Nylon (PA) is the family of polyamides whose molecular backbone carries the repeating amide group [—NHCO—], making it a thermoplastic resin that competes directly with metals and wood in structural applications [S1]. Production volumes concentrate on aliphatic grades — PA6, PA66, PA11, PA12 — with nomenclature set by the carbon count of the constituent monomers [S1].
For spec work, PA is a substitute candidate whenever a designer needs a metal-like strength with one-third the density, tolerates injection moulding at scale, and accepts a moisture-conditioned operating envelope. The catch sits in the amide bond itself: that same polar linkage that gives nylon its toughness is also why it absorbs water, creeps under sustained load, and degrades under UV without protection. The rest of this piece maps the trade-offs against PA grades, additives, and the standards that govern them.
What the PA family actually covers
Polyamides split into three structural classes — aliphatic PA, aliphatic-aromatic PA, and aromatic PA — with the aliphatic sub-family holding the largest share of volume and the broadest industrial use [S1]. Aliphatic PA grades are named by the carbon count of their monomers: PA6 (caprolactam, 6 carbons), PA66 (hexamethylenediamine + adipic acid, 6+6), PA11 (castor-oil-derived, 11 carbons), PA12 (laurylactam, 12 carbons), and so on [S1].
Because every grade carries the same amide group, they share a behaviour set — polar, hydrogen-bonding, hygroscopic — but the spacing of those amide groups along the chain is what differentiates them. Longer aliphatic segments between amides mean lower moisture pickup, better dimensional stability, and lower density, at the cost of peak strength and heat resistance. That single lever — amide spacing — drives most of the comparison table later in this article.
Mechanical and chemical strengths in real numbers
PA66 in dry-as-moulded (DAM) condition typically delivers tensile strength in the 80–85 MPa range with a density around 1.14 g/cm³, giving a specific gravity roughly 1/7 that of mild steel — the headline property that puts PA on every metal-replacement shortlist [S1]. PA6 sits a notch below on strength but offers easier processing and a lower melting point (~220 °C for PA6 vs ~265 °C for PA66), which is why PA6 dominates large thin-wall mouldings while PA66 dominates under-hood and structural parts.
Abrasion resistance is the second headline: PA66 wears better than most engineering plastics in sliding contact, which is why unfilled and glass-filled PA66 grades are common in gear, cam, and bushing work. Chemically, nylon resists aliphatic hydrocarbons, oils, fuels, ketones, and many dilute alkalis at room temperature, but is attacked by strong mineral acids, oxidising agents, and phenols — a set of bounds that has to be checked against any chemical-service datasheet before substitution [S1].
For thermal service, glass-fibre-reinforced PA66 (typically 30% or 33% GF) raises the heat-deflection temperature under load (HDT at 1.82 MPa) to roughly 250 °C, opening continuous-use temperatures in the 150–170 °C band depending on the OEM rating and conditioning state. Glass fibre also cuts creep, which is the limiter on most unfilled PA in sustained-load service.
Where nylon fails: moisture, creep, UV, and notch sensitivity
Moisture absorption is the single most consequential weakness. PA6 can absorb up to roughly 9–10% water by mass at saturation, PA66 around 7–8%, and even PA12 — the lowest in the family — pulls in 1.5–2% at equilibrium in water [S1]. Water plasticises the resin: tensile strength drops 30–50% from DAM to conditioned state, modulus halves, and dimensions swell 1–3% linearly. Any tolerance tighter than ±0.5% on a moulded part needs either glass fill, a lower-absorption grade (PA11/PA12), or a moisture-management design rule.
Creep under sustained load is the second limit. Unfilled PA66 has an apparent creep modulus that decays noticeably above ~50 °C, so any design holding a constant load for thousands of hours needs a creep-modulus curve, not a short-term tensile number. Glass-fibre reinforcement and heat-stabilised grades are the standard mitigation; specifying PA in a constant-load, warm, wet environment without checking creep data is the classic failure pattern.
UV and weathering are the third limit. Unprotected PA yellows, loses tensile strength, and embrittles within months of outdoor exposure because the amide bond is photo-oxidatively cleaved. Carbon-black-filled or paint-coated PA6/PA66 grades pass 2–5 year outdoor service in automotive and agricultural applications; unprotected natural PA does not. Notch sensitivity rounds out the list — PA is notch-sensitive, so sharp internal corners, weld lines, and knit lines need generous radii (typically 0.5× wall or more) to avoid brittle failure under impact.
Selection criteria: matching grade to duty
The decision tree reduces to four parameters: moisture exposure, peak temperature, sustained load, and chemical environment. PA66 GF30 is the default for hot, strong, dry, structural parts; PA6 is the default for large, complex, lower-cost mouldings; PA11 or PA12 is the default when moisture pickup or low-temperature flexibility (−40 °C) drives the spec, and they also pass more aggressive chemical exposure thanks to the longer aliphatic spacer. [S3]
For applications competing with fluoropolymers, the trade-off is straightforward: PTFE wins on chemical resistance and a continuous-use ceiling around 260 °C, but nylon wins on mechanical strength, creep, and cost. A typical spec line for a chemical-process seal or lined-pipe support bushing compares nylon's tensile and wear against PTFE's near-zero moisture and universal chemical passivity — and many plants end up running both, PTFE for the wetted seal, glass-filled PA for the structural bracket.
