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ThermoPore's X-Pore™ material listing serves as an excellent starting point for your development project. Sorted first by raw material type and second by pore size, the X-Pore™ materials listed within this table characterize ThermoPore's material capabilities in very general terms. Of course, ThermoPore possesses ample amounts of material processing capability that can considerably broaden this material offering, so don't limit your design to the material options shown here.
Description of Material Parameters
H20 - In there natural state, ThermoPore's porous materials will not wick water, i.e., they are hydrophobic. However, the material can be rendered hydrophilic through the use of simple raw material additives.
SS - A special additive can be incorporated into a porous plastic part's structure to cause the parts to be "self-sealing". You can learn more about this novel material additive and how it functions at ThermoTV.
Pore Size is determined through Mercury Intrusion Porsimetry. Due to mercury's high surface energy, the porous plastic does not seek to be "wetted" by the mercury. Put another way, the mercury does not naturally enter to pores of the porous plastic (as water does with a typical kitchen sponge). A mercury intrusion porsimeter, however, applies a know amount of force onto the liquid mercury in order to infuse the mercury into the pores of the porous plastic. Generally speaking, it takes more force to push the mercury into pores of smaller diameter and the mercury intrusion porsimeter measures and records the applied force and the amount of mercury infused into the porous plastic's structure. Analytical analysis of the curve creates a pore size distribution that is "bell" shaped in nature. The pore size value listed above represents the average pore size - but there are often times pore +/- 20% of this size within the porous plastic in lesser and lesser quantity as you travel away from the average.
Pore Volume is determined through Mercury Intrusion Porsimetry testing. During this test, the sample's exterior (or envelop) volume is measured. Next, the volumetric amount of mercury introduced into the sample is record. The pore volume of the porous plastic is expressed as the ratio of air volume to plastic volume as a percentage. Therefore, a pore volume of 45% would describe a porous plastic article that was 55% plastic, and 45% void of plastic.
Water Intrusion Pressure
As described above for Bubble Point, porous materials have an affinity for "wetting" fluids, i.e., fluids with low surface energies. Increase the fluid's surface energy or decrease the surface energy of the porous material's surface, however, and the opposite is true. Porous materials with hydrophobic characteristics have a natural ability to withstand or hold back water intrusion into their structure. However, their ability is a function of three variables: the relative surface energy differences between the porous plastic and the challenging fluid, and 2) the pore size of the porous plastic, and 3) the amount of pressure that the fluid is exhibiting onto to porous plastic. If we use water as the challenging fluid and a polyolefin as the base polymer for the porous plastic, a material's water intrusion pressure increases as the pore size decreases. The amount of pressure that the material's is capable of incurring just prior to water intrusion is referred to as the water intrusion pressure. Water Intrusion tests are very valuable in hydrophobic venting applications where the largest through-hole might dictate a vent's ability to perform in various applications.
Air Filtration Efficiency describes a material's ability to capture particles of various sizes. This test involves the preparation of an upstream sample (also referred to as the influent sample) with a known concentration of particulate of know diameter. Typically, the upstream sample will be characterized by particle count data and particle size data. Next, the supply air is moved through the porous plastic. Many of the particles do not make their way through the porous plastic. As a result, there are fewer particles present in the down stream sample (also referred to as the effluent sample). The ratio of the number of particles of a specific particle size (diameter) in the influent versus the number of particles of a specific particle size (diameter) in the effluent can be expressed as a percentage. This percentage represents the penetration percentage. However, filtration efficiencies are typically expressed in terms of percent capture which is equal to one minus the penetration percentage.
Filtration efficiencies change with different flow rates. Flow rates are typically expressed in terms of "face velocity" which is calculated by dividing the volumetric flow rate by the size of the test sample. This yields a face velocity unit that is simply distance / unit time (for example, inches/sec., feet/minute, cm/sec., or m/sec.). Because of the fact that a material's filtration efficiency varies with changes in the face velocity of the influent stream, air filtration efficiency specs need to reference the test condition's face velocity, particle size, and capture efficiency. A typical format for filtration efficiency takes the following format: 99.9% efficient for particles with a diameter between .01 and .2 microns at a face velocity of 3 ft./min.
Air Permeability describes the resistance that air incurs as it attempts to travels through a porous material. Materials with tight pore structure usually create more resistance to air flow than materials with more open or larger pore sizes. Air Permeability is typically expressed with three parameters: air flow rate, differential pressure, and time. Because air permeability is not linear with different face velocities, the proper specification of air permeability should include both differential pressure and face velocity values. In one scenario, the differential pressure can be held constant and the face velocity can be recorded (3 ft/min @ ΔP of 1.2" H2O). In a second scenario, the face velocity can be held constant and the differential pressure can be recorded (ΔP of 2.6" H2O @ face velocity of 3 ft/min). In yet a third scenario a Gurley number can be referenced. The Gurley number is equal to the amount of time (seconds) required for a known volume of air to pass through a known sample size of material when a constant pressure is applied to the influent air stream (i.e., a Gurley number of 23 sec.).