PA6 vs PA66 is the first decision in almost every nylon part. The two polyamides look alike on a datasheet, but PA66 melts about 40 °C higher, takes up a little less water and holds its stiffness better when hot, while PA6 is tougher, easier to process, gives a better surface and costs less. The right choice depends on service temperature, moisture and total part cost, not on price per kilogram alone. As a formulation development partner, SYCHEM TECH supplies both and the additives that go with them.
Short answer: choose PA66 for continuous service above about 140 °C, reflow soldering, tight tolerances in hot and humid conditions, and wear parts that run hot. Choose PA6 for impact-prone parts, visible surfaces, film, fiber and monofilament, and any part where the heat headroom of PA66 is not needed and cost matters.
How PA6 and PA66 differ in structure
Both are nylons: chains of carbon units linked by amide groups that hydrogen-bond to neighbouring chains. PA6 (polycaprolactam) is made from a single monomer, caprolactam, so it repeats one six-carbon unit. PA66 (polyhexamethylene adipamide) is made from two monomers, hexamethylenediamine and adipic acid, each with six carbons, which alternate along the chain.
The PA66 chain is more regular, so its amide groups line up more easily between chains. The crystals pack more tightly and melt at about 255–265 °C, against 220–225 °C for PA6. That difference in melting point is the root of most of the others: heat resistance, processing temperature and dimensional stability.
PA6 vs PA66 comparison table
| Property (typical, unfilled, dry) | PA6 | PA66 |
|---|---|---|
| Made from | Caprolactam (one monomer) | Hexamethylenediamine + adipic acid |
| Melting point | 220–225 °C | 255–265 °C |
| Density | About 1.13 g/cm³ | About 1.14 g/cm³ |
| Tensile strength | 70–85 MPa | 80–95 MPa |
| HDT at 1.8 MPa | About 55–75 °C | About 70–90 °C |
| Moisture at 23 °C / 50% RH | About 2.7–3.0% | About 2.3–2.6% |
| Moisture at saturation | About 9.5% | About 8.5% |
| Toughness and impact | Higher | Lower |
| Melt and mold temperature | Melt 230–260 °C, mold 60–80 °C | Melt 280–300 °C, mold 80–100 °C |
| Surface finish | Better; easier on visible parts | Good |
| Price | Lower, typically 10–20% below PA66 | Higher |
Values are typical ranges for unfilled grades, not specifications. Viscosity, glass fiber, heat stabilizer and conditioning move every property, so ask for the TDS of the exact grade before you lock a formula. Density is almost identical, so prices per kilogram compare fairly; the real cost differences come from processing, glass loading and cycle time.
PA6 vs PA66 moisture absorption: the hidden variable
Both nylons absorb water from the air, and water acts as a plasticizer. A dry-as-molded part is stiff and strong; a conditioned part is tougher but softer, with tensile strength and modulus noticeably lower. Every 1% of absorbed water also swells PA6 by roughly 0.2–0.3% in dimension.
PA66 absorbs a little less water and reaches equilibrium more slowly, which is one reason tight-tolerance connectors and precision housings still lean toward it. For either resin, measure conditioned parts after the moisture content has stabilized, not straight off the press, and dry the pellets properly before molding.
PA6 vs PA66 with glass fiber and additives
Unfilled nylon rarely reaches the plant floor. Adding 30% glass fiber lifts the HDT of both resins above 200 °C, and PA6 GF30 reaches a tensile strength of roughly 160–185 MPa when dry. Glass fiber therefore closes much of the gap at room temperature, but not at the top of the temperature range, where the melting point still sets the limit.
| Requirement | PA6 GF30 | PA66 GF30 |
|---|---|---|
| Structural stiffness at room temperature | Good choice | Good choice |
| Continuous service around 120–130 °C | Usually sufficient | Sufficient |
| Continuous service around 150 °C, peaks near 180 °C | Marginal | Standard choice, heat-stabilized |
| Reflow soldering near 260 °C | Not suitable | Needed (or a higher-temperature polyamide) |
| Surface appearance with glass fiber | Better | Good |
| Material cost | Lower | Higher |
Additives then tune each compound:
- Impact modification. MAH-grafted POE toughening agents bond to the nylon and raise notched and cold impact; around 15–25% gives super-tough nylon. Our SY-NE suits PA6, SY-NP and SY-NK suit PA6 and PA66, and SY-NT is designed for use down to −50 °C. See compatibilizer vs toughening agent for why the graft matters.
