25 May 2020

PUR/PIR sector challenges - lambda value

What is the lambda value and why is it so important in the thermal insulation industry? Is there a limit to which it can be reduced? Apart from being a physical property, the thermal conductivity coefficient which is so often talked about in the thermal insulation industry, determines the class of an insulation material, which in turn directly affects the market position and price of a given product. It is no wonder, after all, as the value of an insulation product should be determined by the product's ability to reduce heat loss. The question is: How far does this ability go?

The lambda value and polyol production

The thermal conductivity of insulation foam rests on the conductivity of the polyurethane cell gas mixture, the conductivity of the solid polymer and the thermal radiation occurring between the cells.
Nearly all segments of the polyurethane industry struggle to reduce the thermal conductivity coefficient in a finished polyurethane barrier. Polyol manufacturers modify the polymer matrix to increase the effect that the lambda value of the material constituting the cell wall has on the insulating power of the finished foam.

The lambda value and other system components

Meanwhile, stabiliser manufacturers are working on the ratio of hydrophobic chains to hydrophilic heads of the silicone molecule by connecting it with increasingly complex chemical groups, all just to appropriately modify the surface tension and affect the structure, size and shape of the forming cells, which have a direct impact on the conductivity of the finished foam.

It is worth pointing out that silicone stabilisers are not the only surfactants applied in polyurethane systems to improve the thermal conductivity coefficient. These also include surfactants which are not categorised as silicone: mainly alcohol alkoxylates and fatty acid alkoxylates. We can therefore imagine the vast number of potential molecule combinations that affect the formation of polyurethane cells and, eventually, the thermal conductivity value.


This raises the question: To what extent can stabiliser manufacturers regulate this effect by means of particle structure? Polyurethane production is the resultant of numerous physicochemical phenomena. Here, mixing and the influence of pressure and temperature must be taken into account. It is worth keeping in mind that not all laboratory discoveries generate a symmetrical change in large-scale production.

What affects the thermal conductivity of polyurethane foam?

In general terms, thermal radiation is reduced when the cells are made smaller, and thermal conductivity in a solid polymer is decreased by reducing foam density. However, it is primarily the conductivity of the cell gas mixture that determines the thermal conductivity of foam. About 65-80% of the insulating power of foam results from the insulating properties of the cell gas mixture, while cell size and density combined account for the remainder.

The future of the thermal insulation industryIs there a perfect gas whose conductivity will be low enough to significantly reduce the thermal conductivity of polyurethane? At this point, it should be noted that such a gas must also change its state of aggregation within the temperature range used to form polyurethane.
One should bear in mind that only a close correlation between three phenomena: thermal radiation, conductivity of the cell gases, and conductivity of a solid body, which in this case is the polymer constituting the cell walls, will generate the expected result in the thermal conductivity of polyurethane foam. There seems to be a large number of possible variations.

However, to what extent are able to reduce cell size or decrease foam density without losing other important physical and mechanical properties? Let us remember that there are limitations imposed by physics in this aspect. Are we then able to override its existing laws? Will we discover a gas with extremely low intrinsic thermal conductivity that will be both eco-friendly and biologically inert? They say that necessity is the mother of invention...


Author: Monika Cybulska-Kucharska
Process Engineer


A graduate of chemical technology at the University of Technology and Life Sciences in Bydgoszcz. A technologist who is guided by the motto that humbleness and experience is a guarantee of success. My adventure with the polyol-polyurethane market lasts almost 8 years! I actively participate in many projects aimed at improving Purinova's products. Privately, she is an amateur of small and large journeys and an enthusiast of arranging puzzles.