For temperatures other than ambient, the thermal performance and the relative variation with pressure changes in a complex but predictable fashion.  Heat transfer through these porous materials occurs via the sum of three different mechanisms; 1) solid phase conduction, 2) gas phase conduction, and 3) radiation.  As the operating temperature increases, solid phase conduction increases with a fairly weak temperature dependence.  In the gas phase conduction, two changes occur:  First, the thermal conductivity of the bulk gases increases in almost direct proportion to absolute temperature.  Secondly, increasing temperature results in an increase in the mean free path of the gas.  Therefore, higher temperatures yield higher bulk gas conductivity but this is partly offset by a reduction in the effective gas phase conduction at a particular pressure level due to a greater percentage of molecules striking the pore walls rather than other gas molecules.  The third component, radiation, can be highly temperature dependent depending upon whether the insulation is sufficiently opacified with infrared absorbing and/or scattering materials. A graph of both thermal conductivity and R-value for NanoPore™ based insulation products and it's relation to temperature is shown in the figure below (note temperatures are in Kelvin).

For the lowest temperature, performance improves at all pressure values because of reduced solid phase conduction, gas phase conduction, and radiation.  However, the characteristic decrease with pressure from the Knudsen effect actually shifts to lower pressures because of the shorter mean free path (~20 nm at 100 K versus ~60 nm at 300 K). At higher temperatures and moderate vacuum (>100 mbar), the thermal performance is virtually independent of temperature since the higher bulk gas conduction, solid phase conduction, and radiation terms are balanced by a decrease in the effective gas phase conductivity arising from the enhanced Knudsen effect caused by the larger mean free path.

NanoPore™ Thermal Insulation for High Temperature

NanoPore™ HP-150 can be used at temperatures up to 300°C, but for higher temperature use NanoPore produces a special high temperature insulation, NanoPore™ HT-170 which can be used up to 800°C and above in some cases. For applications with highly specific performance requirements, custom grades of NanoPore™ Thermal Insulation can provided to meet a project's special needs.

High Temperature Implications for VIPs

NanoPore™ VIPs can be used at temperatures below 120-150°C, depending on the required lifetime of the product. Special high temperature barrier materials can be used for longer lifetimes at high temperature but generally speaking higher temperature applications will shorten VIP lifetime.

A key property of a VIP insert material, other than its’ thermal performance, is the filler's mechanical stability  under a one bar load at the operating temperature.  Changes in physical dimensions during use at temperature results in loss of thermal performance, higher probability of premature barrier failure, and loss of mechanical integrity for the entire insulation system. NanoPore based insulation products exhibit excellent mechanical stability at temperature with almost no thermal expansion problems or shrinkage and collapse that can occur polyurethane or open-cell polystyrene based VIPs which can occur at temperatures as low as 80°C. This stability is not surprising when one considers that slightly modified versions of NanoPore™ are used as insulation at very high temperature (>800°C).