Polypropylene has a broad chemical resistance profile, which is why it is a common choice for filter screens. It also has specific, well-defined weaknesses, and those weaknesses get worse as temperature rises. Knowing where they are matters more than the general reassurance.
Where PP does well
PP resists most aqueous solutions, dilute and concentrated acids and alkalis within its temperature range, most inorganic salts, and many organic solvents at room temperature. For water treatment, food processing liquids, and many general industrial streams it is a sound choice and usually the cheapest one that works.
Where it does not
Strong oxidising agents. Concentrated nitric acid, chromic acid and similar oxidisers attack PP progressively. The damage is cumulative and it embrittles the material.
Chlorinated and aromatic hydrocarbons, and some other organic solvents, cause swelling or stress cracking. Swelling changes the aperture, which quietly changes the filtration.
Some oils and greases at elevated temperature, which can be absorbed and cause softening.
High temperature in general. This is the one that catches people out, because a chemical that PP handles at 20 C may be a problem at 60 C.
Temperature is the multiplier
Chemical resistance ratings are quoted at a temperature, usually ambient, and they degrade as temperature rises. The mechanism is simple: reactions proceed faster and the polymer is more permeable. A screen that is comfortable in a cold process can fail in months in the same chemistry hot.
If your process runs warm, rate the material at the operating temperature rather than at room temperature. This is the single most common cause of screens that fail much faster than expected.
What chemical attack looks like
- Embrittlement – the screen cracks at the wire crossings or where it is clamped
- Swelling or softening – the geometry changes and apertures close up
- Discolouration – often the first visible sign, and worth photographing
- Stress cracking at points of mechanical stress, which is why it often starts at the frame
How to specify safely
- List everything in the stream, including cleaning agents and sanitisers, not just the process fluid
- Give the maximum temperature, including any cleaning or sterilising cycle, rather than the normal operating temperature
- Give concentrations, because resistance is concentration dependent
- Say whether exposure is continuous or intermittent – intermittent exposure at high concentration is often survivable
When in doubt
Immersion testing is cheap. Put a sample in the actual fluid at the actual temperature for a few weeks and check for weight change, dimensional change and brittleness. It is a far better guide than a compatibility chart, because charts are general and your fluid is specific.
The cleaning agent is a separate exposure
Screens are usually specified against the process fluid and then cleaned with something else entirely, often something considerably more aggressive. A screen that sees a mild process stream and a caustic clean-in-place cycle is living in the caustic, not in the process stream.
This is one of the most common reasons for compatibility surprises, and it is worth listing the cleaning agents, their concentrations and their temperatures alongside the process conditions when you specify.
Intermittent aggressive exposure is often survivable where continuous exposure would not be, so duration and frequency matter as much as concentration. Tell us the cycle rather than just the chemistry.
What we will tell you
We supply PP filter screens and we will ask about the full chemistry and temperature before quoting. If your conditions are outside what PP handles comfortably, we would rather say so than sell you screens that fail in a season – and in a few cases the honest answer is a different polymer.