News - Nylon Yarn Physical Testing: Tenacity, Elongation, Shrinkage and Evenness Specifications

Nylon Yarn Physical Testing: Tenacity, Elongation, Shrinkage and Evenness Specifications for B2B Buyers

When a knit fabric mill quotes a tight delivery window to a sportswear brand, the last thing it needs is a yarn lot that breaks on the knitting machine or shrinks unevenly after dyeing. Most quality disputes between overseas buyers and Chinese yarn suppliers are not about price. They are about whether the physical numbers on the certificate of analysis (COA) actually match the rolls arriving at the warehouse. This article explains how nylon yarn strength, stretch, shrinkage, twist, evenness and oil content are measured, what tolerances a B2B buyer should write into a specification sheet, and how to verify a lot before it is loaded into the truck. We focus on bio-based nylon filament, low-melt bonding yarn and conventional polyamide textured yarn, but the testing logic applies to almost any continuous filament nylon you import.

Tenacity and Elongation at Break: How the Numbers Are Produced

Tenacity is the most quoted mechanical property on any nylon COA, and also the most frequently misunderstood. It is expressed in centinewtons per decitex (cN/dtex), which normalizes breaking force to the linear density of the yarn so that a 50 denier and a 150 denier sample can be compared directly. Conventional nylon 6 and nylon 66 flat filament typically reaches 4.0 to 5.5 cN/dtex, while bio-based PA56 drawn filament normally falls in the 3.8 to 4.8 cN/dtex band because of its slightly lower crystallinity. Drawn textured yarn (DTY) reads lower, often 3.2 to 4.4 cN/dtex, because the crimp structure absorbs part of the load.

Two standards dominate the laboratory. ASTM D2256 covers the single-strand test for yarn Breaking Tenacity, and ISO 2062 is the international equivalent for the determination of breaking force and elongation at break of individual yarns. Both are run on a constant-rate-of-extension (CRE) tensile tester. The operator mounts a single end, applies a small pre-tension to straighten it, and pulls until rupture. For polyamide filament the standard gauge length is a 500 mm gauge length with a crosshead speed of 500 mm/min and a 0.5 cN per dtex pre-tension. Deviation from these settings moves the result, so the COA should state the gauge and speed used.

Elongation at break is recorded in the same pull. Flat nylon filament elongates 25% to 45%, while textured yarn stretches 30% to 60% before snapping. A tight specification for a warp-knit lining might state tenacity 4.2 ± 0.3 cN/dtex and elongation 38% ± 5%. The coefficient of variation (CV%) across ten to twenty breaks per lot should stay under 8%; a CV above 12% signals uneven drawing or weak spots that will cause end breaks in production. Buyers should ask for both the mean and the CV, not only the average, because a good average can hide a bimodal distribution.

Conditioning and Test Environment: Why 20°C and 65% RH Matter

Polyamide is hygroscopic. It can absorb up to 4.5% moisture at standard atmosphere, and that moisture acts as a plasticizer, lowering measured tenacity and raising elongation. This is why conditioning is not a formality. The accepted procedure is to store samples at 20 ± 2°C and 65 ± 4% RH for a minimum of 16 hours, and preferably 24 hours, before testing. Samples rushed from a dry container or a humid warehouse give numbers that do not represent the yarn your factory will run.

The same lot measured at 30°C and 80% RH can show tenacity roughly 6% to 9% lower than the same lot measured at 20°C and 65% RH. For a buyer comparing a supplier’s COA against an in-house re-test, a difference of that size is often an environment artifact rather than a real defect. Always confirm the conditioning record before escalating a claim. Laboratories that follow ISO 139 and ASTM D1776 will note the equilibrium conditions on the report; those that do not should be treated with caution.

Boiling Water Shrinkage and Dry Heat Shrinkage: Sizing Stability on the Knitting Machine

Shrinkage decides whether your finished panel keeps its dimensions after the autoclave, the dye bath or the heat setting frame. Two methods are relevant for nylon. Boiling water shrinkage (BWS) is measured by marking a 1000 mm length of yarn, immersing it in 100°C water for 30 minutes without tension, drying flat, and measuring the final length. The formula is (L0 minus L1) divided by L0, expressed as a percentage. Flat nylon filament usually shows BWS below 2%, while textured yarn runs boiling water shrinkage of 3% to 8%. Low-melt bonding yarn is a special case: its sheath component is designed to shrink and fuse, so BWS is part of the bonding mechanism rather than a defect.

Dry heat shrinkage (DHS) is measured in a forced-air oven, commonly at 180°C for 15 minutes with the yarn relaxed. Nylon 66 tolerates higher heat than nylon 6, so its DHS at a given temperature is typically lower. For a double-jersey sports fabric the buyer may specify BWS 4.5% ± 1.0% and DHS at 180°C below 4%. If BWS is too high, the greige fabric relaxes after steaming and the finished garment comes up short against the marker. If it is too low on a textured yarn, the fabric may feel boardy and lose recovery. A single shrinkage number on the COA is not enough; request both BWS and DHS with the exact test temperature stated.

