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Is TPU Better Than PVC for Dry Bags?

The short answer, with the exception stated

If the bag will be folded, squeezed, carried in the cold or kept for more than three seasons, buy TPU. If it is a single-season item, a giveaway, a cooler liner that lives in the shade, or a bag that will be replaced rather than repaired, PVC is a rational choice and the money saved is real. The mistake is buying PVC for a bag that will be used hard, because it does not fail gracefully; it stiffens, then it cracks at the fold, and by then the seam is usually going too.

Where the answer flips is a bag that never flexes and never sees cold. A rigid-sided cooler liner, a bucket bag stored indoors, a promotional tote used at a pool for one summer: on those, PVC gives you the same waterproof performance for roughly two thirds of the money. The two properties that TPU buys you, low-temperature flexibility and plasticiser-free ageing, are worth nothing if the bag never gets cold and is not expected to last.

What the two films actually are

Polyvinyl chloride is a rigid polymer made flexible by adding plasticiser, typically 25% to 40% by weight, and it is that additive package rather than the PVC itself that governs how the material behaves. Thermoplastic polyurethane is a block copolymer that is elastomeric as polymerised: hard segments give it strength, soft segments give it stretch, and no additive is required to make it flexible. That single difference explains almost every property gap that follows.

In a dry bag the polymer is only half the construction. Both films are normally bonded to a woven face fabric, nylon or polyester at 420D or 500D, which carries the tensile load and takes the abrasion while the film does the waterproofing. Build quality varies hugely at this interface: a laminated film is fused into the weave under heat and pressure, while a coated film sits on the surface and can delaminate in a hot car. The lamination versus coating note covers how to tell them apart on a sample.

Thickness matters as much as chemistry. A 0.3 mm film is fine for a 5L grab bag and marginal for anything dragged across a concreted slipway; 0.5 mm is the working default for a bag that gets used weekly. Film choice interacts with polymer choice, because a 0.5 mm PVC coating is heavier and stiffer than a 0.5 mm TPU film for the same puncture resistance, which is why the weight gap in the next section is larger than the thickness numbers alone suggest.

Weight: grams per square metre and grams per bag

Translated into something you can feel: a 20L dry bag in 0.5 mm TPU weighs about 480 g empty, and the same bag in 0.5 mm PVC weighs about 640 g. That is 160 g of nothing, carried on every trip, for the same volume and roughly the same puncture resistance. On a kayak deck or a paddleboard, where a bag is carried in the hand rather than on the back, 160 g is small; on a 12 km portage with three bags it is half a kilogram.

The weight gap also compounds with the closure. A stiffer film needs a wider, heavier roll-top mouth to fold cleanly, and PVC bags often carry a heavier stiffener bar and a chunkier buckle to make the fold hold. So the real difference between two finished bags of the same nominal size is usually 180 to 240 g rather than the 160 g the fabric table suggests. Our film thickness guide gives the full weight-versus-durability curve.

Cold behaviour: the axis PVC loses outright

This is the single clearest reason to pay for TPU, and it is also the reason the two materials are not interchangeable in a winter range. A PVC bag that seals perfectly at 20 °C can refuse to seal at minus 8 °C, because the film will not take the tight radius of a three-fold roll. The failure looks like user error and is not: the closure is geometrically incapable of closing on itself when the film will not bend.

Cold crack is measured by bending a conditioned specimen around a mandrel at falling temperatures and recording the temperature at which the film cracks. Polyether-grade TPU typically passes below minus 40 °C, which is past the point where anyone is folding a bag; flexible PVC loses plasticiser mobility and cracks somewhere near minus 15 to minus 20 °C depending on the additive package. Test method detail and grade selection are set out in the cold crack guide.

Abrasion, puncture and tear

Those figures come from the abrasive-wheel and puncture methods maintained by ASTM, and they are worth quoting in a specification because they are comparable between brands in a way that marketing adjectives are not. Abrasion is the failure mode that kills most bags, and TPU wins it by a wide margin because the elastomer recovers after each contact where a plasticised coating wears away. On a bag dragged across a slipway or a boat deck, that shows up as a difference of years rather than months. Puncture resistance is closer than people expect, and it is dominated by thickness rather than by polymer: a 0.6 mm PVC coating can out-puncture a 0.3 mm TPU film.

Tear propagation is the one to think about for field repair. TPU resists the growth of an existing nick, so a 10 mm cut made by a fishing hook tends to stay a 10 mm cut through a season of handling. PVC propagates more readily, which is why a small puncture in a PVC bag often becomes a long split by the end of a trip. That behaviour, not the initial puncture force, is what decides whether a bag is repairable or finished.

Welding and what happens at the seam

Both polymers can be joined by radio-frequency welding, because both have polar molecular structures that heat under a high-frequency field. In practice TPU welds more forgivingly: the process window between an under-weld and an over-weld is roughly 15% to 20% wider, which means a production line holds consistent seam strength across a shift. PVC welds well when it is new and progressively worse as the plasticiser content drifts with age and batch.

