I’ve spent enough time in plants and on OEM lines to know that when nylon pneumatic tubing starts swelling or getting rubbery in spots, it’s never “random.” It’s usually a precise intersection of chemistry, humidity, and heat—often amplified by pressure spikes and installation decisions. I’ve seen fittings back out, ferrules lose bite, and Cv drop from a few soft sections that trace back to steam, compressor discharge heat, or an innocent glycol-laced mist from upstream processes. If you’ve noticed inconsistent tube feel, diameter changes, or push-to-connects weeping, you’re dealing with a materials and conditions problem—one we can solve with a disciplined approach.
Nylon tubing swells or softens when moisture or certain chemicals plasticize the polymer, lowering its glass transition temperature and rigidity. Elevated temperatures (hot water, steam, near compressors/ovens) accelerate absorption and hydrolytic effects, while pressure surges exploit the softened state, causing dimensional growth and reduced burst strength. Nylon 6 absorbs more moisture than Nylon 12; switching to PA12, PTFE, or reinforced/lined constructions improves chemical resistance and dimensional stability. Pre-conditioning, barrier liners, and derating for temperature/humidity prevent swelling-related failures.
In the sections below, I break down the typical exposures that drive material changes, how temperature cycling makes matters worse, what pressure/surge conditions to check, and when it’s time to upgrade to PA12, PTFE, or reinforced tubing. I’ll also share practical design and maintenance tactics—pre-conditioning, fluid compatibility control, and fitting strategies—to keep lines leak-proof and predictable.
What exposure to oils, solvents, or moisture is causing material changes in my lines?
Hygroscopic behavior: moisture is the primary culprit
- Nylon is hygroscopic; it absorbs moisture from ambient air and water. In service, moisture acts as a plasticizer—reducing stiffness, increasing elongation, and causing measurable diameter growth.
- Hot water or steam dramatically speeds uptake; the tubing can soften and lose pressure resistance in those zones.
- Rapid transitions from dry to humid environments cause dimensional instability. Pre-conditioning tubing to service humidity before installation reduces “swelling shocks.”
Chemical ingress and plasticization
- Certain chemicals penetrate and plasticize nylon’s matrix, leading to swelling and softness. Common offenders:
- Alcohols (ethanol, isopropanol), glycols (ethylene/propylene), some aromatic/aliphatic hydrocarbons.
- Strong acids/bases at elevated temperature.
- Chlorinated solvents (especially damaging).
- Concentration, temperature, and dwell time matter; warm, persistent mist or film is worse than cold, short contact.
- Residual compressor oils and additives can migrate into the tube wall over time, especially near discharge or in poorly filtered systems.
FRL and air prep impacts
- Inadequate filtration and separation allow aerosols (oil, condensate) and fine contaminants to reach tubing.
- Desiccant driers can swing humidity from very low to normal plant levels, driving dimensional changes if tubing wasn’t pre-conditioned.
- Over-lubrication from mist lubricators can elevate hydrocarbon exposure; compatibility must be verified.
Practical checks I run on-site
- Wipe test: soft sections often feel “waxy” and may show gloss change; check for condensate or chemical film.
- Proximity survey: note routes near washdown, steam lines, ovens, chemical baths, or compressor discharge piping.
- Compatibility cross-check: validate all media against nylon compatibility charts; look for alcohol/glycol cleaning steps.
How does temperature cycling near compressors or ovens affect my tubing?
Temperature-humidity coupling lowers nylon’s effective rigidity
- Elevated temperature reduces nylon’s glass transition temperature in the presence of moisture. A humid, warm environment can push nylon from “semi-rigid” to “flexible” at typical plant temperatures.
- Thermal cycling drives repeated moisture diffusion in/out of the polymer, exacerbating dimensional change and stress at fittings.
Localized hot spots create weak links
- Near compressors, receivers, or aftercoolers: short sections can see transient heat and oil aerosols.
- Near ovens, autoclaves, or steam cleaning: hot, humid zones plasticize nylon rapidly, reducing burst pressure and kink resistance.
- Routing above hot manifolds or under heat lamps: radiant heating softens tubing without raising bulk line temperature readings.
Failure modes I watch for
- Push-to-connect blow-offs: softened tube surface reduces bite; ferrule-based fittings lose clamping force.
- Ovalization and micro-kinks at bend points after heat cycles.
- Drift in Cv/flow due to diameter growth; control loops become sluggish.
Mitigations
- Thermal shields, standoff brackets, and rerouting away from heat sources.
- Use higher-crystallinity grades or PA12; consider fluoropolymer liners (PTFE, FEP) for barrier performance.
- Derate pressure for temperature and humidity; set conservative surge limits.
Are my pressure and surge conditions exceeding the tubing’s ratings in my setup?
Pressure ratings need derating in wet/hot service
- Nylon’s burst and working pressure decrease with moisture content and temperature. A section exposed to hot condensate can fall below spec even if the system gauge looks “in limits.”
