ナイロンの吸湿：効果、測定、制御

Understanding moisture absorption in nylon — how it affects mechanical properties, dimensional stability, processing, and design strategies to manage it.

Nylon’s Moisture Sensitivity: A Critical Design Variable

Of all common engineering thermoplastics, nylon absorbs the most moisture. PA6 reaches 9.5% by weight at saturation, while PA66 reaches 8.5%. This is not a minor property — it fundamentally alters the material’s mechanical behavior, dimensions, electrical properties, and appearance.

Designers and engineers who ignore moisture absorption face a common failure mode: parts that fit perfectly when molded (dry) but swell, distort, or change flexibility after exposure to normal humidity. A nylon gear that meshes perfectly in the factory may bind and wear prematurely in a humid warehouse. Understanding and controlling moisture effects is essential for reliable nylon part design.

Mechanism: How Nylon Absorbs Water

Nylon’s molecular structure contains amide groups (-CONH-) that form hydrogen bonds with water molecules. This is the same hydrogen-bonding mechanism that makes nylon strong in the first place — but water molecules occupy hydrogen-bonding sites that were previously linking nylon chains together, reducing inter-chain forces.

Absorption Rate:
The rate of moisture absorption depends on thickness and humidity:

1mm film	2 hours	8 hours

2mm sheet	8 hours	36 hours
6mm plate	3 days	12 days

This is why thin-walled parts reach equilibrium faster — and why thick sections can retain dry conditions in the core while the surface is saturated.

Drying Reverses the Process:
Heating nylon above 80°C drives off absorbed moisture. At 100°C for 4-6 hours, PA6 and PA66 reach dry-as-molded condition. However, the dimensional changes from drying are not fully reversed — the part does not shrink back to its dry-molded dimensions because the polymer chains have reorganized.

Effects on Mechanical Properties

引張強度	95 MPa	75 MPa	-21%
引張弾性率	3.2 GPa	2.0 GPa	-37%
曲げ弾性率	3.0 GPa	1.8 GPa	-40%

破断伸度	80%	180%	+125%
Hardness (Shore D)	82	74	-10%

Key insight: Moisture acts as a plasticizer for nylon. The material becomes softer, weaker in tension and flexure, but significantly tougher. Impact resistance nearly doubles in conditioned nylon vs. dry nylon.

デザインの意味合い: If you design to dry property values, your conditioned parts will be 20-40% weaker than calculated. Always design to the highest moisture condition the part will experience in service.

Glass fiber reinforcement mitigates moisture effects — GF30 grades show only 10-15% strength reduction from dry to conditioned (vs. 20-25% for unfilled). The glass fiber network is unaffected by moisture; only the nylon matrix is plasticized.

Dimensional Effects and Warpage

Moisture absorption causes linear expansion in nylon:

Dimensional Change from Dry to Saturated:

グレード	Linear Expansion per % Moisture
PA6	0.25-0.30%
PA66	0.22-0.28%

PA12	0.12%
PA66-GF30	0.07%

A PA6 bushing with 50mm OD, molded dry, will expand to approximately 50.35mm at 50% RH saturation (0.35% change × 50mm = 0.175mm). If the assembly requires 50.0-50.1mm fit, this is a critical tolerance issue.

Unequal Moisture Distribution Causes Warpage:
In thick sections, the outer surface absorbs moisture while the core remains dry. This creates differential swelling — the surface wants to expand while the core resists. The result is warpage (bowing, distortion) even in parts with symmetrical geometry.

Design Strategies:
1. Anneal before final dimensioning — Heat treat parts at 120-130°C for 1-2 hours to crystallize and stabilize dimensions before machining or assembly
2. Condition to equilibrium — Allow parts to reach uniform moisture content before final assembly
3. Use GF or CF reinforcement — Fiber reinforcement reduces moisture-induced expansion by 70-80%
4. Specify PA12 — At 1.5% saturation vs. 8-9% for PA6/66, PA12’s dimensional change is negligible

Processing: Drying Requirements

Excess moisture in nylon during injection molding causes catastrophic defects:

Moisture Defects:
- Bubbles and voids: Steam formed during injection creates internal voids
- Silver streaks: Water vapor flashing off during injection creates surface streaks
- Reduced molecular weight: Hydrolysis during processing weakens the material
- Reduced mechanical properties: Even if surface looks good, the material is degraded

Required Drying Parameters:

素材	Drying Temperature	Drying Time	Max Moisture Content
PA6	80°C	4～6時間	0.20%
PA66	80-85°C	4～6時間	0.15%

PA12	80°C	3-4 hours	0.10%
PA66-GF30	85°C	4～6時間	0.12%

Drying Equipment: Desiccant dryers are mandatory for nylon. Hot air dryers are insufficient because they cannot remove moisture below the surface. Desiccant dryers with dew point below -40°C are required.

Moisture Analyzers: Use Karl Fischer titration or loss-on-drying to verify material moisture before processing critical parts. Most production facilities check every batch.

PA12 vs. PA6/PA66: When to Choose Low-Moisture Grades

For applications where moisture is unavoidable, PA12 is the logical choice:

Applications where PA12’s low absorption is essential:
- Underwater or marine components: PA12 maintains properties in submerged conditions where PA6/66 would absorb 5-8%
- Outdoor exposed parts: PA12’s 1.5% saturation vs. 8-9% means far less dimensional change through seasonal humidity cycles
- Food processing (steam cleaning): PA12 resists steam exposure better than PA6/66
- Fluid metering components: Dimensional stability in humid air is critical for precision metering
- Cable conduits: PA12 handles underground moisture without swelling

Cost vs. Benefit:
PA12 costs approximately 2-3× more than PA66. The premium is justified when:
1. Field failures from moisture-induced swelling are costly
2. Dimensional tolerances are tight (±0.05mm or tighter)
3. The part is exposed to water, humidity, or steam
4. Assembly requires parts at equilibrium condition before fit testing

Hybrid Approach:
For many applications, PA66-GF30 achieves a practical balance: the glass fiber reinforcement reduces moisture absorption by ~70% (effective absorption drops from 8.5% to ~2.5%), and the GF network limits dimensional change. This is why PA66-GF30 is the automotive default — it handles under-hood humidity without the premium cost of PA12.

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