For sliding-wear gears, bushings, and wear pads, the practical comparison is between unfilled PA66 (low cost, moderate PV limit), PA66 with MoS₂ or graphite internal lubricant (lower friction, higher PV), and cast PA6 (large-section parts, lower residual stress). All three run quieter and need less lubrication than metal, but the PV limit — typically 0.05–0.1 MPa·m/s for unfilled grades — sets the upper bound on load × speed.
Standards, processing, and the moisture-management rule
ISO 1043-1 labels the family "PA," and ISO 1874 grades individual types by property blocks. ASTM D6779 covers PA moulding and extrusion materials. For processing, moisture content before moulding is non-negotiable: PA6 and PA66 must be dried to below 0.02% (200 ppm) before injection moulding, otherwise hydrolytic chain scission drops molecular weight and the part fails impact testing — a common production-line defect with a clean fingerprint of silver streaks and brittle fracture. [S2]
For chemical-process equipment using nylon as a liner, support, or valve seat, ASTM and ISO standards govern dimensional and pressure classes rather than the resin itself; the resin selection then references ISO 1874 or OEM datasheets. When nylon is specified alongside metals in a piping or instrument assembly, the moisture-swell behaviour has to be carried into the mechanical fit design — a NPT-threaded PA fitting, for instance, can seal differently in a dry shop than after six months on a humid process line.
For instrumentation hardware that integrates nylon housings or cable glands with metal pressure transmitters or flow meters, the same moisture rule applies: nylon's dielectric and mechanical properties shift between dry-as-shipped and conditioned state, so impact and IP-rating tests should be specified on conditioned samples if the field environment is wet. This is also why many instrument manufacturers rate their nylon-cable-gland assemblies at IP68 only after a preconditioning step.
Reinforcement and additive systems that move the trade-off curve
Glass-fibre reinforcement at 30–33% by mass is the workhorse: it roughly doubles tensile strength and modulus, lifts HDT by 60–100 °C, and cuts creep by half to two-thirds, at the cost of mould-wear on tooling and a slight loss in elongation at break. Carbon-fibre reinforcement adds stiffness, electrical conductivity (useful for static-dissipative ATEX Category 2 housings), and lower mass — at a 3–5× price premium. [S3]
Impact modifiers (typically elastomeric tougheners such as EPDM-grafted or maleated polyolefins) restore some of the elongation lost in glass-filled grades, producing the "toughened" PA66 grades used in crash-relevant automotive structural parts. Heat stabilisers (copper halide + KI/Aryl systems) push continuous-use temperature upward by 20–40 °C and are the standard additive for under-hood service. UV stabilisers (HALS plus carbon black or selected pigments) are the only path to multi-year outdoor service; the standard carbon-black loading is 1.5–2% for full protection.
For flame performance, halogenated and phosphorus-based systems push unfilled PA66 to UL94 V-2 and glass-filled PA66 to V-0 at typical thicknesses (1.5–3.0 mm). Specifying UL94 V-0 at 0.8 mm requires careful grade selection, and a "V-0 at 1.5 mm" rating should not be assumed to transfer to thinner sections.
Where nylon beats alternatives, and where it loses outright
Nylon (polyamide, PA) is a thermoplastic resin widely used as a substitute for traditional materials such as metals and wood in various structural applications. It also wins against most other engineering plastics on toughness and wear. [S3]
Nylon loses outright against stainless steel and other alloys on continuous high temperature (>200 °C), on long-term creep under sustained static load in warm/wet conditions, on UV without protection, and on chemical resistance to strong acids and oxidisers. It also loses to acetal (POM) on dimensional stability in wet environments, and to PTFE on chemical and thermal ceilings — though at a fraction of the cost. The practical spec line is: pick PA when you need strength + toughness + mouldability at low-to-mid temperature in a non-aggressive chemical; pick something else when any one of those bounds is exceeded.
A common selection mistake is to read the dry-as-moulded tensile number and ignore the conditioned state. A PA66 part specified to 60 MPa tensile in a 50 °C, 70% RH environment is running with a 30–40% safety-margin haircut from the start; if the application is also UV-exposed outdoors without carbon black, the safety factor erodes further over time. The fix is straightforward — use conditioned-state property data for design, glass-fill for creep, and carbon-black or paint for UV — and the result is one of the most cost-effective engineering plastics in the catalogue.
Maintenance, failure modes, and field signals
Three failure signatures dominate field returns. First, brittle fracture at a notch or weld line, typically from a sharp internal corner or a knit line opposite a gate — the cure is radius and gate redesign, not material substitution. Second, dimensional swell and fastener loosening in a wet, warm service — the cure is a lower-absorption grade (PA12, PA11) or a glass-filled grade with a designed moisture gap. Third, surface chalking and cracking on outdoor parts — the cure is carbon-black or paint, plus a UV-stable grade.
Trackable signals for a spec engineer in 2026: the ongoing substitution of PA66 in metal-replacement structural brackets, the use of PA12 in flexible fuel lines and pneumatic tubing as R134a and HFOs push refrigerant-system materials harder, and the use of carbon-fibre-filled PA in static-dissipative pressure sensor housings for ATEX/IECEx zones. None of these moves changes the underlying trade-off — moisture, creep, and UV remain the three limits — but they refine which grade and which additive package belongs on the BOM. When the operating envelope exceeds any one of those three limits, the spec shifts off PA to acetal, PPS, or a fluoropolymer; when it does not, glass-filled PA66 is still the lowest-cost path to a metal-strength part.
Background reading: Truck-Mounted Crane Types and Classifications: A 2026 Spec Map.