer is thin, the substrate adhesion has a greater chance to resist the tendency to shrink. As the foam thickness increases, the adhesive forces have less and less effect.
The outer edge of the foam layer shrinks more than the inner one causing the foam to curl up at the edges.
This curling could lead to the pulling away of the foam from the substrate.
To overcome the shrinkage and cracking in cold weather, spraying thin layers rather than the full thickness in one pass could be the applicable technique of choice.
Follow the spray equipment manufacturer’s safe operation guidelines for all spray operations.
Every spray unit is slightly different and you will need to adjust your preheater and hose temperatures accordingly for each Polyurethane Foam System In addition, adjust your impingement pressures to have desired spray pattern.
Use a fixed ratio (one-to-one) having positive displacement pumps connected to a common drive.
Always spray perpendicular to the surface in one inch lifts.
Spraying perpendicular to the surface helps to minimize overspray.
One inch foam lifts give optimum foam performance and yield.
Thin foam lifts give poor chemical reaction since not enough exothermic reaction will be present.
Too thick foam lifts, while giving great yield, leads to weak foam, cracking and scorching.
Allow foam lifts to cool somewhat before successive foam lifts are applied.
(Exception: High Density foams may require smaller foam lift thickness.) Do not walk on foam immediately after application – this can cause blisters.