Polyurethane

In polyurethane manufacture, for many applications, the catalysts of choice for catalysing the reaction between a polyol and an isocyanate composition, i.e., for hardening or curing polyurethane (PU) materials, have long been organic mercury compounds. This is because, for a wide range of polyurethane materials, these catalysts provide a robust and desirable “reaction profile” characterised by:

• an initial induction period in which the reaction is either very slow or does not take place, which continues for sufficient time to permit the “system” (combination of polyurethane materials and catalyst) to be mixed and cast (or sprayed); and
• a subsequent rapid reaction period during which the product cures, taking on its final properties (shape, hardness, flexibility, strength, etc.).

Like any catalyst used in PU elastomer systems, the mercury catalyst is incorporated into the polymer structure and remains in the final product. Over time – and accelerated by exposure to harsh environments, UV, abrasion, etc. – the polymer structure breaks down and mercury is released.

Mercury in PU products already attracted attention some years ago. According to an investigation by the Minnesota (USA) Department of Health, some PU elastomer flooring manufactured from about 1960 through at least 1980 contained up to 0.1% mercury in phenylmercuric acetate or other organo-mercuric salts that were used as catalysts (Reiner 2005, as cited by MDH 2006). Ambient mercury concentrations in school gyms ranged from 0.13 to 2.9 Pg/m3, and in 5 of 6 gyms was above the RfC level of 0.3 Pg/m3 established by US EPA as the exposure level below which no adverse health effect is expected (MDH 2006). A separate investigation in Ohio (USA) showed that PU elastomer floors in schools also emitted mercury is excess of the 0.3 Pg/m3 RfC level (Newhouse 2003).

It is estimated that 300-350 tonnes of mercury catalyst may be used globally in PU elastomer applications, of which some 60-105 tonnes in the EU (industry communications; SRI 2006). If one assumes the mercury catalyst is added to a system at an average of 0.5-0.6%, then approximately 55,000 - 65,000 tonnes of PU elastomers globally are catalysed with mercury each year. Assuming the global market for PU elastomers is 1.6 million tonnes, this suggests that around 4% of that global market uses mercury catalysts. As a percentage this is not high, but it represents over 100 tonnes of mercury consumption worldwide, and 20-35 tonnes of mercury consumption with PU elastomers in the EU27+2. The mercury catalyst mainly ends up in the final product, and it is roughly estimated that the mercury consumption in PU elastomer end products corresponds to the consumption during production of 20-35 tonnes within the EU27+2.

Tin and amine catalysts are alternatives to Hg catalysts for some PU elastomer applications, titanium and zirconium compounds have been introduced for others, while bismuth, zinc, platinum, palladium, hafnium, etc., compounds are marketed for still others. In fact, known mercury-free catalysts could be used for nearly all elastomer applications, but some reduction in the key performance characteristics of activity, selectivity, catalyst lifetime, etc., may have to be accommodated until the best system is identified for a given application. (Shepherd 2008).

As suggested, a large number of Hg-free catalysts for PU elastomers have been developed as alternatives to mercury – the large number reflecting the fact that there does not appear to be a “drop-in” substitute for mercury catalysts that can be used in so many different systems, that confers similarly desirable curing properties, and that is so forgiving and easy to adjust to the needs of the user.

Despite these challenges, it should be stressed that perfectly viable substitutes to mercury catalysts are already in use for over 95% of PU elastomer systems, and have been in use for many years. The cost of most mercury-free catalysts is quite competitive with the typical mercury catalyst cost, and even more so if one takes account of waste disposal costs, environmental and other customer concerns.

The cost of Thorcat 535 has increased significantly in recent years, and is presently in the range of €40-50/kg, compared to €25-35/kg for medium-priced mercury-free catalysts, and €10-20/kg for cheap mercury-free catalysts (IMCD 2008). A bismuth catalyst would be fairly close to the cost of Thorcat (2008, COWI, Concorde)