- Surge events (valve slam, deadhead conditions, rapid cylinder decel) superimpose transient peaks that exploit softened tubing.
What I verify during audits
- Actual line transients with a fast-response pressure logger; sustained spikes >10–20% above nominal can cause localized expansion.
- Regulator setpoints versus at-load pressures; undersized regulators or long small-ID runs cause oscillations.
- Flow path Cv and valve actuation timing; aggressive on/off solenoids without snubbers generate hydraulic-like hammer in air systems.
Controls to stabilize pressure behavior
- Add accumulators or snubbers near fast-acting valves to tame surges.
- Tune exhaust restrictors and cylinder speed controls.
- Confirm FRL sizing; under-sized filters/regulators increase dynamic pressure drop and oscillation.
Table: Typical nylon tubing derating factors (indicative, verify with your supplier)
| Condition | Effect on Working Pressure | Notes |
|---|---|---|
| Dry, 20–25°C | Baseline | Use manufacturer rating |
| Humid (≥60% RH), 25°C | −10% to −20% | Moisture plasticization |
| Hot water/steam contact, 60–80°C | −20% to −40% | Rapid softening; check burst ratings |
| Chemical exposure (alcohol/glycol) | Variable −10% to −30% | Depends on concentration and dwell |
Should I switch to PA12, PTFE, or reinforced options for better chemical resistance?
Material selection: match environment to polymer
Table: Material comparison for swelling/softening control
| Tubing Type | Moisture Uptake | Chemical Resistance | Temp Capability | Flex/Bend Stability | Typical Use Case |
|---|---|---|---|---|---|
| Nylon 6 (PA6) | Higher | Moderate; sensitive to alcohols/glycols | Up to ~80–100°C (service) | Good dry; degrades humid/hot | General pneumatics in dry environments |
| Nylon 12 (PA12) | Lower | Better than PA6; improved hydrocarbon resistance | Similar service range | More dimensionally stable | Plant air with humidity; better near oils |
| PTFE (Fluoropolymer) | Negligible | Excellent broad-spectrum | Up to ~200–230°C | Can be kink-prone unless supported | Harsh chemicals, high temp, low permeation |
| Reinforced (braid/armored PA) | Lower under load | Depends on liner (PA/PU/PTFE) | Higher working pressure | Resists surge-induced growth | Dynamic lines, pressure spikes, mechanical abuse |
| Lined (PA outer / PTFE inner) | Outer: PA; Inner: very low | Inner protects against chemicals; barrier to ingress | Inner follows PTFE capability | Maintains fitting grip with PA outer | Chemical barrier with good fitting compatibility |
When I recommend each option
- PA12 over PA6: your first move when humidity and mild oils are present. Better dimensional stability and lower water uptake.
- PTFE (or FEP/PFA): for aggressive solvents, high temps, or zero-swelling requirements; pair with compatible fittings and support to avoid kinks.
- Reinforced PA or PA/PTFE lined: for surge-heavy circuits, long runs near heat, or where fitting retention is critical under varying conditions.
Design tactics to lock in reliability
- Pre-condition nylon tubing to service humidity before cutting/assembly to stabilize dimensions.
- Specify barrier liners (fluoropolymer) where chemical ingress or moisture is unavoidable.
- Use fittings with deeper collets or compression ferrules rated for softer tubing states; verify grip on conditioned samples.
- Document compatible fluids (cleaners, lubricants, washdowns) and keep them away from vulnerable runs.
- Apply temperature/humidity derating in the pressure spec; validate with on-line transient measurements.
Quick troubleshooting workflow I follow
- Map soft/swollen sections to environmental features (heat, washdown, chemical stations).
- Check FRL performance, compressor carryover, and humidity swings; review maintenance fluids.
- Log pressure transients at suspect nodes.
- Pull a coupon for mass gain test after controlled exposure (moisture or chemical).
- Implement rerouting/shielding and material upgrade (PA12/PTFE/liner) with fitting validation.
Visual: barrier-liner concept for nylon tubing
Conclusion
If your nylon tubing is swelling or softening in certain sections, you’re seeing the combined effects of moisture/chemical plasticization, temperature acceleration, and pressure dynamics acting on a polymer that’s sensitive to its environment. Moisture uptake—especially with heat—lowers nylon’s effective rigidity and burst strength, while oils, alcohols, and glycols can further plasticize the matrix. Pressure surges then exploit these softened regions, leading to dimensional growth and fitting issues. The fix is straightforward: control exposure (air prep, rerouting, shielding), derate for temperature/humidity, pre-condition tubing, and upgrade materials where needed—PA12 for better humidity/oil resistance, PTFE or lined/reinforced constructions for harsh chemistry or surges. With those steps, you’ll return your pneumatic lines to leak-proof, stable performance.
