Thermal insulation foam is a versatile material that can be used in a wide range of applications. It provides excellent temperature control, sound reduction and can also help to prevent the growth of mildew or mold. Foams are widely used in insulating homes and offices as well as being a great option for packaging products.
The R-value of thermal insulation foam varies from 3.2-3.8 per inch for open cell polyurethane (oc) spray foam to 5-6.5 per inch for closed cell polyurethane spray foam and is therefore a good choice for thermal and moisture protection in buildings. It can be sprayed directly onto roof tiles or concrete slabs, into wall cavities or into holes drilled into walls.
A variety of biobased materials are used to make these foams, including EPS/XPS, PU, and PLA. Each of these materials have unique structural characteristics that impact their thermal conductivity. Moreover, many biobased foams have been reported to exhibit different cell size distributions and are not consistent with a homogeneous, closed-cell structure.
It is important to note that the available data on thermal conductivity of these experimental foams are limited in both number and range. This makes it difficult to assess trends and predict how specific heat transfer mechanisms influence the performance of these foams. However, the following analysis aims to identify potential trends present in these data and provide preliminary insights into how these variables affect the overall performance of biobased foams.
The relationship between foam structure and thermal conductivity is shown to be a power law, with the resulting value asymptotically approaching the thermal conductivity of air in a linear fashion. This is largely attributed to the ability of the cells in the foam to act as barriers against the phonon transport of air. This can be seen in Fig. 2, which shows a graph of average thermal conductivity data for PLA, cellulose, and partially biobased PU foams.
In a similar manner, the thermal conductivity of foams fabricated from a blend of two or more biobased materials is often displayed as a power law. This behavior is also reflected in a variety of other structural characteristics, such as cell size and density.
Compared to other types of polyurethane foams, partially biobased PU foams tend to display more consistent thermal conductivity performance, albeit in limited ranges. In contrast, fully biobased PU foams may show large fluctuations in their thermal conductivity.
This could be caused by a combination of factors, such as anisotropy in the foam structure, which can lead to cell elongation in one direction, or by adjusting the blend components or additives. Both can significantly affect the final thermal conductivity of a biobased foam, but they are not always reported in the literature.
A more recent, more accurate approach to predicting the thermal conductivity of these foams is the use of mathematical models. These models are able to account for the effects of both the thermal conductivity of the bulk material and the individual heat transfer mechanisms within the foam.