MLI Blankets consist of multiple layers of highly reflective, low emittance or “E” materials. Similar to “Low E” windows in your house, emittance measures a material’s ability to reflect solar energy. A material that has low emittance properties will have highly reflective properties like a mirror and would deflect heat and specifically in a space environment, solar radiation.
For optimum insulation performance, successive metalized layers are separated by materials with low-thermal conductivity like Nomex or polyester netting. One layer of the MLI structure may reflect as much as 97 percent of incident radiation and adding additional layers decreases absorptance between layers exponentially until it is completely dissipated by the time the energy would reach the protected component.
The simplest way is to support and separate layers using crinkled or embossed film. This ensures that adjacent layers are connected at only a few points. To avoid metal-to-metal contacts of high thermal conductivity, the film must be metalized on only one side.
A more common and more effective method is to separate and support layers of two-side-aluminized film using a thin layer of a poor thermal conductor. Fiberglass material, paper, cloth, and various polymer non-woven or meshes can be used. An additional benefit of this method is that the non-conductive layer provides added puncture resistance and overall creates a more robust construction.
Additionally, the two-side-aluminized films offer lower absorptance because they reflect radiation on both the first and second surface.
Most MLI blankets are based on three foundations, polyester, polyimide or fluorocarbon films. These base materials are metalized, coated, laminated, perforated, crinkled or embossed or some combination to meet different product specifications. For perspective, an MLI film manufacturer could have a product portfolio of over 400 products based on the three aforementioned film types.
The films are the base layer for TPS and are selected for a few key properties including physical, thermal, electrical and chemical. See chart below for data.
Some MLI blankets use coated films in addition to metalization. Various coatings are used for corrosion resistance, controlling emittance and bonding dissimilar substrates together.