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Polyester vs Nylon Dry Bag Shells

What the two fibres actually are at the polymer level

Polyester in this context is polyethylene terephthalate, usually written PET, extruded as filament yarn and woven into a plain or ripstop construction. Nylon is a polyamide, supplied in outdoor fabrics as PA 6 or PA 6.6, and the difference between those two grades matters more than most buyers realise: PA 6.6 melts near 260 degrees Celsius while PA 6 softens around 215 to 220 degrees. Both are thermoplastics, which is why both can be welded and heat-set, and both are hydrophobic compared with cotton but not compared with each other.

The chemical difference that drives almost everything else is the amide group. In a polyamide chain the amide groups attract water molecules and hold them, which is why nylon takes up moisture from the air and why it swells very slightly when wet. A polyester chain has ester groups instead, which interact far less with water. That single substitution explains the regain gap, the drying gap, the stretch gap and part of the dyeing difference between the two families.

Neither fibre is waterproof on its own. Waterproofing comes from a TPU or PVC film bonded to the back of the weave, or from a coating applied to it, and the shell is there to protect that film from puncture and abrasion. Choosing a face fabric is therefore a mechanical decision with a chemical edge, not a waterproofing decision. Our lamination and coating comparison covers the film side of the same sandwich.

Tenacity, tear and abrasion: the numbers that decide a shell

Nylon is the stronger fibre on paper. Filament PA 6.6 runs at roughly 6.0 to 9.0 grams per denier of tenacity against 4.5 to 7.0 for PET, and in a coated 500D construction that translates to a tongue tear of about 70 to 100 newtons for nylon against 45 to 60 newtons for polyester. Elongation at break follows the same direction: 20 to 40 percent for nylon against 15 to 25 percent for polyester, which is why a nylon bag feels slightly more forgiving when it is stuffed past its comfort limit.

Abrasion is where the gap shows in the field rather than in the lab. On a Martindale tester with a standard wool abradant, a 500D polyester face with a polyurethane coating typically reaches an end point near 30,000 to 40,000 cycles, while an equivalent 500D nylon reaches 50,000 to 70,000. That is the difference between a bag whose base corners go matte and grey after two seasons on a beach and one that still looks new after four. If your bag is dragged rather than carried, the fibre choice is worth more than an extra 100 g of fabric weight.

Water uptake, drying time and the wet-bag problem

Moisture regain is measured at 20 degrees Celsius and 65 percent relative humidity, and the gap between the two fibres is roughly tenfold: 0.4 percent for polyester against 4.0 to 4.5 percent for nylon 6.6. In a 500D shell weighing 180 grams per square metre, a 15L bag with about 0.35 square metres of fabric carries under a gram of absorbed water in polyester and around 15 grams in nylon. That sounds trivial, and on the scales it is, but the consequences show up in drying time and dimensional stability rather than weight.

A nylon shell that has soaked in a capsize takes noticeably longer to dry because water is held inside the fibre as well as between the yarns, and a bag stored damp because of that is the one that grows mildew. Nylon also loses 10 to 15 percent of its tensile strength when fully saturated and recovers when it dries, while polyester loses under 5 percent and barely changes dimension at all. For a bag that is emptied, rinsed and hung up the same day, none of this matters; for a bag thrown in a boot for a week, it is the whole story.

UV behaviour and colour fastness under sun and salt

Polyester is the more UV-resistant fibre, and the margin is large enough to plan around. After 300 hours of xenon-arc exposure to a standard grey-scale end point, a coated 500D polyester typically retains 85 to 90 percent of its tensile strength, while a comparable nylon retains 60 to 70 percent. Outdoors that maps loosely to a summer of daily deck exposure: the polyester shell is tired but serviceable, and the nylon shell has started to chalk at the folds. UV absorbers and a pigmented finish narrow the gap, but they do not close it.

Colour behaves differently too, because the two fibres take dye through different routes. Polyester is dyed with disperse dyes that diffuse into the fibre at high temperature, which gives excellent wash and light fastness and makes sublimation printing possible directly onto the face. Nylon is dyed with acid dyes, which give brighter shades but generally lower light fastness, and printed nylon usually needs a pigment or transfer route instead. If your range depends on a printed graphic, that is a process difference as much as an aesthetic one; our colour fastness notes cover saltwater behaviour in more detail.

How each face bonds to TPU and PVC film

Adhesion between the woven face and the waterproof film is a surface-energy problem, and both fibres need preparation. Polyester yarn is usually spun with a finish that must be scoured off before lamination, and it bonds well to both TPU and PVC with a standard adhesive tie layer. Nylon has a slightly higher surface energy and bonds readily to TPU, but it holds absorbed moisture that has to be driven off before the laminating nip, otherwise the bond line can produce bubbles during welding later.

The melting point difference then sets the weld window. A TPU film welds under radio-frequency energy in a range where the film flows but the face should not, roughly 150 to 200 degrees Celsius at the interface. A polyester face with a 255 degree melting point sits comfortably above that window; a PA 6 face softening near 215 degrees is much closer to it, and an over-heated weld on a nylon 6 face can glaze the yarn and leave a shiny, brittle seam. This is the reason many production partners specify polyester faces for high-volume RF welding and reserve nylon for grades and processes where the window is tightly controlled.

