Ask Dr. Dave / Columns

Ask Dr. Dave: October 2013

We want to bond steel to foamed polystyrene in large bond areas.

Question: We want to bond steel to foamed polystyrene in large bond areas, and we want to use a cost-effective adhesive that doesn’t take long to dry. Is a hot melt the way to go?

Answer: Solvent-based contact cements, such as those based on chloroprene rubber, were traditionally used for this application, but many users are now reluctant to use them because of new emissions regulations, safety concerns or disposal issues. Water-based adhesives can be used, but they tend to cure slowly because of the difficulty of evaporating the water.

The modern way of bonding these materials is to use a reactive hot-melt polyurethane. These adhesives are applied molten like a conventional hot melt, but then cure by reaction with atmospheric moisture to form a crosslinked thermoset system. Most parts have sufficient strength to be handled after a few minutes. The adhesives can be formulated to have different open times and different rates of bond strength buildup. Such adhesives are widely used in the bonding of large panels, such as in garage door construction and the walls of recreational vehicles. They are available in cartridges, pails and drums to suit a wide range of users. For large bond areas, spraying or roll coating is often the best application method.

Question: We manufacture cast-iron, water-filled radiator systems from imported parts and have a large scrap rate due to pinholes that lead to leaks. Is there a cost-effective way of sealing these holes?

 Answer: You could smear a two-part epoxy adhesive over the holes, but this would likely be a slow, labor-intensive process. Another alternative would be to fill the radiators with an anaerobic or heat-cured, methacrylate-based impregnation resin and then apply air pressure to fill the holes. However, one effective technique that I have used on similar parts is to heat the area around the holes to about 100°C and then spray a low-viscosity anaerobic sealant over the area. As the part cools, the sealant will penetrate the porosity to create an effective, water-resistant seal.

Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.  

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