Twist per Meter and Twist Contraction in Textured Nylon

Twist is the number of turns per meter (TPM) inserted into the yarn, and it controls bulk, handle and the way two ends grip each other in plating or in a low-melt interlace. Air-jet textured yarn carries little true twist, often 0 to 50 TPM, while a conventional ring-twisted or false-twist set yarn may carry twist per meter between 80 and 600 depending on the target hand. A 75 denier interlace for a fine circular knit could be set at 120 TPM, whereas a coarse chenille-core binding yarn for upholstery might reach 550 TPM.

Twist contraction is the length the yarn loses when twist is inserted, expressed as a percentage of the original untwisted length. A yarn supplied at 1000 mm before twisting may measure 960 mm after 300 TPM, a contraction of 4%. This matters because the supplier quotes denier on the twisted yarn, but the knitting machine consumes length, so over-twisted lots shorten the meters per kilogram you actually receive. The COA should report TPM as measured by ASTM D1423 (the untwist-retwist method) or the direct capstan method, with a tolerance such as ± 20 TPM on a 200 TPM spec. Inconsistent TPM between cones causes stripes in plain jersey that no dyer can correct.

Evenness CV% and USTER Reference Levels

Evenness describes how uniform the yarn mass is along its length. It is measured on a capacitive evenness tester that records the linear density variation continuously, then reports the CV% of that variation. For fine nylon filament a USTER-style reference level of 25% (the historical USTER Statistics percentile) might correspond to a CV% around 1.0% to 2.0%, while coarse yarns are permitted wider variation. A practical buying specification for a quality circular knit is an evenness CV% below 1.5% for 50 to 100 denier nylon, rising to below 2.2% for 200 denier and above.

Beyond the mean CV%, the evenness report lists thin places, thick places and neps per 1000 meters. A thin place is typically a 30% to 50% reduction in mass over a short length; thick places and neps are local lumps that show as faults on the fabric face. For a clean sports mesh the buyer may cap thin places at 5 per 1000 m and neps at 3 per 1000 m. Reference levels shift by denier and by year of the USTER Statistics edition, so when you cite a USTER percentile on a purchase order, name the edition (for example USTER Statistics 2021) so the supplier and the lab are calibrated to the same benchmark.

Oil Pickup (OPU) and Knitability

Oil pickup, sometimes called oil content or finish level, is the percentage of spin finish and antistatic agent on the yarn surface, measured by solvent extraction and reported as OPU%. The finish reduces friction on the ceramic guides and controls static during high-speed unwinding. Too little finish and the yarn generates static, fluffs and breaks; too much and it migrates, stains the needles, and blocks the dye. For most warp and weft knitting of nylon the workable window is an oil pickup of 1.2% to 2.0%, with flat yarn on the lower side and textured yarn on the higher side.

The COA should state OPU as a range, not a single point, because the finish is applied in a spray that varies by 0.2% to 0.3% cone to cone. A buyer running fine 40 gauge machines should demand OPU 1.5% ± 0.3% and verify it on arrival, since a lot at 0.9% will run with frequent end breaks while a lot at 2.6% will leave oily rings on the fabric. The finish chemistry also matters: bio-based nylon often ships with a plant-derived ester finish that behaves differently from a mineral oil finish under heat, so request the finish type on the COA when knitability is critical.

What Every COA Must List: Fields, Tolerances and Units

A certificate of analysis is only useful if it records the right fields in the right units. A minimal but defensible COA for a nylon filament lot should include the following, each with the test method and the agreed tolerance:

Lot or batch number and the production date, plus the number of cones in the lot.
Product type (for example bio-based PA56 DTY) and the luster (bright, semi-dull, full-dull).
Nominal and measured denier or dtex, with filament count, tolerance ± 2.5% on linear density.
Tenacity in cN/dtex, mean and CV%, with the gauge length and speed used.
Elongation at break in percent, mean and CV%.
Boiling water shrinkage and dry heat shrinkage at the stated temperature, tolerance ± 1.0%.
Twist in TPM and twist contraction in percent, where applicable.
Evenness CV% and the thin/thick/nep counts per 1000 m.
Oil pickup (OPU%) and the finish type.
Conditioning atmosphere and the equilibration time before test.
Test standard referenced and the issuing laboratory name and date.

The tolerance column is where most contracts are won or lost. A vague COA that prints only “tenacity 4.3 cN/dtex” without an accepted range gives the supplier cover to ship a lot at 3.6 cN/dtex and argue it was “within normal variation.” Write the allowable band into the purchase order and make the COA a contractual attachment, not a courtesy document.

Acceptance Sampling: Where to Cut the Package and How to Balance

A lot is rarely uniform from the outer wrap to the core, so sampling position changes the result. For a cylindrical cone, the surface yarn has seen more handling and sometimes more exposure, while the inner layers may retain more finish. A sound procedure pulls samples from three depths on each of several randomly selected cones: the outer 5%, the middle band, and the inner 10% near the core. Combine these into a composite for evenness and OPU, but keep them separate for a tenacity-by-depth check when stripe risk is high. For a 5-ton order split across 400 cones, an AQL 2.5 inspection plan would typically examine around 20 to 32 cones, pulling one test end from each.

