Like all isocyanates, MDI is highly reactive and can irritate eyes, skin and respiratory organs. It can have a sensitizing effect if inhaled or in contact with skin – frequent use can lead to allergies. MDI is suspected of being carcinogenic. There is no danger provided a craftsperson meticulously follows the manufacturer’s safety recommendations. However, inexperienced DIY enthusiasts are often careless. Lawmakers have now reacted: as from 2009, polyurethane-based installation foams will fall under a higher hazard class in Germany and some other European countries. They’ll then only be available for professional use, and will disappear from self-service shelves in DIY stores.

α-Silanes provide the basis for the first isocyanate-free insulation foams. STP-U is a polyurethane-based prepolymer whose isocyanate group is bonded firmly to a silyl group. Crosslinking takes place by way of silane chemistry, and free isocyanates do not occur. Soudal, an important construction chemicals manufacturer, has been using the novel technology since 2004. “Our Soudafoam SMX® is every bit as good as the well-known polyurethane foam and also meets the new, stricter environmental regulations,” says Vic Swerts, head of this family-owned Belgian company. Indeed, STP-U-based insulation foams boast excellent product characteristics: they cure fast, do not continue swelling after application and do not yellow.

Here, therefore, the silicon atom and X are attached to the same carbon atom (C), known by chemists as the α-carbon. As a result, the chemically active alkoxy groups (OR), which are also attached to the silicon atom, are in close proximity to the reactive X group. This activates them, making them able to react much faster. This is the underlying principle of the α-effect.

In the presence of moisture, e.g. air moisture, the OR groups are split (hydrolyzed, in chemical terms). This enables them to react with each other and form a stable silicon-oxygen bond (siloxane bond).This is the basis of the crosslinking reaction occurring in adhesives and foams containing α-silane binders. As α-silanes are so reactive, crosslinking is very fast, with no need for the organo-tin catalysts that accelerate this reaction with γ-silanes.

A broad range of α-silane-terminated polymers (STP) is now available, of which the α-silane-terminated polyethers (STP-E) are especially interesting. There is broad scope for customizing properties of the end product by varying the X group and the kind of polymer to which X is attached, as well as the number of alkoxy groups (OR) and their chemical nature.