Why Moisture Absorption is the Defining Characteristic of Nylon
No other engineering thermoplastic absorbs moisture as dramatically as nylon. PA6 can absorb up to 9.5% by weight at saturation — more than any other common engineering plastic. Even PA66, with its tighter molecular packing, reaches 8.5%. PA12 reaches only 1.5%, making it the nylon of choice for wet environments.
This moisture absorption is not a defect — it is a fundamental property arising from the hydrogen-bonding amide groups in the polyamide chain. Water molecules penetrate the amorphous regions of the polymer and plasticize it, fundamentally altering mechanical, thermal, and dimensional properties.
For buyers and engineers specifying nylon, understanding moisture effects is not optional. Every nylon datasheet value — tensile strength, modulus, impact resistance — is measured on conditioned specimens. The same part, tested dry-as-molded, can show tensile strength 40% higher but impact resistance 70% lower than conditioned values.
How Nylon Absorbs Moisture: The Science
Moisture absorption in nylon follows a classic Fickian diffusion process with two phases:
- Phase 1 (rapid): Surface saturation within hours — parts appear “wet” but interior is still dry. This is the most dangerous phase for dimensional instability and surface blistering.
- Phase 2 (slow): True equilibrium through bulk diffusion — can take weeks or months for thick sections (>10mm wall thickness)
At 50% relative humidity (23°C), PA6 reaches approximately 2.5% moisture content within 24 hours. Full saturation at 95% RH can take 2-4 weeks. The actual equilibrium moisture content depends on:
| Nylon Grade | Max Moisture Absorption (%) | Equilibrium at 50% RH | Equilibrium at 95% RH |
|---|---|---|---|
| PA6 | 9.0-9.5% | 2.5-3.0% | 8.5-9.0% |
| PA66 | 8.0-8.5% | 2.0-2.5% | 7.5-8.0% |
| PA12 | 1.4-1.6% | 0.7-0.9% | 1.3-1.5% |
| PA6-GF30 | 6.0-7.0% | 1.5-2.0% | 5.5-6.5% |
| PA66-GF30 | 5.0-6.0% | 1.3-1.8% | 4.5-5.5% |
| PA46 | 7.0-7.5% | 2.0-2.3% | 6.5-7.0% |
Effects on Mechanical Properties
Moisture absorption simultaneously improves some properties while degrading others — a trade-off that must be understood for accurate part design.
| 속성 | Dry As Molded | Conditioned (50% RH) | Change |
|---|---|---|---|
| 인장 강도(MPa) | 80-90 | 55-65 | -28% |
| Tensile Modulus (MPa) | 3000-3300 | 1800-2100 | -35% |
| 휴식 시 연신율 | 40-60% | 150-250% | +250% |
| Izod Impact (kJ/m²) | 3-5 | 6-10 | +100% |
| 굴곡 탄성률(MPa) | 2800-3100 | 1500-1800 | -40% |
| HDT (1.82 MPa, °C) | 65-75 | 55-65 | -15°C |
Effects on Dimensional Stability
Moisture absorption causes dimensional changes that can be significant for precision parts. The dimensional change is not linear — it follows a differential swelling pattern between skin (which equilibrates first) and core (which remains dry).
- Linear expansion: PA6 expands approximately 0.15-0.20% per 1% moisture absorbed in the flow direction, and 0.40-0.50% in the transverse direction
- Anisotropic swelling: Similar to shrinkage anisotropy, moisture swelling is greater perpendicular to the mold flow direction
- Warpage from differential drying: Parts dried unevenly after conditioning can warp as surface and core dry at different rates
- Threaded inserts: Metal inserts in nylon do not change dimension with humidity — dimensional changes occur only in the nylon surrounding the insert, creating stress on the interface
Measurement Methods: How to Measure Nylon Moisture Content
Accurate moisture measurement is essential for quality control in nylon processing and for verifying material condition before use.
| Method | Principle | Accuracy | Time | Best Use |
|---|---|---|---|---|
| Loss on Drying (LOD) | Weigh before/after oven drying at 103°C | ±0.1% | 2-4 hours | Quick production QC |
| Karl Fischer Titration | Chemical reaction with water molecules | ±0.01% | 10-20 min | Precision verification |
| Coulometric KF | Electrolytic Karl Fischer | ±0.001% | 15-30 min | R&D, spec testing |
| Moisture Analyzer | Automated LOD with integrated balance | ±0.05% | 20-40 min | Inline production QC |
| Dew Point Hygrometer | Equilibrium with desiccant | ±0.3% | 30-60 min | Dryer monitoring |
Control Strategies: Drying and Storage Best Practices
Effective moisture control requires a systematic approach from material storage through processing:
- Desiccant dryer: The standard for nylon drying. Use desiccant with dew point below -40°C (ideally -70°C). Maintain air dew point of -40°C throughout the drying cycle. Target 0.1-0.2% residual moisture for PA66, 0.2% for PA6.
- Drying temperatures: 80°C for PA6 and PA66 (higher temperatures risk oxidative degradation), 70-75°C for PA12
- Drying time: 4-6 hours minimum for properly sealed material; 8-12 hours for material exposed to humid air or with wall thickness >15mm
- Hopper design: Use sealed hoppers with nitrogen purge to prevent re-absorption during production runs
- Material shelf life after drying: Once dried, nylon re-absorbs moisture at approximately 0.1% per hour at 23°C/50% RH — dried material must be used within 2-4 hours or re-dried
Frequently Asked Questions
Q: How long does it take for nylon to reach equilibrium moisture content?
A: At 23°C/50% RH, 3mm thick nylon parts reach equilibrium in approximately 4-5 days. At 23°C/95% RH, equilibrium is reached in 2-3 days. Thick sections (>6mm) may require weeks. Accelerate conditioning by immersing in 50°C water for 24-48 hours, then air-drying at 23°C for 4 hours.
Q: Can I prevent moisture absorption completely?
A: No. Nylon is inherently hygroscopic due to its amide bonds. Surface coatings only delay absorption—they cannot prevent it. Design for conditioned properties or consider alternatives like POM, PBT, or polyolefins for applications requiring zero moisture absorption.
Q: How do I dry nylon before processing?
A: Dry PA6 at 80°C for 4-6 hours (moisture <0.2%). Dry PA66 at 85°C for 6-8 hours. Use dehumidifying dryers with -40°C dewpoint. Avoid drying above 100°C which can cause oxidation. Re-dry if material is exposed to ambient air for more than 30 minutes.
Q: What’s the effect of moisture on dimensions?
A: Nylon swells 0.2-0.3% per 1% moisture absorption. Going from dry (0.2%) to conditioned (2.5%) at 50% RH causes approximately 0.5-0.7% dimensional growth. For tight-tolerance parts, either design for conditioned size, or post-machine after conditioning.