- Heat ageing. Parts in hot air need a heat stabilizer: copper-halide systems for long service near 150 °C, or phenolic antioxidants for lighter duty and light colours.
- Flame retardancy. Halogen-free UL94 V-0 in glass-fiber PA66 usually relies on aluminium phosphinate systems at around 15–20%. For other polymers, see our flame retardants.
- Mold release and flow. EBS wax works as an internal lubricant and release agent in PA6 and PA66 at low dosage.
Browse PA6 and PA66 grades, including glass-fiber, flame-retardant and impact-modified types, on our PA resin page.
When PA6 is the better choice
- Impact-prone parts and parts that must stay tough in the cold, often with a toughening agent
- Visible parts that need a good surface, including glass-fiber parts
- Thin walls and complex shapes, where the wider processing window helps
- Film (including BOPA for food packaging), fiber, monofilament and large cast shapes
- Parts at up to about 120–130 °C, such as charge-air cooler covers, battery brackets and clips, often in GF30
- Terminal blocks, low-voltage housings and switch parts in flame-retardant PA6
When PA66 is the better choice
- Under-hood parts at about 150 °C continuous with peaks near 180 °C: intake manifolds, thermostat housings, radiator end tanks
- Connectors and breakers that see reflow soldering near 260 °C or glow-wire testing
- Precision parts that must hold tolerances in hot, humid service
- Cable ties, gears, bearings and pulleys that run against friction heat
- Any application where an OEM specification already names PA66
PA6 vs PA66 cost: compare per part, not per kilogram
PA6 is usually 10–20% cheaper per kilogram than PA66, and the gap moves with caprolactam and adipic acid supply. The resin price is only part of the bill. PA66 needs higher melt and mold temperatures, while PA6 may need more conditioning time or a thicker wall to match stiffness. Convert every quote to cost per finished part, including glass loading, additives, cycle time and scrap, and validate any PA66-to-PA6 switch with heat-ageing and moisture tests first.
Frequently asked questions
Is PA66 stronger than PA6?
Slightly, when dry: about 80–95 MPa against 70–85 MPa unfilled, and PA66 keeps more stiffness when hot. PA6 is the tougher of the two at room temperature. With glass fiber, the room-temperature gap is small.
Can PA6 replace PA66 to save money?
Often, for parts that stay below about 120–130 °C and do not see reflow soldering. Above that, or in hot and humid precision parts, stay with PA66. Always confirm with heat-ageing and moisture-swelling tests before switching.
Which absorbs more water, PA6 or PA66?
PA6. At 23 °C and 50% relative humidity, budget about 2.7–3.0% for PA6 and 2.3–2.6% for PA66 in unfilled grades. Glass-fiber grades absorb less in proportion to their resin content.
Can PA6 and PA66 be blended?
Yes. They are compatible, and PA66/6 copolymers are made for film and blow molding. Adding PA6 to PA66 lowers the melting point and heat resistance, so a blend behaves between the two.
What should we tell a supplier to get the right grade?
The part and wall thickness, service temperature, UL or glow-wire requirements, colour, annual volume and regrind ratio. With that, a supplier can recommend the resin, glass content, heat stabilizer and flame-retardant package in one round.
Get a PA6 or PA66 recommendation
Still weighing PA6 vs PA66? Send us the part, its service temperature and its impact, flame or tolerance targets. We will suggest a resin grade and additive package, email you the TDS and arrange samples. Contact our technical team or write to info@sychemtech.com.
Related products: our PA6 and PA66 resins, SY series toughening agents for nylon, antioxidants and EBS wax, and the full engineering plastics range. For styrenic engineering resins, see the ABS grade selection guide.
