Question: We want to bond steel to foamed polystyrene. What do you recommend?

Answer: Solvent-based contact cements, such as those based on chloroprene rubber, were traditionally used for this application. They work very well and are available in spray or aerosol versions. However, many users are now reluctant to use them because of emissions regulations, safety concerns or disposal issues. Water-based adhesives can be used for this application but tend to cure dry slowly because of the difficulty of evaporating the water.

The modern method of bonding these materials is to use a hot-melt adhesive. The best performance comes from using reactive hot-melt polyurethanes. These adhesives are applied molten like a conventional hot melt and give instant fixturing with good initial strength (referred to as “green strength”) but then cure slowly by reaction with atmospheric moisture to form a crosslinked thermoset system. 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 insulated garage door construction incorporating polystyrene foam and the walls of recreational vehicles. They are available in cartridges, pails and drums to suit a range of users.

Question: Are cyanoacrylate adhesives thermoplastics?

Answer: Yes. As you probably know, a thermoplastic is a plastic that softens on heating and hardens again on cooling. The problem with using thermoplastics as adhesives is that they have relatively poor heat resistance and they creep under loads. Most cyanoacrylates will soften at about 100°C.There are grades with additives that will improve the heat resistance and monomers, such as allyl cyanoacrylates that have excellent heat resistance after heat curing. Some special toughened cyanoacrylates do act as true structural adhesives, although they tend to be expensive compared to structural epoxies, for example. ASI

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.