Nylon, polyester, polypropylene or cotton: how to choose webbing for the product it goes into

"Which webbing is stronger" is a question with no answer

Breaking strength describes exactly one moment: how a new, dry piece of webbing fails while clamped in a test machine. The finished product does not live that way. It gets wet, it stands in the sun, it holds load for weeks, it chafes against a buckle, it lives next to chemicals, and it has to come out in a color the customer recognizes.

So before I name a fiber, I ask about something else. Whether the load is steady or shock. Whether it gets wet constantly, occasionally or never. How many years outdoors. Peak contact temperature — and whether anything slides across the webbing under load. And whether the color has to match a physical standard.

Short answer up front, so you do not have to read to the end. Static load, sun, moisture, dimensional stability — polyester. Shock, impact, abrasion in the dry — nylon, which the European standards call polyamide. Polypropylene where there is no sustained load at all and where nobody is matching color. Cotton for the hand, for natural content, because it does not melt — and never where it stays wet for a long time.

Now the reasoning, because every one of those points has cost somebody a batch.

You buy meters and pay for kilograms

Polypropylene density is 0.90–0.91 g/cm³ — the only one of the four that floats. Nylon is 1.14, polyester 1.38. At the same cross-section, a kilogram of PP yields roughly 1.5 times more meters than a kilogram of PET. You cut and sew meters, you haggle over kilograms, and the arithmetic comes out a factor of one and a half wrong.

The second line on that invoice is water, which you also pay for. Commercial moisture regains are listed in ASTM D1909: cotton 7.0%, nylon 4.5%, polyester 0.4%, polypropylene 0.0%. A kilogram of cotton webbing carries about 70 grams of moisture pulled out of the air. The same roll weighed in a heated shop in winter and in a damp warehouse in summer will not give the same figure. Nothing is missing, and there is nothing to argue about.

The third trap sounds logical, which is why it lives so long: "PP is light, so its specific strength must be good." It is not. Fiber tenacity is measured in grams per denier, meaning force already divided by linear density — the lightness is inside the number. nylon 6,6 8.5–10 g/den, PET 7.0–9.0, PP 5.0–7.0, cotton 3.0–5.0. Per unit mass polypropylene comes in about one and a half times weaker than nylon, and once the lower density is accounted for, the gap per unit cross-section is wider still.

The least useful figure on the datasheet

Two webbings with identical rated breaking strength are two different products.

If a nylon and a polyester webbing break at the same force, the nylon will stretch further on the way there, and it takes more work to break it. In plain terms: for the same number on the datasheet, nylon absorbs the shock while polyester passes the impact straight into the structure. Elongation at break: nylon 6,6 18–30%, PET 12–20%. Under working load a polyester sling stretches around 3%, a nylon one 8–10%.

There is a second layer that buyers usually never see at all. That is creep — permanent elongation under sustained constant load. Polyester holds best, nylon is mid-range and collapses wet, polypropylene is worst by a wide margin. With PP the cause is physical: the glass transition sits roughly between −20 and 0 °C, so at room temperature the amorphous regions are already mobile and the chains slip past one another slowly. Order of magnitude: about 1.5% strain per year at 5 MPa and room temperature, 2.5% at 60 °C. The rigging rule of thumb is simple — sustained load no higher than 15–20% of breaking strength.

The clearest illustration is packaging strapping. PP strap stretches about 25% as it is applied and then recovers about 10%. PET elongates roughly six times less, so it holds tension for the whole trip while PP goes slack within a few days. The same creep, measured in the carrier's money.

Wet webbing is a different material

Moisture regain: PP 0.01–0.1%, polyester about 0.4%, nylon 4.2–5%, cotton 8.5%.

Wet nylon keeps 85–90% of its dry strength. But the 10–15% loss is not the main event. Elongation at break climbs from 15–30% to 30–45%, and creep resistance goes from mid-range to poor. The mechanism is plasticization: water works its way in between the amide groups and breaks the hydrogen bonds holding the chains to each other. How serious that is shows up in the thermal data — the glass transition temperatures of dry nylon 6 and of nylon 6 holding 4.9% water are more than 70 °C apart. Once wet, nylon slides out of its rigid state into a soft one. A strap in the rain does not break; it sags. And it dries back to spec, so the loss is reversible.

Cotton does the opposite: swelling cellulose relieves internal stress, and the wet fiber is 20–25% stronger than the dry one. A rare case where the natural fiber beats the technical one on a hard number.

Polyester and polypropylene barely react to water at all.

It is telling that the automotive seat belt standards never name a polymer — they build it out of conditions instead. UN R16 calls for width of no less than 46 mm at a load of 980 daN, breaking force of no less than 1470 daN with no more than 10% spread between specimens, and no less than 75% of the mean breaking force after every conditioning: light, cold at −30 °C, heat at 60 °C and 65% humidity, and three hours' immersion in water at 20 °C with the break carried out within the following ten minutes. FMVSS 209 adds no less than 75% after abrasion and no less than 85% after exposure to micro-organisms. So when someone asks for "webbing like seat belt webbing", the conversation is not about a breaking figure — it is about whether the product holds its width and its shade through a wet cycle.

