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Characteristics of Microporous Insulation
Physical characteristics
Microporous insulation is essentially a finely ground, powdery substance comprised of a blend of ceramic powders and fibers. These materials, along with other additives that fluctuate with the application for which they are being employed, combine to form a high temperature material that provides excellent thermal stability, low thermal diffusivity and a thermal conductivity that is lower than still air.
Often, the microporous material is encased in a fabric or metal shell for improved handling and added strength, but it can also be quilted (for improved flexibility), easily molded, machined or fabricated for intricate part requirements. The material can also be pressed into metal castings or casings and then hollowed out in order to provide insulation for piping, electronics equipment and power units. It is ideal for any high temperature environment (up to its continuous service temperature of 1800°F), has been used historically in aerospace, automotive, industrial and commercial markets where space and weight conservation are critical necessities without compromising thermal performance.
Density and thickness considerations
The thermal performance of microporous material is somewhat dependent upon core formulation density. Because microporous seeks to minimize all three modes of heat transfer, increasing or decreasing the material’s core density above or below a certain point increases the potential of heat transfer in one or more modes. Although the standard density of a microporous product is largely dependent upon its product form (i.e. board, panel, flexible, etc.), most standard products tend to achieve greater maximization of the microporous material’s benefits at between 16 – 18 lb/ft3. Increasing the density beyond this point progressively increases the particle-to-particle contact within the mix, and therefore, the possibility for solid conduction of heat through the material. Similarly, decreasing the density below this range progressively increases the amount of void space between the particles, and therefore, the possibility for convection currents to exist within the material.
Unlike many conventional and fibrous forms of insulation, increasing the thickness of the material may not necessarily provide greater thermal performance unless the density of the material is simultaneously increased as well. The graph below illustrates this relationship between thickness and density.
Performance Comparison graph
Please note that manufacturing parameters may require certain products to be produced in higher or lower densities. Typically, board and panel products may be manufactured at 17 and/or 18 lb/ft3 densities to achieve greater strength, while most quilted products must be manufactured at a slightly lower density (i.e. 16 lb/ft3) in order to maintain the flexibility of the product.
Compression resistance
In general, compression resistance of microporous material is largely dependent upon the product’s core formulation density. Naturally, as density of a product increases, the compression resistance increases as well. As density decreases, the material is less compression resistant. As a result, high density board products offer excellent compression resistance where applications with large weight/force baring loads are employed or expected.The graph below illustrates the relative compression resistance of products within ThermoDyne’s DynaGuard industrial product line.
Compression Resistance graph
Approximate linear shrinkage
Although the actual linear shrinkage may vary slightly with the material and product form, a general thumb-rule for most forms of microporous insulation is that linear shrinkage is < 1.5% when tested at 1800°F for 4 hours.
Uniformity
Due to the manufacturing process of ThermoDyne microporous products, variations in material uniformity and core density distribution are extremely small, and within the allowable error associated with conductivity measuring equipment.
Anistropic nature of the material
A test performed in 1992, by Purdue University’s School of Medical Engineering, determined that most microporous materials have a slight anistropic quality, which gives the material a certain performance when heat passes through it in one direction, and another performance when heat passes through it in another direction.
Although this quality may be affected by particle size and fiber length, heat energy is estimated to have up to a 6 – 8 % easier path through the material when traveling “with” the fiber of the material as opposed to when it is traveling (as intended) against the fiber grain.
Hydroscopic/Hydrophilic nature of the material
The porous structure of microporous insulation material is, by nature, extremely hydroscopic. As a result, it absorbs liquid quickly and readily, and, when saturated, suffers an irreversible loss of its superior thermal performance properties (approximately 25+%, depending upon the product form).
However, natural humidity and water vapor (testing performed using 100% humidity at 100°F for 24 hrs) does not affect the microporous structure.
Hydrophobic options to combat the material’s inherent hydroscopic qualities are available in many product forms, particularly in environments such as ladle-liners and molten metal applications where the microporous material is used as a backup lining and may see some contact with water. These hydrophobic materials give added protection to the material’s microporous structure, and have service temperature use limits of approximately 900°F. After the hydrophobic components bur
Material performance in various atmospheres and altitudes/pressures:
As is the case with most insulation materials, the thermal performance is largely dependent upon certain environmental factors such as altitude (or atmospheric pressure) and the presence of various gasses. The relative performance of microporous materials is illustrated in the graphs below, and shows an example 20pcf microporous material operating in three gasses (Air, Helium, Hydrogen) at three altitudes (Sea Level/760mm Hg, 60,000ft/55mm Hg, and 100,000ft/8mm Hg).
Material at Sea Level graph
Material at 60,000 ft graph
Material at 100,000 ft graph
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