What does water do?
| Material | In water and washdown |
|---|---|
| Acetal (POM) | Very little water absorption (about 0.2% at saturation), so it holds tolerances in humid places. The copolymer resists hot water better than the homopolymer |
| UHMW-PE | Does not absorb water, and is a common choice for food processing and wet environments |
| Nylon | Absorbs about 1.5 to 3.0% water by weight at equilibrium in ambient humidity, and takes up more when soaked, so a bore can close. Take the soaked figure from the grade's datasheet (Q1-03) |
| Polyurethane | Steam, and long exposure to humid conditions above about 70 °C, damage it irreversibly |
What do caustic and acid cleaners do?
It depends on the chemical, how strong it is and how hot. Two examples show how much.
UHMW-PE. A supplier's chemical resistance chart rates it excellent in water, alkalis (such as sodium hydroxide), salts and oils, and excellent to good in diluted acids. Aromatic solvents are limited, chlorinated solvents limited to not recommended, and strong oxidisers (such as concentrated nitric acid) not recommended. The same chart stresses that it is a general guide, and that final selection must consider concentration, temperature, mechanical load, exposure time and the cleaning process.
Acetal. Published ratings for sodium hydroxide (caustic soda) change with strength and temperature (ChemicalResistance.org): very good at 1% at both 20 °C and 50 °C; very good at 30 to 50% at 20 °C but only limited at 50 °C; and limited at 60% and for concentrated solutions even at 20 °C. Verify which concentration your process uses, and test under real service conditions when heated.
So a bushing that survives a mild cleaner at room temperature may not survive a stronger one used hot. Get the cleaner's name and strength, the temperature, how long it is in contact, and how often.
Two more points matter for acetal. First, find out whether the grade is a homopolymer or a copolymer. The copolymer is the better one in hot water and in strong bases, so a rating table that does not say which it covers is incomplete. Second, check chlorine-based sanitisers separately, because they are common in food plants. Hot chlorinated water is a known failure mode for homopolymer acetal. Take the actual rating for your sanitiser, concentration and temperature from the grade's maker.
How does chemical resistance depend on conditions?
How a plastic behaves toward a chemical depends on temperature, the length of exposure, and the type and amount of mechanical loading. Sometimes the surrounding medium can even take on the role of a lubricant. A bushing under load in a hot cleaner is a harder case than the same polymer soaking unloaded in the cold, because stress opens up cracks that a chemical can then work into.
What else changes in a wash?
- Size. Wet and hot washing changes the size of nylon and polyurethane, which affects the fit (Q1-03).
- Lubricant. A wash tends to flush grease out of the clearance and carry water and cleaner in. A dry-running grade does not depend on grease (Q1-06).
- Shaft. A wet shaft needs corrosion protection (Q1-05, K06-Q1-08).
- Sunlight. If the bushing is also in the sun, nylon and acetal that are not UV-stabilised are not suited to long outdoor exposure. Ask for a stabilised grade and check its datasheet.
What to give the supplier
The cleaner and its concentration, the temperature of the wash, the contact time and how often, whether steam or hot water is used, the load and speed, and whether the bushing touches food (Q1-08).