Foam IR Absorption

We want the insulation foam to absorb as much of the heat energy as possible to reduce the radiative fraction of overall thermal conductivity (see Foam-Conductivity). As discovered by Max Planck, every object emits radiation with wavelengths specific to its temperature. The energy of the radiation can be calculated using the Planck equation below. At each wavelength λ we need to know the Spectral Radiance L at temperature T (in Kelvin) using Planck's constant h, Boltzmann's constant k, the speed of light c and the frequency of the radiation ν:

`L=(2hc^2)/(λ^5(e^((hν)/(kT))-1))`

Using the T C slide, you can set the temperature of your interest (for instance, 20 °C as in a comfortable room). The blue line will show how much energy is carried through radiation with specific wavelengths.

To see if a material is good at absorbing heat radiation at the chosen temperature we need to know the absorption A of the material at these wavelengths. This can be seen from a standard FT-IR spectrum of the material. Confusingly we tend to plot the Planck graph in terms of wavelength μm and the FT-IR spectrum in terms of cm^-1. But the X-axis button will allow you to toggle the X scale units between μm and cm^-1

The graph provides FT-IR spectra of a few common insulation materials. Each spectrum is normalized to 1 as we are interested in getting a feel for the relative merits of different foam materials. The "1" for the spectra is defined as the largest peak in the classic FT-IR spectrum from 4000 to 400 cm^-1 to make the comparisons somewhat fair. Eventually the spectra must be normalized for constant sample thickness or maximum absorption.

Finally, the app integrates absorption and calculates the relative total energy absorbed by the material. This number is not absolute but shows quite nicely that some materials are better than others at absorbing heat radiation, so may have lower thermal conductivity. As is immediately obvious, and as is well known, PE and PS foams have low absorption of mid-IR radiation, while PU and Silica both show significantly higher absorption..

Your own data files

The app can check heat absorbing capacity of any material. All you need is the FT-IR spectrum of the material in a simple comma-separated values, tab-separated values, or .dpt format. Be aware that to get a reliable answer, you need to correct the baseline of the spectrum. The app makes no attempt to do any correction other than remove negative values. Then the file can be loaded directly into the app using the 'Choose File' option. If you don't have a FT-IR spectrometer, we have and we can take spectra of your materials and analyse them for you. Just Contact Us