For aerospace design, whenever heat is a problem the ideal thermal insulation will be...
Not All Microporous Insulations Are The Same
Category: Microporous Insulation | 16/04/2007 - 11:31:21
The more astute designers know that the best high temperature insulation is a microporous insulation that can be as much as four times more efficient at high temperatures than conventional insulation materials. However, very few designers realise that not all microporous insulation materials are equal and the differences in capability, and in health and safety issues, become more apparent when they are pushed to the limit and exposed to their maximum rated temperatures.
A microporous insulation is by definition around 90% air, but the air is contained in minute cells, or pockets, that are smaller in average size than the mean free path of an air molecule. In heat transfer, gaseous conduction occurs when gas molecules collide and transfer their kinetic energy. The mean free path is the average distance they need to travel before they hit another molecule. If the collisions are prevented, as in a microporous insulation, heat transfer through the gas is dramatically reduced. When you realise that the dimension of the mean free path of an air molecule at STP is around 93 nm (3.66 x 10-6 inches) you begin to appreciate the microscopic scale at which this all happens. It's truly nanotechnology in insulation.
Illustration (i) : Comparison TC chart.: All materials rated by manufactures for 1000°C operation
The main constituent of a microporous insulation is usually pyrogenic silica which is present as very small amorphous particles. In Microtherm® these range in size from 5 - 25 nm and the silica has a low intrinsic thermal conductivity of around 1.4 W/m.K, meaning it's a good insulation anyway. By size comparison, the diameter of a human hair can be anything from 1000 to more than 7000 times bigger. These small particles chemically bond to form long particle chains which then tangle around each other as the insulation is mixed and formed. On a microscopic scale they form long convoluted heat conduction paths through the insulation. Now, solid conduction of heat through a material occurs when adjacent molecules vibrate together and transfer energy and it is influenced by two separate dimensional factors. The rate of solid conduction is directly proportional to the cross sectional area of the conduction path, and inversely proportional to the length of that conduction path. In Microtherm®, as with other microporous insulations, all the properties of the silica structure combine beneficially to give an exceptionally low rate of solid conduction.
To give Microtherm® integrity for moulding and machining and to give it handling strength it also contains a small percentage of chopped glass filament reinforcement. The insulation is classified by the World Health Organisation as free of respirable fibres as defined in the European Dangerous Substances Directive Amendment, 97/69/EC.
The other more important ingredient is the opacifier which is a fine mineral oxide powder that gives the insulation the ability to block the movement of infrared radiation almost completely. We know from the laws of physics, from the Stephan-Boltzmann fourth power law to be precise, that radiative heat losses from a surface are directly proportional to the fourth power of the temperature difference. At temperatures above 100 oC (around 212 oF) radiation becomes the dominant mode of heat transfer and increases rapidly as things get hotter.
Illustration (ii) : Chart - effect of opacifier on performance of Microtherm®
Infrared radiation is a form of electromagnetic radiation with a wavelength longer than visible light but shorter than microwave. It is just outside the red end of the visible spectrum and exists over a range of wavelengths divided into "near", "mid", and "far" IR. Far infrared waves are thermal and any object which has a temperature above absolute zero will radiate in the infrared.
The small particles of the mineral oxide opacifier are dispersed uniformly throughout the Microtherm® and work by refracting (bending) the IR wave at the particle surface and changing its direction. The particle size is close to the wavelength of the IR to optimise the effect. Repeated scattering of the wave occurs, mostly close to the surface of the Microtherm®. A portion of the IR wave is actually scattered so much that it actually goes back out of the insulation. Although the insulation is an almost perfect block to IR radiation, inevitably a small portion of the radiation does penetrate. However, the efficiency with which Microtherm® blocks the transmission of IR radiation is the reason for its remarkable high temperature performance.
All of these facts so far are simply an overview of the physics relating to any microporous insulation, or nanoporous as some material specialists prefer.
However, if we consider in greater detail the interaction of product quality and thermal performance it soon becomes very apparent just why Microtherm® outperforms other microporous insulations. It is the recognized global market leader in microporous applications with more than double the market share of its closest rival.
All Microtherm® constituent materials are used under rigid quality control. Pyrogenic silica, sometimes known as fumed silica, is sourced to an optimised particle size and surface area for the best low thermal conductivity/low density combination. These silicas are created with high surface area - enough that 30 g (just under 2 oz) will have a combined surface area sufficient to cover a soccer pitch. This improves their effectiveness for their common applications such as for catalysts and adsorbents, pharmaceuticals, and computer chips. Microtherm® likewise benefits from that exceptional surface area.
Glass filament is available in various types and can be used in a range of diameters and chopped lengths (selected for Microtherm® in accordance with stringent health and safety considerations).
Similarly, a range of different mineral oxides can be chosen as opacifier alternatives.
By always selecting the optimum specification for all constituent materials, Microtherm® is made to a consistent high quality with a thermal conductivity as low as, or lower, than other microporous insulations. This applies across the full temperature range capability up to its maximum rated temperature for continuous full immersion exposure of 1000 oC (1832 oF). If this temperature capability is not high enough, Microtherm® Super A can be used continuously up to 1200 oC (2192 oF).
However, at these very high temperatures there is another performance aspect where Microtherm® excels. As with all insulation materials, a small amount of irreversible lateral shrinkage will occur at the maximum temperature limit and above. As the temperature increases, the particles of silica begin to sinter and fuse together, changing the nature of the structure and increasing the solid conduction component of heat transfer. With Microtherm® insulation this shrinkage is extremely slight and rarely has any influence on the effective performance. This shrinkage is actually much less than for other, competing microporous insulation products at the same temperatures.
Illustration (iii) : Shrinkage comparison of insulations at 1000 oC (1832°F).
Microtherm measure shrinkage according to internationally recognized standards - ASTM C356, BS-EN 1094-6, and ISO 2477. In each case, 24 hour full immersion exposure conditions are assessed. These are much more severe assessments than methods commonly used by other manufacturers and they ensure that our specified maximum service temperatures for our products are always on the cautious side. The quoted maximum exposure temperatures for Microtherm® insulation products are deliberately selected for continuous exposure conditions with due reference to maintaining very low shrinkage.
Unfortunately, there is also a health and safety consideration at temperatures of 1000 oC (1832 oF) and higher, and once again, here, Microtherm® excels.
Amorphous fumed silica particles are absolutely safe and used in a wide variety of every day applications. However, in the case of microporous silica based insulation material exposed to extreme temperatures there can occasionally be a problem.
After prolonged exposure to conditions of very high temperature exceeding the manufacturer's temperature limits recommended for continuous use, it is possible that the silica will devitrify and some crystalline silica in the form of crystobalite may form. This has been classified as a human lung carcinogen.
All microporous insulations can be affected in this way although some are appreciably safer than others. Microtherm is possibly the safest.
After rigorous analysis it was found that with Microtherm products exposed to 1000 °C (1832 oF) for several weeks barely detectable levels of crystalline silica could be observed. In other testing by the Institute of Occupational Medicine no respirable crystalline silica was detected. Similar analyses of some of our competitors' materials have revealed in some cases much higher levels of crystobalite under equivalent exposure conditions.
The conclusions are apparent. Microtherm® is the market leader in microporous insulation applications because it is demonstrably the best in thermal protection performance, product quality, and user safety. Why use anything but the best?
Further information on Microtherm® products and applications can be found on www.microthermgroup.com.