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Polyamide 6 (PA6) and polyamide 66 (PA66) are both aliphatic semicrystalline thermoplastics derived from the same empirical formula C₆H₁₁NO, but their repeating units differ in symmetry and hydrogen-bonding registry. PA6 is produced by hydrolytic ring-opening polymerization of ε-caprolactam, yielding a chain with one amide group per hexanoate repeat. PA66 is produced by condensation of hexamethylene diamine and adipic acid, yielding a more regular arrangement of hydrogen-bonded sheets. This structural difference raises the melting point of PA66 to 260–265 °C by ISO 11357-3, compared with 220–225 °C for PA6, even though the two polymers are isomers. The higher melting point and greater chain symmetry of PA66 translate into higher tensile modulus, improved creep resistance, and lower equilibrium moisture absorption, whereas PA6 often provides easier flow at lower barrel temperatures and superior dry-as-molded impact toughness. Selection between these polymers is governed by maximum continuous-use temperature, exposure to moisture or glycol, dimensional tolerance after conditioning, and the available processing window. Material designations are defined in ISO 1043-1; conditioning and mechanical testing are specified in ISO 291 and ISO 527-1/-2, respectively.

Thermal Degradation Pathways and Melt Residence Time in PA6 Versus PA66

On a co-rotating twin-screw extruder with L/D ratio 40:1 and atmospheric venting, PA6 is typically processed at melt temperatures of 230–250 °C, while PA66 requires 260–290 °C to maintain homogeneous melt flow. The higher processing temperature of PA66 narrows the oxidative stability window; thermal degradation begins as chain scission at the amide C–N bond, followed by cyclopentanone formation and eventual gel or carbon speck formation. Residence times longer than 8–10 min at 290 °C produce yellowing and a measurable drop in relative viscosity of approximately 5–10% in unfilled grades; PA6 at 250 °C tolerates a slightly longer residence time before equivalent viscosity loss. Injection moulding of PA66 therefore demands precise shot-size control and barrel capacity utilization below 50–70% of maximum shot weight to limit hold-up in the compression zone. Pneumatic conveying after drying must use dried air or nitrogen with a dew point below -40 °C to prevent re-absorption; transfer lines exceeding 20 m justify hopper isolation valves and secondary dryers. In injection moulding, PA6 flows readily with barrel temperatures of 230–270 °C, whereas PA66 normally requires 270–300 °C at the nozzle. Melt temperature should not exceed 310 °C for PA66 or 275 °C for PA6 without considering thermal stabilizer packages and short residence times. The processing window narrows further when flame-retardant packages based on red phosphorus or halogenated synergists are used; exothermic decomposition can occur near 300 °C in PA66 grades, and the documented processing window may be limited to ±5 °C in thin-walled connector housings with wall thickness below 0.8 mm.

Dry-as-molded tensile properties measured according to ISO 527-1/-2 at 23 °C show PA66 tensile yield stress between 75 MPa and 85 MPa, while PA6 general-purpose grades fall between 70 MPa and 80 MPa. Tensile modulus for PA66 ranges from 2,800 MPa to 3,500 MPa, approximately 10–15% higher than PA6 at 2,500–3,200 MPa. At 80 °C, PA66 typically retains 60–70% of its room-temperature tensile strength, while PA6 may retain 50–60%. Creep resistance measured on injection-moulded tensile bars per ISO 899-1 under 20 MPa stress at 60 °C for 1,000 h shows PA66 retaining a creep modulus of 1,200–1,500 MPa, whereas PA6 typically falls to 900–1,200 MPa; exact values depend on molecular weight and nucleating additives. Notched Charpy impact measured per ISO 179-1/1eA in the dry state gives 5–8 kJ/m² for PA6 and 4–7 kJ/m² for PA66; PA6 is generally tougher when dry. The comparative dataset below consolidates the properties that most frequently control initial screening.

Property	Test method	PA6	PA66
Density	ISO 1183-1	1.13–1.14 g/cm³	1.13–1.15 g/cm³
Melting temperature, DSC	ISO 11357-3	220–225 °C	260–265 °C
Glass transition temperature, dry	ISO 6721-7	45–55 °C	50–60 °C
Tensile yield stress, dry	ISO 527-1/-2	70–80 MPa	75–85 MPa
Tensile modulus, dry	ISO 527-1/-2	2,500–3,200 MPa	2,800–3,500 MPa
Notched Charpy impact, dry, 23 °C	ISO 179-1/1eA	5–8 kJ/m²	4–7 kJ/m²
Water absorption at 50% RH, 23 °C	ISO 62	2.5–2.8%	2.0–2.5%
Unfilled mold shrinkage	ISO 294-4	0.7–1.5%	0.8–1.6%

Which Material Retains Impact Strength After Conditioning at 50% RH?

