Heat Reflecting Insulation: the Concept of Emissivity in Thermal Packaging Materials
Have you ever wondered why cold chain packaging companies use aluminum layered box liners or aluminum bubble mailers? It’s the same reason Marathon runners wear those metallic space blankets at the end of the race, why it’s terrible to get into a car with a dark colored interior in the summer and why the Apollo spacecraft employed aluminum foil as a heat shield.
To understand these phenomena let’s first go over radiant heat. Heat can transferred be from one object to another in a few difference forms: conductive heat, convective heat, or radiant heat. In very basic terms, conductive is when heat is passed from one object to another by contact. Think about a hot skillet passing heat to frying bacon. Convective heat is heat that is passed from on object to another through a fluid (fluid can be liquid or gas). Think about hard boiling an egg; the hot pot transfers heat through the water to the egg. Finally, radiant heat is when heat is transferred via electromagnetic waves. That just means heat transference in the form of visible light, infrared, etc. The sun emits radiant heat, microwaves emit radiant heat and any object that is hot will emit radiant heat.
When we try and insulate our perishables from heat we care about protecting against all three methods of heat transfer but the reflective properties of insulation do a very good job in mitigating the effects of radiant heat.
When we try and gauge a material’s ability to insulate against radiant heat, the most important property is called emissivity.
When radiant heat hits an object, three things can happen. The radiant heat can be reflected, absorbed, or transmitted. From an insulation perspective, if the radiant heat is absorbed it makes the insulation hot, therefore increasing the conductive heat transfer to the payload it is protecting. If the radiant heat is transmitted, well then it’s just passing that heat along to the payload and that’s not great either. The best outcome would be reflecting that radiant heat back out.
The property of emissivity measures the amount of radiant heat that is reflected vs absorbed or transmitted (emitted) and is measured on a scale of 0 to 1. Emissivity of 1 represents all radiant heat being absorbed or transmitted and 0 represents all radiant heat being reflected. Smooth, shiny materials like aluminum foil or polished silver will have very low emissivity values, close to 0. Rough, unpolished materials like asphalt or brick will have emissivity values close to 1.
Cold chain packaging companies use aluminum foil box liners because the aluminum foil has a low emissivity, meaning it reflects back thermal radiation and reduces heat transfer from outside the package to the perishables you’re trying to keep cold. Marathon runners are trying to stay warm at the end of their race but they can still leverage the low emissivity properties of a metallic surface because their space blanket is reflecting their body heat back at them. Those black leather seats in your car that scorch and scald sunburnt skin have a high emissivity value, meaning they are absorbing that radiant heat that is coming in from the hot summer sun. In the vacuum of outer space, since there is no conductive or convective heat transfer, the space craft’s low-emissivity heat shields are protecting against radiant heat, e.g. solar flares from the sun.
OK, so how can we use this to protect perishables? Minus Works has produced a product to capitalize on the insulating qualities of low emissivity material. The CORDILLERO Series is an innovative product that consists of a gel pack contained in a low emissivity metallic coating. We offer it in a standalone pack or in a saddle-bag /linked chain format. The chain is designed to be wrapped around the perishable payload, providing refrigerant benefits as well as thermally reflective insulation.
Like other Minus Works products, the CORDILLERO is engineered for the environment and uses recycled material in the film containment and 100% renewable, plant-based inputs in the refrigerant gel.
Bonus Points: Emissivity, also changes with surface temperature. According to the Stefan-Boltzman Law and blackbody radiation theory, the colder the surface temperature, the lower the emissivity for metals. Because the CORDILLERO Series uses a radiant barrier outside of the gel refrigerant, the cold gel actually further lowers the emissivity of our material, allowing the product to reflect even more heat and be an even better insulator.
 According to Kirchhoff’s Law of Thermal Radiation, heat emissivity and heat absorption are equal for opaque materials.
 Shininess is not always a good indicator of emissivity because when we perceive something as shiny, we are only seeing it in the visible light spectrum whereas with thermal radiation we should also consider the infrared spectrum. Emissivity is wavelength dependent. An example of this dynamic would be white paint – it reflects a lot of visible light but it emits a whole lot of infrared radiation, giving it a high emissivity and making it a poor surface for a radiant barrier.