I’ve seen nylon tubing behave beautifully in lab conditions and then turn stubbornly rigid once it’s installed on real machines. When a line that was supple during commissioning becomes stiff 12–18 months into service, it’s not just inconvenient—it can drive leaks, pull fittings out of tolerance, and force unplanned maintenance. In my experience, the root causes are rarely singular; they’re a cocktail of air quality, thermal and UV exposure, polymer chemistry, and storage/handling practices that quietly push nylon toward higher crystallinity and lower ductility.
Nylon tubing hardens over time primarily due to moisture loss (nylon is hygroscopic), thermal post-crystallization at elevated temperatures, and UV/oxidative aging that embrittles the polymer chains. Dry compressed air from aggressive dryers pulls out plasticizing water, heat accelerates crystallinity, and UV plus oxygen drives chain scission/crosslinking—together producing a stiffer, less flexible tube. Mitigation includes tuning air preparation, shielding or stabilizing against UV, and selecting conditioned or UV-stabilized nylon or alternative materials for exposed runs.
Below I break down how oxidation, heat, and UV embrittle nylon in real plant environments, how to verify your air preparation is protecting tubing rather than aging it, the storage and handling fixes that matter, and when it’s time to move to UV-stabilized nylon or alternative polymers like polyurethane for outdoor service.
How do oxidation, heat, and UV contribute to embrittlement in my environment?
Hygroscopic behavior and post-crystallization
- Nylon 6/12 tubing absorbs water in storage; that water acts as a molecular plasticizer. When you put the tube into a dry, well‑prepared compressed air system, you unintentionally “dry” the nylon. As moisture leaves, polymer chains pack more tightly, crystallinity rises, and modulus increases—perceptibly harder, less kink‑resistant tubing.
- Elevated temperatures (near manifolds, ovens, rooftops, engine bays) accelerate post‑crystallization. Repeated thermal cycling densifies the microstructure and relaxes residual stresses left from extrusion, making the tube stiffer over months.
UV and oxidative aging
- UV radiation breaks nylon’s amide-containing chains, generating free radicals. In the presence of oxygen (and occasional ozone near corona sources), you see chain scission and some crosslinking. The macroscopic result: a harder surface, micro‑cracking, and loss of impact resistance. Unstabilized nylon outdoors can show a “case-hardened” feel even while the core remains more ductile.
- Oils, solvents, and certain cleaning agents can extract minor plasticizers or react with nylon; even if nylon isn’t heavily plasticized, surface extraction plus oxidation compounds embrittlement.
Material differences matter
- Nylon 12 absorbs less moisture than Nylon 6, so it’s less prone to drying‑induced stiffening, but it’s still vulnerable to UV and heat.
- Black, UV‑stabilized grades with carbon black or HALS (hindered amine light stabilizers) markedly slow photodegradation compared to natural/clear nylon.
Quick comparison: nylon types for aging resistance
| Property / Behavior | Nylon 6 Tubing | Nylon 12 Tubing | UV-Stabilized Nylon (Black) |
|---|---|---|---|
| Moisture absorption | Higher | Lower | Lower (similar base polymer) |
| Drying-induced stiffening | More pronounced | Less pronounced | Less pronounced |
| UV resistance | Poor without stabilizers | Poor without stabilizers | Good (carbon black/HALS) |
| Heat/post-crystallization aging | Moderate–High | Moderate | Moderate (UV protected only) |
Are my dryers and filters removing enough moisture to protect the tubing?
The paradox of “too dry” air
- In pneumatics, we typically target low dew points to protect valves, FRLs, and actuators from corrosion and ice. For nylon tubing, extremely dry air (−40 °C PDP) accelerates de‑plasticization. I don’t recommend compromising system dryness, but you should understand that hardening is a predictable side effect.
Practical air preparation checks
- Verify dew point at point‑of‑use, not just at the dryer discharge. Long outdoor runs can re‑cool air and change local dew point.
- Ensure coalescing filters (0.01–0.1 µm) are catching aerosols; oil mist can interact with nylon surfaces. Replace elements on differential pressure rise, not just calendar time.
- If embrittlement is localized near heat sources, add point‑of‑use FRLs with fine filtration and regulate pressure and temperature to reduce thermal cycling on the tubing.
Balancing protection and material life
- For critical flexible runs, consider conditioned nylon tubing (pre‑moisturized) or swap to polyurethane (PU) in areas where flexibility is paramount; keep the system dry for component health while selecting tubing that tolerates dryness.