Sunlight eats the strength, and nothing shows on the surface

The most honest numbers here come from the multi-year WSTDA industry program, which measured real slings rather than fiber specimens. In polyester most of the loss lands in the first twelve months — around 30% — and the curve then flattens. Nylon degrades steadily across all 36 months and ends at 50–60% with no sign of a plateau. That is why slings in permanent sunlight have to be proof-tested at twice the working load at least every six months, and they are retired on the calendar rather than on appearance: two years for one- and two-ply, three years for three- and four-ply.

Unstabilized polypropylene loses noticeable strength after roughly 500 hours of direct sun. UV stabilization is not optional for it, and the stabilizer package is something to make the yarn supplier document rather than take on trust.

Now the interesting part, because almost nobody weighs it when choosing a color. In nylon webbing tested to MIL-DTL-4088 (xenon, 0.83 W/m²·nm at 340 nm, 43 °C, 30% humidity, up to 15 days) navy, black and tan lost 20% of their strength, while white lost 6%. FTIR showed the 1740 cm⁻¹ peak growing: UV is driving hydrolysis. Electron microscopy, meanwhile, saw nothing. The webbing looks new and breaks 20% early. This is a single study and we present it as a single study, but the takeaway is fundamental: the color recipe is part of durability, not part of cosmetics.

The flip side. In solution-dyed yarn the pigment is locked inside the polymer across the full cross-section, and the top outdoor pigments reach 7–8 on the blue wool scale. Cheap solution-dyed polypropylene can outlast expensive dyed nylon in the sun.

The permitted limit sits nowhere near the melting point

Melting: PP 160–170 °C, and softening starts as early as around 150 °C. nylon 6 about 220 °C, nylon 6,6 and PET 255–265 °C. Cotton does not melt — it chars.

The permitted limits look nothing like that. EN 1492-1: polyester and nylon −40 to +100 °C, polypropylene −40 to +80 °C. EN 12195-2 allows polyester up to +120 °C for cargo restraint. US sling practice is tighter — no higher than 90 °C. The gap between melting and permitted is a factor of two, and it is not a safety margin: it is the slow thermal degradation zone, where the material comes apart quietly.

Which gives a practical rule for anything exported: a temperature range quoted without the name of the standard behind it is a meaningless number in an offer.

The strictest example is fire and rescue equipment. NFPA 1983 screens materials on a single parameter: melting point no lower than 204 °C. That throws out polypropylene, and with it high-performance Dyneema at 144–152 °C, even though per unit mass it is several times stronger than steel.

Polyester has a limit of its own that nobody warns you about. Its hydrolysis is governed by the glass transition, and that sits around 70–80 °C: below it the process crawls, above it accelerates sharply. Autoclaves, steam sterilization and hot industrial laundering are real risks. Ask the customer for the process temperature before the specification is signed, not afterwards.

One detail that often decides the sewing process: nylon, PET and PP cut and seal with a hot knife; cotton does not — it needs an overlock or a sealed end.

Chemically, nylon and polyester are mirror images

Nylon takes alkalis, hydrocarbons, oils and fuels — and acids kill it. Polyester is the reverse: it takes acids, oxidizers and bleach, and degrades in hot alkali. Cotton behaves like nylon: alkali is fine (mercerization is built on that), acids destroy it. Polypropylene is the most inert of the four — the full pH range at room temperature, sulfuric acid up to 50%, nitric up to 40% — but it is vulnerable to strong oxidizers and to chlorinated and aromatic hydrocarbons.

What alkali does to polyester is not a scare story, it is a production process: 5% NaOH at 90 °C for one hour gives about 15% mass loss; at 100 °C over the same hour, more than 22%.

The bluntest prohibition is written into OSHA 1910.184: nylon slings are not to be used where acids or phenolics are present; polyester and polypropylene slings are not to be used where caustics are present.

So the question to put to a customer is not the industry, it is the environment. Wash-down bays, concrete, soda ash — not polyester. Battery rooms and acid wash lines — not nylon and not cotton. There is no universal answer to "which fiber is more resistant"; the correct answer is always "resistant to what".

On abrasion we make no promises

General textile comparisons put nylon first, and it wins there not on strength but on elongation and on how much work of rupture it can absorb. Rope and sling references rank it differently: polyester very good, dry nylon very good, polypropylene fair, wet nylon poor. There is industry data on webbing itself in which polyester retained more residual strength than nylon, and the explanation is geometric — stiffer filaments pack more tightly, so the abradant stays on the surface.

A general fiber ranking is not a promise about a specific webbing. We deliberately do not quote Martindale cycle counts from other people's tables: the spread between sources is wider than the difference between the fibers, and passing such numbers to a customer means signing off on somebody else's error. If abrasion is critical, you test that webbing, in that construction, in the state — dry or wet — it will actually work in.

What is known and useful: wet nylon falls to the worst rating on the list. The PPE market has already accounted for it — ANSI/ASSP Z359.11 rates load-bearing webbing not only new (no less than 22.2 kN) but after abrasive conditioning as well (no less than 16.0 kN).

