A datasheet gives one temperature figure per polymer, and that figure is almost always a dry-heat value. It tells you what the material survives in hot air. It does not tell you what happens in hot water, hot steam or a hot caustic wash – and that is where most “unexplained” filter fabric failures actually come from.
Two different mechanisms
Dry heat attacks a polymer thermally: the chains soften, the fabric loses tensile strength, it creeps under load and it may shrink. The failure is gradual and fairly predictable. You see it as a cloth that goes slack, stretches out of its fixing and eventually tears at the highest-stress point.
Wet heat adds a chemical mechanism on top. In the presence of water at elevated temperature, some polymers hydrolyse – water molecules break the chain bonds. The fabric does not soften, it embrittles. It can look completely intact and still crumble when you flex it between your fingers. And because the mechanism is chemical rather than thermal, it happens well below the dry-heat limit printed on the datasheet.
Polyester is the classic case. In dry hot service it is a workhorse to around 150 °C continuous. Put the same fabric into hot water above roughly 80 °C, or into a hot alkaline wash, and hydrolysis becomes the dominant ageing mechanism. The datasheet figure is not wrong – it is simply answering a different question. Cotton has the same caveat: nominally usable to around 100 °C, but a wet environment above about 80 °C needs hydrolysis resistance to be considered explicitly.
The symptoms tell you which one you have
- Fabric slack, stretched, pulled out of the fixing: thermal load. The polymer is above its comfortable working temperature.
- Fabric brittle, cracking, powdering at the flex lines: hydrolysis or chemical attack. Look at the wash step, not the process temperature.
- Seam fails while the fabric is still sound: the sewing thread was specified for a lower class than the fabric.
- Permeability never recovers after a hot clean: the cleaning temperature is doing more damage than the process.
These are diagnostic. A fabric that has gone brittle and one that has gone slack were killed by different things, and swapping to a nominally “higher temperature” polymer only helps in the second case.
Orientation values – and how to read them
As rough continuous-service orientation figures, not guarantees: polypropylene around 90 °C with peaks to 100 °C; polyamide 6 around 120 °C and PA66 around 130 °C; polyester around 150 °C in dry service; PVDF around 150 °C; PPS around 190 °C; aramid around 200 °C; PEEK and PTFE around 260 °C. PP, PVDF, PPS, PEEK and PTFE also carry very broad pH tolerance, which is why they dominate wherever heat and chemistry arrive together.
Read those figures as a starting point for a conversation, not as a selection rule. The real question is always the combination: what temperature, wet or dry, at what pH, under what mechanical load, and what happens during cleaning.
The cleaning cycle is often the real limit
In a surprising number of plants the process temperature is comfortable and the wash temperature is not. A hot caustic clean-in-place, a steam sterilisation step or a hot water backwash can be the single most aggressive event in the whole cycle – and it is the one most often left off the specification sheet. If you tell us the process conditions but not the cleaning conditions, we are specifying against half the duty.
What to send us
Continuous and peak temperature, whether water or steam is present at temperature, pH, the cleaning procedure including its temperature and chemistry, the machine and element format, and the current fabric with a description of how it failed. That last point is worth more than any datasheet: the failure pattern usually identifies the mechanism straight away.
More detail on the polymers and the confection consequences on our high-temperature filtration page, and the material data on the specialty fabrics page. If you have a hot duty that keeps eating elements, send us the numbers.