- If static control is needed, avoid ozone-generating ionizers near nylon tubes; ozone accelerates oxidative embrittlement.
Air preparation targets for mixed priorities
| Parameter | Typical Pneumatic Target | Impact on Nylon Tubing | Mitigation Strategy |
|---|---|---|---|
| Dew point (pressure) | −20 to −40 °C | Faster moisture loss, stiffening | Use PU or conditioned nylon where flexibility is critical |
| Oil carryover | <0.1 mg/m³ | Possible surface interaction | Maintain coalescers; avoid incompatible oils |
| Particulates | <1 µm at precision valves | Surface abrasion, stress concentrators | Keep filters fresh; avoid abrasive dust environments |
What storage and handling practices should I change to reduce aging?
Storage do’s and don’ts
- Store nylon tubing in sealed bags or drums with moderate humidity (40–60% RH), away from heaters or direct sunlight. Unwrapped spools near ovens or rooftops will dry and pre‑age before installation.
- Avoid prolonged storage under tight coil tension; mechanical creep and stress can compact the microstructure.
Installation practices
- Minimize exposure to radiant heat from manifolds, ovens, and motors—use stand-offs or thermal shielding. Repeated thermal spikes drive post‑crystallization.
- Avoid tight bend radii; a stiffer tube will ovalize and form stress whites. Use bend support springs or switch to softer tubing in dynamic sections.
- Protect outdoor runs with UV‑blocking conduit or cable tray covers; even stabilized nylon benefits from shielding to limit surface temperature.
Maintenance routines
- Inspect for surface chalking, gloss loss, micro‑cracks at fittings, and increased pull‑out force—early indicators of embrittlement.
- Replace short, dynamic sections on a timed interval; static trunk lines can last longer if shielded from UV and heat.
Should I move to UV-stabilized or alternative materials for outdoor runs?
When to upgrade material
- If your lines see direct sun, high heat, or very dry air, and flexibility is important (robot dress packs, pick‑and‑place whips), I move away from natural nylon. Black UV‑stabilized nylon helps outdoors but remains susceptible to “drying stiffness.”
- For dynamic/flex-critical runs: Ether‑based polyurethane (PU) offers excellent flexibility, abrasion resistance, and better low‑temperature bend behavior. PU isn’t hygroscopic, so it keeps its feel in dry systems. For extreme chemicals, consider PTFE or FEP; for higher burst/temperature with good UV resistance, stainless steel braided PTFE is robust but less flexible.
Application-driven selection
Outdoor, exposed, moderate chemicals
- UV‑stabilized black nylon 12 or PU with UV‑resistant jacket.
- Add conduit or shield to cut UV and radiant heating.
Dynamic motion, dry air, tight bends
- PU tubing (ester types for abrasion, ether types for hydrolysis resistance). Verify compatibility with lubricants and oils.
High temperature near ovens or engines
- PTFE tubing or silicone (where pressure allows); keep fittings compatible (compression or flare for PTFE; barbed for silicone with clamps).
Aggressive solvents or ozone
- PTFE/FEP; avoid nylon near strong oxidizers.
Fittings and sealing considerations
- With stiffer aging nylon, push‑to‑connect collets can bite harder and induce stress cracking. If embrittlement is observed, consider brass or stainless push‑in fittings with oval collets and proper tube support, or switch to compression fittings for PTFE.
- Maintain leak management: stiffer tubing transmits vibration more readily; recheck Cv impacts through hoses, as harder walls can change micro‑leak behavior at ferrules.
Decision checklist
- Is UV exposure unavoidable? Use black UV‑stabilized nylon or shield the run.
- Is flexibility non-negotiable under very dry air? Choose PU.
- Are chemicals/heat driving failures? Move to PTFE/FEP and matching fittings.
- Are hardening issues localized near heat or ozone? Relocate runs and add shielding; verify nearby electrical equipment isn’t generating ozone.
Conclusion
In the field, nylon hardening is a predictable outcome of three converging factors: drying in low‑humidity compressed air, thermal post‑crystallization from heat and cycling, and UV/oxidative attack on exposed surfaces. I don’t compromise air dryness—valves and actuators need it—but I do adapt tubing selection and layout. Shield outdoor runs, store tubing sealed and away from heat, avoid tight bends, and where flexibility is mission‑critical under dry conditions, move to UV‑stabilized nylon or polyurethane. That combination keeps leak rates low, preserves bend integrity, and extends service life without sacrificing air quality.
