Frequently Asked Questions
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What is the difference between pore size and pore volume?
Pore size and pore volume are measurements typically associated with the following material technologies: porous plastic, sintered bronze, and sintered ceramic. The term pore size is typically used to define the average size or the nominal opening size that exists throughout a porous material. The opening size exists between adjacent spherical particles that have been fused to each other, and the typical unit of measure is the micron, µ.
Figure 1 represents four spheres that are sintered to one another. Imagine that a fluid is traveling through this structure (normal or into your computer monitor). Now, draw a line across the opening that exists between the four spheres. What is the length of the line? If you answered, “It depends on the starting and ending points that you select,” then we agree. The measured opening size does depend on the location of these points.
In Figure 2, Line A is clearly longer than line A'. So, how might you better describe the opening size? You might recommend drawing numerous lines across the opening and representing the size as an average value. Again, we agree and this is how many analytical instruments calculate pore size. Well, the instruments don't actually draw lines as shown in Figure 2, but the instruments effectively do the same thing with fancy algorithms.
There’s another concept to bear in mind when thinking about pore size for sintered parts. The spheres, which represent the raw materials used for porous plastic, sintered bronze, and sintered ceramic articles, vary in diameter, Figure 3. When the raw material’s particle size varies, so does the nominal pore size. So, what we typically see is a pore size distribution curve, Figure 4, which describes the pore size of a material.
Pore Volume is simply the amount of air present in a porous material. A material's pore volume is typically expressed as a percentage (volume of air / total volume of part). If you had in your possession a porous part that measured 1 cm X 1 cm X 1cm in size, then the total volume of the part will be 1 cm3. The total volume of air located inside this1 cm3 part might measure .45 cm3. Therefore, the materials pore volume is .45 cm3/ 1 cm3 = 45%.
Figure 1. Four sintered particles.
Figure 2. Four sintered particles. Distance A, A'
Figure 3. Particel Size Variation
Figure 4. Pore Size Distribution Curve
What is the pore size range for porous plastics?
Generally speaking, pore sizes can range from 10 micron to 250 microns. However, this answer is somewhat material dependent. For example, pore sizes from 10 µ to 150 µ are available with a polyethylene (PE) raw material. Pore sizes from 50 µ to 250 µ are available with a polypropylene (PP) raw material. Parts made from PVDF and PTFE are available with pore sizes of 30 µ.
Is ultra high molecular weight polyethylene (UHMW-PE) suitable for use as a fuel filter?
Nalgene has a very comprehensive chemical compatibility guide that we recommend you leverage to answer a host of chemical compatibility questions. Polyethylene is listed as "High-Density Polyethylene" instead of "Polyethylene". Access the guide by cliching the following link: Chemical Compatibility.
What is the smallest diameter part that can be made (one diameter)?
The answer to this question depends on the part's pore size. Smaller pore sized parts can be made with smaller diameters (and thicknesses). Molded parts with pore sizes in the 10µ - 50µ range can often times be produced that have diameters of .0625" (1.58mm). Porous sheet materials with comparable pore sizes and a thickness of .0625" (1.58mm) have been die cut with diameters of .050" (1.27mm).
What are the most commonly used polymer types for ThermoPore's porous plastics?
Currently, polyethylene and polypropylene constitute 85% - 90% of ThermoPore's sales. However, we are always experimenting with new and novel materials in an effort to leverage the inherent strengths or different polymer families.
What is the difference between molded tube and welded tube?
ThermoPore's molded tube is just that - tube shaped product that has been discretely molded in a tool. The tool has an outside diameter (OD), inside diameter (ID), and overall length (OAL) that corresponds to the parts OD, ID, and OAL. Welded tube, however, is converted from porous plastic sheet stock. Tubes are molded when thicker wall thicknesses are required (greater than .125"/3.18 mm) in thickness. Tubes are welded when thinner wall thickness are required (less than .125"/3.18 mm).
What are the recommended operating temps for porous polyethylene?
The maximum recommended service temperatures vary with conditions: 90°C (194°F), continuous, stressed; 100°C (212°F) continuous, not stressed; 116°C (240°F) intermittent.
What methods are typically used to measure air permeability?
Air Permeability describes the resistance that air incurs as it attempts to travels through a porous material. Materials with small and tight pore structures 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 (e.g.,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 (e.g.,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 (e.g., a Gurley number of 23 sec.).
What parameters influence a material's flow resistance?
The material's thickness can play a significant role. The less obvious parameter, the shape of the raw materials comprising the porous media requires a quick discussion. Fluids (gas or liquid) must make there way around and through the porous materials inner matrix in order to successfully travel through the material. Air flow resistance is commonly expressed the amount of back pressure that is created when a fluid attempts to make along this path. Every change of direction, every turn that the fluid makes in an effort to navigate through the material's matrix requires that work be performed on the fluid. Flow resistance is simply an expression of the amount of work that it takes to force a fluid through a porous material. Decreases in flow resistance are accomplished by minimizing the work required by the fluid. Porous articles made from raw materials with very high aspect ratios tend of decrease flow resistance while maintaining excellent functional properties. Why is this? It's a good question so let's explore.
Let's start with an analogy. Imagine that you are given the task of navigating through two forests, both of which are 100 yards/meters in depth. Your task is to travel a straight line through the forest and minimize your total foot travel. The first forest is a young forest with young trees that are only several inches in diameter. When you look through the forest, you map a course from point "A" to point "B". You have to travel around some of the forest's trees, but for the most part, you're able to navigate through the forest from one side to the other without having to move to far off your straight line. The second forest is old growth with monster redwoods trees- some of which are ten feet in diameter. Again, you map a course from point "A" to point "B" but you encounter a number of larger redwoods in your way. So, you have to circumnavigate each one in order to arrive at your final destination. Circumnavigation requires that you alter your course of travel and this requries extra effort.
When a fluid is trying to accomplish the same navigation through a porous material, circumnavigation requires that work be performed on the fluid. You can minimize this work by using materials of construction that offer very high aspect ratios (high length to diameter ratios...like the young trees).