POLYURETHANE FOAMS

Polyurethane (PU) foams are by far the most popular cellular polymer as they cover more than the half of the total foam market.

Depending on if they are either rigid or flexible these widely used materials are mainly employed as thermal insulators, core of sandwich panels (rigid PU foams) or for comfort related applications (mattresses, seats) in the case of flexible foams. The production of PU foams is based on the reactive mixing of polyol and isocyanate. The comprehension of all the aspects related with the formation, structure and properties of these materials is far to be simple.

These webinars are focused on gaining a better understanding of the relationships between chemical composition, foaming mechanisms, structure, and properties of PU foams.

PART 2 – EXPERIMENTAL TECHNIQUES TO EVALUATE THE FOAMING KINETICS OF PU FOAMS

SPEAKER: MIGUEL ANGEL RODRIGUEZ-PEREZ

In this second part of the webinar, different approximations are considered to evaluate and understand how the changes in the formulation affects the reaction kinetics and foaming mechanisms (nucleation, coalescence, coarsening and drainage) of PU foams. It is also explained how the information obtained from this detailed analysis of the reaction kinetics can be used to develop foams with improved properties. 

When a PU formulation is modified by using, for instance, new polyols, new catalysts, new surfactants, adding particles, modifying the proportions of components, etc., the final foam can show very different properties and, in many times, worst properties. In the last few years, we have developed a collection of experimental techniques to follow the foaming process of PU foams to obtain a better understanding of the final cellular structure and properties of these foams and to optimize the PU formulations. These techniques evaluate on the one hand, the chemical aspects of the foaming process (FTIR spectroscopy and foaming temperature measurements) and on the other hand, the physical aspects of the foaming process (IR- expandometry and X-ray radioscopy).

FTIR spectroscopy allows following the reaction kinetics: evolution of the isocyanate consumption as a function of the foaming time and evolution of the urea, urethane and isocyanurate products (that is, evolution of the blowing, gelling and trimerization reactions) as a function of the foaming time. The analysis of the reaction kinetics, and how they are affected by the changes in the formulation, is a key aspect to understand the expansion, structure, and properties of the final foams. 

The measurements of the internal temperature allow analyzing the differences in reaction kinetics, for instance higher temperatures are normally related with higher isocyanate conversions, and they can also have an important effect on the evolution of the thermal conductivity with time. 

IR-expandometry, allows following the expansion kinetics (evolution of foam height and foam volume with time) as well as the evolution of the surface temperature during the foaming process.

X-ray radioscopy allows analyzing the foaming mechanisms (cell nucleation cell growth, and cell degeneration) produced at all intermediate stages during foaming. In other words, it is possible to analyze the evolution of the foam density as a function of the foaming time as well as the evolution of the cell size and cell nucleation density as a function of the foaming time. With this technique it is possible to evaluate, both qualitatively and quantitatively, how the nucleation, coalescence and coarsening mechanisms are affected by the fact of changing the formulation.