Conditioning to equilibrium at 23 °C and 50% RH according to ISO 291 lowers the glass transition temperature of both materials below room temperature, converting the amorphous phase into a tougher, more ductile state. Water uptake at 50% RH for unfilled PA6 is typically 2.5–2.8%, while PA66 absorbs 2.0–2.5% per ISO 62. The additional moisture in PA6 produces larger glass transition suppression and greater impact-strength gain; conditioned notched Charpy values for PA6 often exceed 15–25 kJ/m², whereas PA66 conditioned values may reach 10–15 kJ/m² or remain partial-break. However, the same moisture uptake reduces PA6 tensile modulus by 25–35% compared with dry-as-molded values, while PA66 modulus loss is typically 20–30%. For load-bearing applications requiring dimensional stability after humidity cycling, PA66 is often selected because the lower equilibrium moisture uptake reduces post-mold expansion and fastener torque relaxation. In impact-critical applications after long outdoor exposure, PA6 can outperform PA66 at low temperatures because its lower crystallinity and higher water content suppress brittle fracture; instrumented puncture tests at -30 °C per ISO 6603-2 may show conditioned PA6 absorbing 20–40% more total energy than conditioned PA66, depending on plate thickness and impactor geometry. The trade-off is that the same moisture gain accelerates hydrolysis at temperatures above 80 °C, so PA6 is not automatically preferred in hot wet environments.

In pressurised radiator end tanks, thermostat housings, and oil filter caps where continuous exposure temperature exceeds 110 °C, glass-fibre-reinforced PA66 is more commonly specified than PA6. In a 50% ethylene glycol/water mixture at 120–130 °C, hydrolytic degradation follows an autoaccelerated mechanism: water protonates the amide carbonyl, chain scission reduces molecular weight, and the weakened surface layer loses tensile strength. PA66 has a lower diffusion coefficient and higher crystalline fraction, which reduce the rate of water ingress; long-term immersion tests per ISO 175 in 130 °C water or 50% glycol show greater than 50% tensile strength retention after 1,000 h for stabilised PA66 grades, whereas unstabilised PA6 may fall below 40% retention under the same conditions. Published data for this specific configuration vary with stabilizer package, filler level, and test-cell oxygen pressure; the ranking is therefore indicative rather than absolute. Hot-air aging per ISO 188 at 120 °C for 1,000 h produces a larger loss in elongation at break for PA6 than PA66 because PA6 has lower melting point and crystallinity. For under-hood components, dimensional tolerances across temperature and humidity cycles are tighter; PA66 with 25–35 wt% short glass fibre and mould temperature of 110–120 °C is used where post-mold warpage must remain controlled, while PA6 equivalents may show greater movement due to higher moisture-induced volume change.

Moisture Uptake, Dimensional Drift, and Hydrolytic Stability Thresholds

The rate of water uptake in unfilled PA6 and PA66 follows Fickian diffusion in the amorphous phase at temperatures below the wet glass transition; immersion in water at 23 °C per ISO 62 yields saturation water absorption of 9.0–10.0% for unfilled PA6 and 7.5–9.0% for unfilled PA66, depending on molecular weight, crystallinity, and additives. The corresponding linear dimensional increase per 1% water absorbed is approximately 0.2–0.3%, meaning a PA6 part with 100 mm length can expand by 0.5–0.8 mm between dry-as-molded and 50% RH equilibrium. PA66 parts expand less, typically 0.4–0.6 mm, because lower moisture uptake offsets a similar hygroexpansion coefficient. Hydrolytic degradation becomes significant above 80 °C; exposure to hot water at 100 °C for 1,000 h may reduce tensile strength by 20–40% in unstabilised PA6 and by 15–30% in unstabilised PA66. For parts operating in saturated steam at 120 °C or higher, both materials require hydrolysis-resistant grades with reduced carboxylic end-group concentration, typically below 20 mmol/kg, and heat-stabilizer packages. Dimensional control after injection moulding also depends on moisture at the granulate; processors confirm a moisture content below 0.10% by Karl Fischer titration before melt processing to prevent surface splay and molecular weight loss. Desiccant-bed dryers with dewpoint below -40 °C are specified for PA66 because even 0.15% residual moisture can reduce tensile strength by 10–15% and produce gate blush in thin-walled parts.