Before any physical test, the samples must be re-conditioned at 20°C and 65% RH for at least 16 hours, because the journey from the supplier’s warehouse to your lab changes the moisture state. Weigh the cone on arrival and compare against the supplier’s net weight; a 3% to 5% weight gap after conditioning suggests a short meterage or a heavy package that was never dried. Photograph the package code and the seal, because the sampling record is the first thing a third party asks for if a dispute reaches arbitration.

Tip: Always request the conditioning time and the actual laboratory temperature and humidity on the COA, because a tenacity reading taken at 30°C and 80% RH can be 6% to 9% lower than the same lot measured at 20°C and 65% RH.

In-House Testing vs Third-Party Laboratories

Most Chinese nylon mills run an internal quality lab equipped with a CRE tensile tester, a BWS rig, a twist tester and an evenness meter, and they issue the COA from that lab. In-house testing is fast and inexpensive, and it is adequate for routine lot release when the buyer has built trust through repeat orders. The weakness is obvious: the supplier tests its own product. When the value of a single lot exceeds a threshold, or when a previous shipment failed, an independent laboratory removes the doubt. SGS, Intertek, Bureau Veritas and TUV routinely run nylon physical tests to ASTM and ISO methods at their offices in major Chinese port cities, with typical turnaround of three to five working days and a fee of a few hundred US dollars per full physical panel.

A practical approach is a split: accept in-house COA for routine lots, but reserve the right to a third-party confirmation on any lot where the buyer’s incoming check disagrees by more than the tolerance band. The purchase order should state who pays for the third-party test if the result confirms the supplier’s COA (buyer pays) versus if it confirms the defect (supplier pays). This single clause ends most arguments before they start.

Non-Conforming Lots: Quarantine, Re-Test and Claim Procedure

When an incoming lot fails one or more specifications, the first step is quarantine. Move the suspect cones to a locked area, photograph the lot code and the failure, and notify the supplier within the claim window written into the contract, commonly 14 to 30 days after arrival. Do not blend the lot into production while the case is open, because mixing destroys the evidence and weakens any future claim.

The next step is a joint or third-party re-test. If the re-test confirms the failure, the remedies usually run in this order: a price concession proportional to the defect, a replacement shipment at the supplier’s cost, or a return and refund for a severe failure such as tenacity 20% below spec. Keep the original COA, the incoming test report, the photos and the communication log, because a claim without a documented chain of evidence is a negotiation, not a settlement. For bio-based PA56 where carbon claims matter, a physical failure also reopens the sustainability documentation, so the QA file should be closed only after both the mechanical and the certification records are reconciled.

FAQ — Physical Specifications for Nylon Yarn Buyers

What tenacity is typical for bio-based PA56 versus nylon 66?

Bio-based PA56 drawn filament usually measures 3.8 to 4.8 cN/dtex, while nylon 66 flat filament reaches 4.5 to 5.5 cN/dtex because of its higher melting point and crystallinity. For most knit applications the PA56 range is sufficient, but for high-abrasion warp knit the buyer may prefer nylon 66 or a blended structure.

How much boiling water shrinkage is acceptable for a circular knit?

For a standard nylon textured yarn a BWS of 3% to 8% is normal, and a buying spec often sets 4.5% ± 1.0%. Flat filament should stay below 2%. If your dye house steams the greige heavily, tighten the upper limit so the finished panel does not come up short against the marker.

Why does the same lot show different tenacity in two labs?

The usual cause is conditioning. A reading at 30°C and 80% RH can sit 6% to 9% below the same lot tested at 20°C and 65% RH, and a different gauge length or crosshead speed adds more spread. Require both labs to report their atmosphere and method so the numbers are comparable.

What OPU level prevents end breaks on fine gauge machines?

Fine 40 gauge circular knits run best at OPU 1.5% ± 0.3%, within a general workable window of 1.2% to 2.0%. Below 1.0% the yarn builds static and breaks; above 2.5% the finish migrates and leaves oily rings that the dyer cannot remove.

How many cones should I sample from a 400-cone order?

Under an AQL 2.5 plan a 400-cone lot typically calls for 20 to 32 cones inspected, pulling one test end from each and sampling from outer, middle and inner depths. For high-stripe-risk orders, increase to 50 cones and keep depth-separated results to trace any variation to its source.

Who pays for a third-party re-test when a lot fails?

The contract should state that if the independent lab confirms the supplier’s COA, the buyer pays; if it confirms the defect, the supplier pays. This removes the incentive to dispute harmless variation and focuses resources on real failures.

Post time: Sep-28-2026

Nylon Yarn Physical Testing: Tenacity, Elongation, Shrinkage and Evenness Specifications