Application — High-Temperature Filtration
Hot Caustic & Alkaline Liquor Filtration
The duty where temperature multiplies chemical attack instead of merely adding to it — and where a fabric rated far above the process temperature can still fail in weeks.
Heat and Alkali Do Not Simply Add Up
A cold caustic liquor is manageable for most polymers. A hot neutral liquid is manageable for many. Put the two together and the behaviour changes qualitatively rather than gradually, because temperature accelerates the chemical reaction that attacks the polymer chain. This is the single most common reason a fabric fails far below its printed temperature rating.
Polyester is the textbook casualty. Rated to around 150 °C in dry service, it hydrolyses in hot alkaline conditions and embrittles rather than softening — a cloth that looks intact cracks when you flex it. Polypropylene, with a much lower nominal rating but genuine chemical stability across the whole pH range, routinely outlasts it in exactly this service until the temperature limit is reached.
Aramid is the other trap. Its 200 °C rating looks like the answer to a hot duty, but its usable pH band is roughly 4 to 11 — so in strongly alkaline liquor it is the wrong material despite the temperature figure. Where the process is both hot and strongly alkaline, PPS, PVDF, PEEK and PTFE are the polymers that carry both properties.
Typical Duties
What We Optimise For
In hot alkaline service the polymer decision is only the beginning. These are the objectives that decide the outcome.
pH Band and Temperature Together
Both properties have to hold at once. Polypropylene to roughly 90 °C across the full pH range; above that PPS, PVDF, PEEK or PTFE. Aramid and polyester are ruled out by chemistry, not by heat.
Sewing Thread in the Same Class
A polyester thread in a hot alkaline element hydrolyses and lets go while the fabric is untouched. Thread material is specified to the fabric, and welded joints replace seams wherever the polymer allows.
Hardware Grade
Eyelets, rings, clamping strips and staples in a grade matched to the liquor. A corroded metal part loses the element just as surely as a torn cloth.
Abrasion in Hot Liquor
Many caustic circuits carry abrasive solids. At temperature the polymer is at its weakest, so abrasion resistance has to be assessed hot, not at ambient.
Cake Release & Scaling
Scale and precipitate build-up is the second failure mode after chemical attack. A smooth, low-anchoring surface makes both washing and descaling far more effective.
Regeneration Procedure
The acid or hot water wash used to descale is frequently the most aggressive event in the cycle. We specify against it, not around it.
Reading the Failure Pattern
A used cloth tells you which mechanism killed it. That matters, because the three patterns below call for three completely different corrections — and only one of them is solved by a higher-temperature polymer.
Slack, stretched, pulled out of the fixing
Thermal overload. The polymer is above its comfortable working temperature and is creeping under load. This is the case where a higher-temperature polymer is the right answer.
Brittle, cracking, powdering at flex lines
Chemical attack — hydrolysis. Look at the liquor and the wash step, not the process temperature. The correction is a chemically stable polymer, which may well have a lower temperature rating.
Seam failed, fabric still sound
The sewing thread was specified a class below the fabric. No fabric upgrade helps until the thread, or a welded joint, is corrected.
What We Need to Specify a Fabric
Related Topics
High-Temperature Filtration
Polymer limits, dry vs. wet heat, confection and cost logic.
Learn MoreChemical Industry
The broader chemical process filtration portfolio.
Learn MoreMining & Minerals
Alumina and hydrometallurgical circuits where caustic liquor is central.
Learn MoreShort Filter Cloth Lifespan
Systematic diagnosis when elements fail early.
Learn MorePressure Leaf Filter Cloths
A common element format for hot liquor polishing.
Learn MoreSpecialty Fabrics — PPS, PVDF, PEEK, PTFE
The polymers that carry both heat and broad pH tolerance.
Learn MoreFrequently Asked Questions
Which filter fabric is used for hot caustic liquor filtration?
Polypropylene is the natural first choice: as an orientation figure it tolerates the full pH range from 1 to 14 and it is hydrolysis-stable, so hot alkali does not attack it chemically. Its limit is temperature — roughly 90 °C continuous, with short peaks to 100 °C. Above that the field narrows to PPS, PVDF, PEEK and PTFE, all of which combine broad pH tolerance with genuinely higher temperature capability. Aramid, despite a 200 °C rating, is limited to roughly pH 4 to 11 and therefore drops out of strongly alkaline service.
Why does polyester fail in hot caustic even below its rated temperature?
Because two different failure mechanisms are being confused. Polyester is rated to around 150 °C in dry service, and that figure describes thermal softening. Hot alkaline conditions attack it chemically instead: hydrolysis breaks the polymer chains, and the fabric embrittles rather than softening. It can look completely sound and still crack when flexed. The rating is not wrong — it simply answers a question about hot air, not about hot caustic.
How do you tell hydrolysis from thermal damage on a used cloth?
The symptom is diagnostic. A fabric that is slack, stretched and pulled out of its fixing has been thermally overloaded. A fabric that is brittle, cracking or powdering at the flex lines has been chemically attacked — look at the liquor and the wash step, not at the process temperature. If the seam has failed while the fabric itself is still sound, the sewing thread was specified a class below the fabric. These three patterns point at three different corrections.
Do the seams and metal parts need special attention in caustic service?
Yes, and this is where a surprising number of good fabric selections still fail. A polyester sewing thread in a hot alkaline element will hydrolyse and let go while the polypropylene or PPS fabric around it is untouched. The same applies to eyelets, rings and clamping strips in an unsuitable stainless grade. Where the polymer allows welding, a welded joint removes the weakest link entirely. We specify thread, welding and hardware as part of the element, not as an accessory.
What do you need in order to specify a fabric?
Liquor composition and concentration, pH, continuous and peak temperature, whether there are oxidising components present, machine type and element format with the fixing method, solids content and particle size including abrasiveness, the required filtrate clarity, the cleaning and regeneration procedure with its temperature, and the current fabric with its observed failure pattern.
Cloths going brittle in hot caustic service?
Send us the liquor, the pH, the temperature and a description of how the fabric failed — the failure pattern usually identifies the mechanism straight away.
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