Weight, hand feel and what the numbers mean in grams

Fibre density runs the other way from strength: PET is about 1.38 grams per cubic centimetre and PA 6.6 about 1.14, so a nylon yarn of the same denier is physically thicker and a finished nylon fabric of the same construction is often slightly heavier per square metre. In practice the difference is small, 5 to 10 percent, and it is usually swamped by the choice of coating weight and film thickness. A 500D polyester with a 0.5 mm TPU film and a 500D nylon with the same film differ by 20 to 40 grams on a 15L body.

Hand feel is where a buyer notices the difference before reading any number. Nylon is softer and drapes better, which makes a bag easier to roll and to stuff into a tight compartment. Polyester is crisper and holds a fold more sharply, which is good for a closure that must lie flat and less pleasant when you are squeezing a bag through a hatch. Once a film is laminated on, both stiffen, and the difference narrows to something most people would struggle to identify blind.

Chemical resistance, hydrolysis and what damages each fibre

Both fibres survive the chemicals a dry bag meets in normal service: salt water, sunscreen, insect repellent, diesel fumes and sunscreen oils. The failure modes are at the extremes. Polyester is vulnerable to hydrolysis in hot alkaline conditions, which is why a polyester bag should not be left soaking in a strong alkaline cleaner, and it is attacked by some solvents used in screen-print inks. Nylon is vulnerable to strong acids, which yellow and weaken the fibre, and it is more readily stained by tannin and by some dyestuffs that migrate from other gear.

Temperature is the other chemical-adjacent limit. A polyester face tolerates a hotter wash and a hotter drying cabinet; a nylon 6 face can glaze or shrink if it meets a heat source above about 150 degrees Celsius for any length of time, which is well within reach of a car dashboard in summer sun or a drying room radiator. If your bags will be dried in a heated cabinet as part of a rental operation, specify the fibre and the maximum cabinet temperature together on the same line of the brief.

Denier, weave and ripstop: where the fibre matters less

Denier and weave construction move the numbers further than the polymer does once you are above roughly 300D. A 500D polyester outperforms a 210D nylon on abrasion despite being the weaker fibre per denier, and a ripstop grid adds tear resistance to either fibre by stopping a propagating tear at the reinforcement yarn. If you are choosing between a 420D polyester and a 420D nylon at the same construction, the fibre decides; if you are choosing between a 210D nylon and a 500D polyester, the construction decides first.

Ripstop is worth a specific note because it is often sold as a nylon feature when it is really a weave feature. A ripstop grid can be woven in polyester or nylon, in 20 denier to 200 denier reinforcement yarns, and it raises tear strength by 30 to 60 percent in both directions for a weight penalty of roughly 5 percent. Our ripstop construction notes and the 420D against 500D comparison take that side of the decision further.

Cost, availability and recycled content

Polyester is the cheaper fibre per kilogram and the more widely available one, largely because PET bottle flake feeds a mature recycling stream. Recycled PET, usually written rPET and certified under a chain-of-custody scheme, is available at 300D to 600D in volume with a lead time close to virgin yarn, and it costs 10 to 25 percent more depending on the certification and the denier. For a range with a recycled-content claim, polyester is the lower-risk route by a wide margin.

Recycled nylon exists and performs identically when it comes from post-industrial waste, but the post-consumer stream is smaller, priced higher and often tied to specific collection programmes such as recovered fishing nets. Expect a 30 to 60 percent premium over virgin PA 6.6 and a longer lead time, and verify the chain of custody documentation rather than the label. Our rPET fabric guide sets out what to ask for and what the certificates actually prove.

Where polyester is plainly the better shell

• Bags that live on a deck or a roof rack all summer: UV retention is the deciding factor.
• Rental and club fleets that are rinsed and dried every day: fast drying beats abrasion life.
• Printed and sublimated graphics: disperse dyes and sublimation work directly on polyester.
• High-volume RF welded programmes: the wider heat window reduces glazed and brittle seams.
• Cooler and food-carrying bags: low regain means less odour retention between uses.
• Any bag that must hold a sharp roll-top fold: polyester holds the crease nylon relaxes out of.

Where nylon is plainly the better shell

• Expedition and raft bags dragged across rock: abrasion and tear lead every other property.
• Ultralight liners and stuff sacks where grams matter more than UV life.
• Compression bags that must be squeezed into small spaces: nylon drapes and recovers.
• Winter and cold-weather use: nylon stays more flexible below minus 20 degrees Celsius.
• Bags with welded handles and heavy lash points: higher tear strength carries point loads.
• Programmes where a softer hand feel is part of the product positioning.

Specifying a shell: what to put on the brief

Six lines decide the outcome, and they should be written as numbers rather than adjectives. Name the fibre and grade, not just nylon or polyester. State the denier and the weave, including whether a ripstop grid is required. Give the fabric weight in grams per square metre with a tolerance, typically plus or minus 5 percent. Name the film, its thickness and the bond method. State the UV and abrasion test methods you want results against, and give the maximum drying temperature the finished bag has to survive.

Test methods matter as much as the figures. Abrasion and tear resistance are normally quoted against ASTM methods and colour fastness and water penetration against ISO methods, so a spec sheet that says abrasion resistant without a method number is not a spec sheet. Ask for the result, the method and the number of cycles, and hold a retention sample from the approved lot so a later shipment can be compared against it rather than against memory.

Our own waterproof line has run since 2014 in an SGS-verified 4,950 m2 facility with seven production lines, and both faces are welded on the same equipment, so the choice is made on the application rather than on what the line can handle.Whichever face you pick, the waterproofing still comes from the film and the weld, which is why the same decision appears in our TPU against PVC comparison and across the TPU waterproof backpack range. Compare the finished specification against the full product line before you commit to a construction.