In the field of high-temperature dust removal, there is a temperature point that is more insidious than the continuous operating temperature and more fatal than instantaneous temperature resistance. Before this temperature, the filter media is as hard and rigid as glass, maintaining a precise pore structure; after this temperature, the fibers suddenly “soften,” losing rigidity, and the filtration accuracy collapses overnight. This temperature point is called the glass transition temperature (Tg).
I. What is the Glass Transition Temperature?
To understand the glass transition temperature, it is necessary to first understand the two states of polymer materials.
Glassy State: At low temperatures, the polymer chains are “frozen” and cannot move freely. The material is as hard and brittle as glass, maintaining a fixed shape. This is the origin of the name “glass transition.”
Elastic State: When the temperature rises above a certain critical point, the polymer chains gain enough energy to begin moving. Materials change from hard to soft, from brittle to elastic, deformable like rubber.
The glass transition temperature (Tg) is the critical temperature at which a polymer material transitions from a glassy state to a highly elastic state.
A classic analogy describes this: the glass transition temperature is like the melting point of a candle—below the melting point, the candle is hard and can stand upright; above the melting point, it becomes a soft lump of wax, still the same material, but no longer able to stand upright.
II. Why is Tg more “insidious” than the continuous use temperature?
The continuous use temperature is usually higher than the glass transition temperature. This is where the trap lies.
Taking several common filter media as examples:
–Polyester (PET): Tg approximately 80-90℃, continuous operating temperature approximately 130-140℃;
– PPS: Tg approximately 90-100℃, continuous operating temperature approximately 160-180℃;
–P84: Tg approximately 315℃ (polyimide has a very high Tg), continuous operating temperature approximately 220-240℃;
–PTFE: Tg approximately 115-120℃ (but PTFE has high crystallinity, making its actual softening behavior more complex), continuous operating temperature approximately 240-260℃;
– Aramid: Tg approximately 270-280℃, continuous operating temperature approximately 200-220℃;
Observing these data reveals that the continuous operating temperature of most filter media is significantly higher than their glass transition temperature. This means that within the temperature range where the filter media “operates normally,” the fibers are actually in a highly elastic state—they are no longer in a rigid glassy state.
III. How does the “significant decrease in filtration accuracy” occur after exceeding the glass transition temperature (Tg)?
This is the most crucial question. After exceeding the glass transition temperature, the decrease in filtration accuracy doesn’t occur suddenly, but rather gradually through three overlapping mechanisms.
Mechanism 1: Fiber Deformation Leads to Pore Enlargement
In the glassy state, fibers are rigid. When dust particles collide with the fibers, the fibers remain stationary, and the particles are intercepted.
In the elastic state, fibers are flexible. When dust-laden airflow passes through the filter media, the airflow exerts a dragging force on the fibers; during pulse backflushing, the reverse airflow exerts an even greater impact force on the fibers. These forces cause the flexible fibers to bend and shift, “squeezing apart” the originally tightly packed fibers, increasing the pore size.
Large particles that could previously be intercepted by the surface fibers now penetrate the enlarged pores and enter the deeper layers of the filter media. Even finer particles penetrate the entire filter media layer. The result is a significant decrease in filtration accuracy—the smallest particle size that the filter media can effectively intercept.
TrennTech, a leading supplier of high-temperature filter cartridges, conducted comparative tests in its laboratory: filtration efficiency of the same batch of PPS filter media was tested at 100℃ (approximately 10℃ above Tg) and 150℃ (approximately 50℃ above Tg). At 150℃, the filtration efficiency for 0.3μm particles decreased from 99.9% to 97.5%—appearing to be only a 2.4 percentage point decrease—but the penetration rate increased from 0.1% to 2.5%, a 25-fold increase.
Mechanism Two: Pulse Backflushing Exacerbates Fiber Displacement
Pulse backflushing is a necessary cleaning method for high-temperature filter cartridges. High-pressure compressed air impacts the filter media from the inside out, causing it to expand and vibrate, shaking off surface dust.
When the filter media is in a highly elastic state, pulse backflushing is a double-edged sword. On the one hand, the softer filter media vibrates more, resulting in more thorough cleaning; on the other hand, each pulse rearranges the fiber positions. After thousands of pulse cycles, the fibers gradually “creep” in the direction of the applied force, causing irreversible changes in the pore structure. This phenomenon manifests macroscopically in filter cartridges as follows: after a period of use, the filter media becomes “loose,” feeling soft and limp to the touch, losing the firmness of new filter media. Microscopically, the porosity of the fiber network increases, and the pore size distribution widens.
Mechanism Three: The Coupling Effect of Thermal Shrinkage and Fiber Displacement
Most filter media, after exceeding their Tg (Transmission Temperature), not only soften but also undergo thermal shrinkage. The fibers attempt to shorten to a more stable length but are constrained by the woven or needle-punched structure. This internal stress is released after the fibers soften, leading to a reduction in the overall size of the filter media, while the relative positions between the fibers undergo more drastic adjustments. Shrinkage combined with displacement results in a complete restructuring of the filter media structure.
IV. The Engineering Significance of Tg in Filter Media Selection
Since most filter media operate at temperatures higher than their Tg, what is the use of the Tg parameter? The answer is: Tg determines how “soft” the filter media becomes at high temperatures and the rate of performance degradation.
1. Higher Tg of filter media results in better dimensional stability at high temperatures.
P84 has a Tg as high as 315℃, and its fibers remain in a glassy state even at its continuous operating temperature of 220-240℃. This means that P84 filter media is rigid within its normal operating temperature range, and the aforementioned fiber displacement and pore enlargement problems will not occur. This is one of the important reasons why P84 is a high-end high-temperature filter media.
PTFE is a special case. Its Tg is about 115-120℃, but PTFE has extremely high crystallinity (over 90%), and the crystalline regions remain rigid before its melting point of 327℃. Therefore, in actual use, even at temperatures much higher than its Tg, the pore structure of PTFE filter media remains relatively stable due to the presence of the crystalline network.
2. The temperature range above Tg determines the rate of performance degradation.
For filter media with lower Tg (such as PPS and polyester), the greater the range of operating temperature above Tg, the faster the performance degradation. This reflects the physical properties of the Arrhenius equation—not only does the chemical reaction rate accelerate with increasing temperature, but the movement rate of polymer chains also increases exponentially with increasing temperature.
An empirical rule is that for every 20°C increase in operating temperature above the glass transition temperature (Tg), the fiber displacement rate approximately doubles, and the rate of decrease in pore structure stability also doubles.
3. Pulse cleaning intensity needs to be matched with Tg.
For filter media with lower Tg, the pressure and frequency of pulse backflushing need to be more carefully considered. Excessively high pulse pressure will accelerate fiber displacement; excessively low pulse frequency will lead to excessive pressure drop. In its engineering practice , TrennTech has established a dedicated pulse parameter database for the Tg of different filter media, optimizing cleaning strategies for each type of filter media.
The glass transition temperature is the inflection point where filter media transitions from “hard” to “soft,” and it is also the starting point where filtration accuracy changes from “precise” to “fuzzy.” Materials engineers consider Tg as an equally important parameter as continuous operating temperature when selecting high-temperature filter media. It’s not because Tg itself is an “upper limit,” but because it reveals the true physical state of the filter media at the operating temperature—whether it’s hard glass or soft rubber.