The standards already chose for you — they just never spelled it out

In slings, color is a legal document. EN 1492-1 requires the material to be identified by label color: nylon green, polyester blue, polypropylene brown; the same code is repeated in EN 12195-2. And the color of the webbing itself encodes the working load limit: 1 t violet, 2 t green, 3 t yellow, 4 t gray, 5 t red, 6 t brown, 8 t blue, 10 t and above orange — with a separate clause forbidding those colors on slings of any other capacity. Missing the shade of five-tonne red is not a cosmetic complaint, it is non-conformity with a safety standard. The Americans landed on something similar independently: in WSTDA-WB-1 the identification yarn running down the center of the webbing is blue for polyester and absent for nylon.

The list of permitted fibers can be a closed one. EN 1492-1 admits only nylon, polyester or polypropylene, only high-tenacity multifilament, only with tenacity of no less than 60 cN/tex; monofilament is outside the scope of the standard, all yarns must be of the same material, and the sewing thread must be the same polymer. A single nylon thread in a polyester sling is already a non-conformity. Geometry is regulated as well (width 25–450 mm, tolerance ±10% up to 100 mm and ±8% above), and so are the ratios: 7× breaking strength over working load limit for slings, and for cargo lashing no more than 7% elongation at LC with breaking force of no less than 3×LC.

In fall protection, natural fibers are banned by the text of the law: OSHA requires ropes and straps used in lanyards, lifelines and the load-bearing components of harnesses to be made of synthetic fiber only.

It is worth seeing how much of all this actually governs dyeing. Military specification A-A-55301 forbids any bleaching of the yarn; it requires the match to the shade standard to hold simultaneously under a filtered 7500 K lamp at 100 foot-candles and under a 2300 K incandescent lamp, which is a direct requirement on metamerism; it sets lightfastness at no less than 4 and dry crocking at no less than 3.5 to AATCC; and for Camouflage Green it specifies spectral reflectance from 600 to 860 nm in 20 nm steps, where a miss at four wavelengths rejects the batch. Add pH of the aqueous extract of 5.0–8.5 and lateral bow of no more than a quarter inch per yard. Military webbing is, first and foremost, dyehouse work with a spectrophotometer.

For the EU there is one more requirement that is easy to walk past, because it is about dye chemistry rather than construction: REACH, Annex XVII, entry 43 — azo dyes capable of releasing certain aromatic amines above 30 mg/kg are prohibited in textiles with direct and prolonged skin contact. The listed examples include bags, straps, gloves and footwear. The method is EN ISO 14362-1, and the check is run in the buyer's own lab.

Why width drifts

Three different causes, and only one of them belongs to the production floor.

Thermal shrinkage. Worst in polypropylene: at normal processing temperatures up to 130 °C shrinkage runs 2.5–5%, monofilament in boiling water can give up to 15% in 20 minutes, and depending on production conditions the figure reaches 30%. Conditioning brings it down to 0.5%. That gap is the answer to why camber in PP webbing cannot be dialed out at the machine: it takes a separate heat-setting stage with its own cycle time and its own cost, and the window for it is narrow — 130–145 °C for 10–60 seconds against a melting point of 165 °C. We have watched camber on PP webbing reach 11 cm per meter after production was finished, on stock nobody had touched.

Moisture. Nylon is hygroscopic and swells mostly across the width, so the same webbing measured in a heated shop in winter and in a damp warehouse in summer gives two different results. That is a property of the polymer, not an oversight. If dimensional stability against hardware is critical for the customer, this is a direct argument for polyester.

Warp tension. Webbing is produced under tension, and the tension changes as the beam runs down — hence the drift from the start of a batch to the end of it. That one is ours to answer for, and it is exactly why the standards carry a width tolerance.

All of it in one table

What to send with the inquiry so the batch does not have to be re-run

Load — static or shock, what value and for how long. This is the choice between polyester and nylon, and it immediately rules polypropylene out of long-term static work.

Wet service — constant, occasional or none. Wet nylon stretches twice as far as dry.

Years outdoors. The same answer gives you the replacement interval to print in the product's own documentation.

Peak contact temperature — and separately, whether the product will see an autoclave, steam treatment, hot laundering, lamination, a heat press or sublimation printing. That last one is a question about dye migration under heat, not about strength.

What is next to it: acids, alkalis, solvents, pH. Not the industry — the environment itself.

Whether anything slides across the webbing under load: a buckle, a cam cleat, an edge.

Color: which system the standard is given in (Pantone TCX or a paper system), whether there is a physical sample, what the webbing has to match inside the finished product, which fiber that part is made of, and under which lighting you will be approving the batch.

Target market. The EU requires REACH conformity on azo dyes for anything in skin contact; slings, PPE and rescue equipment carry their own fiber and marking requirements.

Width, thickness, tolerances — and whether dimensional stability against hardware is critical.

There is no single fiber that does everything, because the properties conflict at the level of physics. The same stretch that makes nylon the best choice for shock makes it the worst for dimensional stability. When both extremes are needed at once, that is solved in the design of the product, not by hunting for a magic fiber.

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Color has its own mechanics and its own reasons for disagreement, covered separately: Why webbing color can miss the shade standard.