For low-voltage connectors, miniature circuit-breaker housings, and terminal blocks, material selection depends heavily on relative thermal index, tracking resistance, and dielectric behaviour after moisture conditioning. PA66 grades commonly demonstrate relative thermal index values of 120–130 °C electrical and 105–115 °C mechanical with impact according to UL 746B, whereas PA6 grades are often rated 115–125 °C electrical and 95–105 °C mechanical with impact. A difference of 5–10 °C in relative thermal index may exclude PA6 from certain appliance or photovoltaic connector applications requiring sustained heat resistance at 125 °C. Unfilled PA6 and PA66 both exhibit comparative tracking index values above 600 V per IEC 60112; glass-fibre-reinforced grades typically fall to 400–500 V, and carbon-black-filled antistatic grades may fall below 300 V. For low-voltage connectors under 400 V, PA66 is usually preferred because it retains higher dielectric strength after moisture conditioning; dielectric strength measured per IEC 60243-1 at 2 mm thickness is typically 25–35 kV/mm for dry unfilled grades, dropping below 15–20 kV/mm after 24 h water immersion. Surface resistivity per IEC 62631-3-2 may fall from 10¹³ Ω to 10⁹ Ω more rapidly in PA6 after conditioning. Flame-retardant versions of both are rated V-0 at 0.8 mm or 1.6 mm per UL 94; PA66 flame-retardant grades generally retain higher heat deflection temperature and tensile strength after thermal aging than PA6 grades at equivalent additive loadings.

When Glass Fibre Reinforcement Is Added at 30 wt%, Weld-Line Strength Diverges

At 30 wt% short-glass-fibre reinforcement, the difference between PA6 and PA66 becomes more pronounced in heat resistance and weld-line performance. Injection-moulded specimens tested per ISO 527-1/-2 show dry tensile strength rising to 160–180 MPa for PA6 GF30 and 175–195 MPa for PA66 GF30; tensile modulus increases to 8,000–9,500 MPa and 9,000–11,000 MPa respectively. Heat deflection temperature under 1.8 MPa per ISO 75-1/-2 is 200–215 °C for PA6 GF30 and 240–250 °C for PA66 GF30. However, fibre orientation at weld lines creates a mechanical weak spot; weld-line tensile strength in PA66 GF30 can drop to 50–60% of the unwelded value, while PA6 GF30 often retains a similar proportion despite lower absolute strength. In hot-tool welding tests on 4 mm plaques, PA66 GF30 permits higher tool temperatures of 290–300 °C and longer hold times to orient glass fibres across the weld plane, but PA6 GF30 welds at 240–260 °C with less thermal degradation risk in the heat-affected zone. Warpage is controlled by mould temperature and gate placement; PA66 GF30 requires mould temperatures above 90 °C to achieve adequate crystallinity and minimise post-mold shrinkage, while PA6 GF30 can be moulded at 70–85 °C. The narrower mould-temperature window for PA66 GF30, typically ±10 °C, justifies oil-heated tooling with individual circuit controllers rather than single-zone water units.

Property	Test method	PA6 GF30	PA66 GF30
Tensile strength, dry	ISO 527-1/-2	160–180 MPa	175–195 MPa
Tensile modulus, dry	ISO 527-1/-2	8,000–9,500 MPa	9,000–11,000 MPa
Flexural modulus, dry	ISO 178	7,500–8,500 MPa	8,500–10,500 MPa
Heat deflection temperature, 1.8 MPa	ISO 75-1/-2	200–215 °C	240–250 °C
Water absorption at 50% RH, 23 °C	ISO 62	1.6–2.0%	1.2–1.6%
Mold shrinkage	ISO 294-4	0.2–0.5%	0.2–0.5%

Because transfer film formation controls wear in dry sliding systems, pin-on-disc testing per ASTM G99 using a hardened steel counterface with Ra 0.1–0.3 µm at sliding speed 0.5 m/s and contact pressure 1 MPa ranks internally lubricated PA66 compounds ahead of equivalent PA6 compounds. PA66 with 15–20 wt% PTFE or 2–4 wt% molybdenum disulfide exhibits steady-state wear factors of 2–5 × 10^-6 mm³/N·m, while PA6 analogues are 20–40% higher under the same conditions. Limiting PV values for unfilled grades in continuous unlubricated operation are approximately 0.15–0.25 MPa·m/s; internally lubricated compounds may reach 0.35–0.50 MPa·m/s. PA6 compounds may require 15–20% derating above 60 °C. Published data for this specific configuration vary with counterface hardness and surface roughness; direct substitution requires component-level wear testing because transfer film formation is sensitive to moulded skin crystallinity. Gears cut from extruded PA66 plate or injection-moulded blanks operate with less tooth-thickness loss after 10⁶ cycles in gear tests based on VDI 2736. For PA6, larger thermal expansion can increase tooth backlash by 0.1–0.2% of pitch diameter when conditioned from dry to 50% RH, whereas PA66 changes less. Failures in PA6 gears in dry running are more often caused by melting and plastic flow at tooth flanks when local flash temperature exceeds 180 °C; PA66 withstands flash temperatures up to 220 °C before local surface melting occurs. These operating boundaries make PA66 the default for high-load dry-running gears, while PA6 remains suitable for lower-speed applications where moisture-induced dimensional growth can be absorbed by larger backlash or flank clearance.