ABSTRACT
Over the last two years, serious concerns have been expressed in the press about human exposures to trace amounts of alkyl tin moieties such as tributyltin (TBT). Organostannic compounds are commonly used as catalysts in polyurethane chemistry. Whereas stannous octoate (SNOCT) exhibits a TBT content below the detection limit of most analysis equipment (< 0.01%), TBT levels as high as 0.6% have been measured in dibutyltin dilaurate (DBTDL). Although DBTDL manufacturers are currently optimising their process to lower the TBT content in DBTDL, the (chemo)pressure exerted on polyurethane foamers is such that DBTDL-free formulations are being actively looked for.
High resilience polyurethane foams based on various polymer polyols - particularly polyols made with pure polystyrene (POSTech*) or poly(styrene/acrylonitrile) - have been developed using DBTDL as the gel catalyst. The direct replacement of DBTDL is not straightforward as foam block dimensional stability issues (often referred to as “cold flow”) are encountered when other catalyst (i.e. SNOCT) are used.
This “cold flow” phenomenon is very much dependent on the size and the weight of the foam block produced, making all small scale laboratory experiments unsuitable for developing novel stable HR foam formulations.
The present research has shown that properties derived from small scale experiments can nevertheless be related to the “cold flowability” of HR formulations. Amongst the various tests investigated, the variation in sag factor from top to bottom of the foam block was found to be the most relevant test method to assess the dimensional stability. :Sag factor is here defined as the force exerted by the foam compressed to 65% of the original thickness divided by that exerted at 25% compression during the compression load deflection (CLD) test.
The test method based on this variation in sag factor through the foam block (vSag) made possible the laboratory examination of formulation parameters for their impact on foam block stability. It also allowed us to develop a wide density range of POSTech-based DBTDL-free HR slabstock foams that are dimensionally stable.
