Novel silica aerogel-polyurethane composites for thermal insulation

Polyurethanes (PUs) are well known for their chemical versatility, which allows to prepare them as either rigid or flexible materials, depending on the chosen starting monomers. They also have an inherently low thermal conductivity lambda (about 0.18 W/mK), which can be further lowered to 0.015-0.030 W/mK in foamed PUs, i.e. porous PUs filled with gases having a low thermal conductivity. However, in PU foames the filling gas tends to escape with time upon relatively short times (days/weeks), causing a progressive increase of lambda towards the 0.18 W/mK value typical of the solid (i.e., non-foamed) PU. In order to achieve an effective thermal insulation performance able to last in time, it is hence desirable to identify appropriate PU-based formulations. One possible strategy is to realize a composite with solid PU as the matrix and a solid filler characterized by very low lambda, such as silica aerogel (SA), which when taken alone can reach indefinitely lasting thermal conductivities down to 0.015 W/mK. Here we present some results of this approach, related to the thermal behavior of a composite based on elastomeric PU as the matrix and silica aerogel as the filler. In more detail, we found that it is possible to realize homogeneous composites with SA loadings up to 20% W/W, and that such composites have a thermal conductivity about 13% lower than that of the pristine (i.e., pure), starting PU. Tests on the evolution of the thermal conductivities of such composites are ongoing, with preliminary results indicating that the lower lambda of these composites does not decay appreciably over time spans of about two weeks.