Microtherm® SlimVac - the leading vacuum insulation panel (VIP).
A technically superior product based on the best thermal insulation in the world.
What happens when you take Microtherm®, the best insulation in the world, seal it inside a “state of the art” multi-layer polymer barrier material and then draw a deep vacuum?
Answer – you create SlimVac® the best vacuum insulation panel currently available.
A simple fact - heat will naturally move from a warm or a hot region to a cooler region to achieve thermal balance. To stop or to slow down this movement of heat we can introduce an insulation into the transfer path. We in the Microtherm Group, with our innovative thermal insulation solutions, have been doing this very effectively for more than forty years. We are recognized as offering the benchmark systems in high temperature thermal protection and the best insulation solutions right down to cryogenic temperatures.
Another simple fact - the better the insulation – the more efficiently the movement of heat is blocked.
The best high temperature insulation of all is a microporous insulation such as Microtherm® which has a thermal conductivity that is so low it is almost down to the lowest theoretically possible according to the known laws of physics. This performance is maintained right down to cryogenic levels.
However, for lower temperature applications, Microtherm® is also used as the insulation core for SlimVac®, the leading vacuum insulation panel.
A microporous insulation is defined in ASTM C168 as -"Material in the form of compacted powder or fibers with an average interconnecting pore size comparable to or below the mean free path of air molecules at standard atmospheric pressure. Microporous insulation may contain opacifiers to reduce the amount of radiant heat transmitted."
Now, the physics - heat transfer can occur through solid conduction, gaseous conduction, convection and radiation, usually the overall heat transfer comes from a combined effect of all of them.
The page Microporous Insulation elsewhere on this web site covers the physics in more detail and explains just why Microtherm® is so very good at all temperatures from cryogenic up to 1000 °C (1830 °F).
However, by encasing Microtherm® in a multi layer polymer material with ultra low permeability, and then evacuating the composite panel to a few mbar pressure and sealing it, we create a true superinsulation panel – SlimVac®. Amazingly, SlimVac® has a thermal conductivity roughly five times lower even than basic Microtherm®. It is arguably the best vacuum insulation panel available and to explain why it is such a technically efficient insulation we must go back to the physics of heat transfer.
Solid conduction in Microtherm® is very low for two reasons.
- The nano-sized particles making up Microtherm® form into chains that are chemically bonded but have very restricted contact with one another, limiting thermal pathways (amount of heat conducted is directly proportional to the cross section of the conduction path i.e. small diameter particles/very small contact area/very low heat flow).
- The heat paths through the chains that form the solid matrix are very tortuous, and therefore long. This decreases the rate at which heat can flow by solid conduction (amount of heat conducted is inversely proportional to the length of the conduction path).
Because the evacuation process removes gas molecules and does not affect physical structure, SlimVac® panels have the same resistance to solid conduction as basic Microtherm®. In other words, SlimVac® is an excellent barrier to this mode of heat transfer.
Gaseous conduction in Microtherm® is restricted by the microporous effect. This is unique to Microtherm® and other microporous insulations and is the property which gives these materials their step-change decrease in thermal conductivity over conventional insulation materials. Unfortunately, not even Microtherm® is perfect and a small amount of gaseous conduction still occurs.
However, take that same Microtherm® and encase it and evacuate down to a few mbar pressure and all gaseous conduction is stopped. This is the simple technical reason for the dramatic lowering of thermal conductivity in a SlimVac® panel. Insulation simply does not get any better than this.
Convection cannot take place within a microporous insulation because the trapped air voids in the structure are far too small to allow any effective fluid flow movement of air molecules.
Radiation does have an effect even at the modest ambient temperatures at which SlimVac® panels are normally used. In Microtherm®, radiative heat transfer is blocked by the inclusion of an opacifier in the insulation structure. At high temperatures it is the reason for the impressive performance difference between Microtherm® and conventional insulation materials. At lower temperatures radiation is not so noticeable but by comparing thermal conductivity in vacuum insulation panels made both with, and without, opacifier, we have demonstrated that our TC is around 30% lower thanks to the opacifier.
The other important constituent of Microtherm® is the glass filament reinforcement. This helps to give the insulation strength for handling and also for resistance to compression. Good compression resistance is important in a vacuum insulation panel core as it maintains the panel thickness when it is subjected to atmospheric pressure after removal from the vacuum chamber. As there is no air inside the panel, all compression must be resisted by the physical structure of the insulation. It is worth remembering that atmospheric pressure acting on 1 m² of surface is equivalent to over 10 tonnes of applied load. Good compression resistance means that a lower density (lighter) insulation core can be used.
For SlimVac®, the best insulation available is encased in the very best of multi-layer polymer barrier bags. Modern barrier materials have ultra low permeation rates for air and water vapour and ensure long effective working lives for vacuum insulation panels. The permeation rates of these materials are so low that the equipment used to measure the values is simply not sensitive enough – the barrier materials are in fact too good! Values are always given as less than a particular figure.
The manufacturer’s quoted figures are:
OTR (oxygen transmission rate) < 0.0005 cc/m²/day.
WVTR (water vapour transmission rate) < 0.01 gm/m²/day.
With an outer covering of this quality, the weakest part of the enclosing bag is the welded seal where the inner sealant layer of two skins is fused together by a combination of temperature and pressure. The slightest imperfection in this weld will cause a potential leakage path into the evacuated panel.
Microtherm® cores are covered in woven E-glass cloth before insertion into the barrier bag. This provides a low friction surface for easy insertion and also ensures that no dust is deposited on the sections of the sealant layer that will be fused together. SlimVac® panels are absolutely reliable for a working life of many decades as described in Microtherm® SlimVac – the long life VIP.
Independent testing by FIW München which is a leading consulting services provider specialising in the thermal performance of buildings concluded that even after 15 years of use a SlimVac panel should still have a TC of only 5.3 mW/m.K. A copy of their report is available on this web site for downloading at Cert_FIW_testreport_Slimvac_EN.pdf.
A final hidden benefit for users of SlimVac® panels applies for building applications where the superb insulation performance is used to achieve ultra low heat loss rates through the structure of a building.
If the occupiers of the building should be unfortunate enough to suffer a fire they will have the reassurance of knowing that even though the SlimVac® components of the structure may fail as vacuum panels due to the excess of heat causing the polymer outer covering to be destroyed, the Microtherm® core material will continue to operate as the best passive fire protection on the market. For more information see Microtherm® - a perfect solution for passive fire protection elsewhere on this web site.
In every respect, SlimVac® offers the very best of quality and performance.