Seam strength is where the difference becomes structural. A well-made TPU weld is stronger than the film beside it, so a pull test breaks the base material at 380 to 460 N per 50 mm rather than peeling the join. A bonded or solvent-welded PVC seam usually fails at the join at 220 to 300 N, and the failure mode is peel rather than tear, which is the dangerous one because it propagates along the seam. See RF welding explained for the process detail.

UV, salt and five years on a deck

Accelerated UV figures need interpreting carefully: 500 hours in a test chamber is not a fixed number of months in the sun, because intensity and spectrum differ from a real deck. What the table shows reliably is the direction and the relative margin. TPU retains its mechanical properties; PVC loses elongation first, and loss of elongation is what turns a flexible bag into a brittle one that splits at the fold line.

Salt is gentler on the polymer than most people assume and harsher on the hardware. Neither film is damaged by salt water itself; the damage comes from salt crystals left in the fold line, which abrade the film every time the bag is rolled. Rinse with fresh water, dry in shade, and store loosely rolled, and both materials will outlast their hardware. Hardware corrosion is the separate problem covered in corrosion resistance in dry bag hardware.

Plasticisers, odour and skin contact

The plasticiser question is the one that shows up in retail vendor forms and it is specific to PVC. Flexible PVC relies on additives to stay soft, and some phthalate plasticisers are restricted in toys, food contact and childcare articles in the European Union and in several US states. Modern formulations use alternative plasticisers, but the compliance burden sits with the brand to document the actual formulation rather than the polymer name.

TPU contains no added plasticiser, which removes the migration mechanism entirely and is why phthalate-free TPU is the default specification for food-contact and child-facing products. Odour follows the same chemistry: a new PVC bag smells of its additive package for two to six weeks and the smell returns on a hot day, while a new TPU bag is close to odourless. Guidance on plasticisers and restricted substances published by the EPA is a useful starting point for US programmes.

Migration testing is the practical way to settle the question for a specific bag. Overall migration into a food simulant, specific migration for regulated substances, and a sensory check for odour are three separate tests, and a certificate that only covers one of them does not answer the other two. The phthalate-free TPU note and the migration testing guide explain what each report does and does not prove.

Ageing in storage: hydrolysis and plasticiser loss

Two slow mechanisms shorten the shelf life of a stored bag. Polyester-based TPU can hydrolyse in warm, humid storage, because moisture attacks the ester linkages and the film gradually softens and loses strength; polyether grades resist this and are the right choice for tropical markets. PVC does not hydrolyse, but it loses plasticiser steadily, and the loss accelerates above about 40 °C, which is a normal temperature inside a car or a container in summer.

The practical rule for a bag that is stored rather than used is the same for both: cool, dry, dark, and not folded tightly. A bag left rolled tightly for two years in a hot garage develops a permanent crease that becomes the failure line in year three, regardless of polymer. Store loosely rolled with the mouth open and the buckle undone, and both materials will sit for years without measurable loss of performance.

Cost per bag and cost per year

On a 20L roll-top, 0.5 mm TPU typically lands at USD 16 to 24 per unit at a 500-piece order and the PVC equivalent at USD 9 to 13, a gap of roughly 40%. Divide by service life and the gap closes: TPU at USD 20 over eight years costs about USD 2.50 per year of use, and PVC at USD 11 over four years costs about USD 2.75 per year, before counting the cost of the two replacements and the trips they interrupt.

That arithmetic favours TPU more strongly as use intensifies. A rental fleet washing and drying bags weekly sees PVC stiffen inside two seasons, so the per-year gap widens to something like USD 2.20 against USD 3.60. For a single-season promotional handout the calculation genuinely favours PVC, and this is the one case where the cheaper film is also the correct engineering answer. Grade-by-grade life figures are in the service life comparison.

Repairability in the field

Both films can be patched, and the difference is in how well the patch holds. A TPU patch bonded to a TPU film with heat and pressure becomes effectively one piece of material; pull tests on a 60 mm patch typically reach 70% to 85% of the base film strength after a few minutes of cooling. A PVC patch relies on a solvent or an adhesive, and the bond strength depends heavily on how much plasticiser has already left the surface.

That means a PVC bag becomes harder to repair as it ages, precisely when it needs repairing most. A three-year-old PVC bag has a plasticiser-rich surface that resists adhesion, and the field fix is to wipe with solvent, roughen lightly and use a flexible contact adhesive rather than a heat patch. TPU repairs are simpler: clean, dry, apply a TPU patch with heat for about 30 seconds, and air test the result by squeezing the closed bag.

End of life and what happens to the material

Recyclability favours TPU, with caveats. Clean TPU offcuts and mono-material TPU bags can be reground and reprocessed, and the material holds enough value that a take-back stream is commercially plausible. PVC is harder: the chlorine content and the additive package mean it needs a dedicated stream, and mixed PVC textilosan is generally not accepted in general recycling.

The honest answer for both is that a laminated bag is a composite, and composites are hard to separate. A TPU film bonded to a polyester face cannot be split into clean streams at scale today, so the realistic end-of-life options are reuse, repair and second life rather than closed-loop recycling. Design choices that help are mono-polymer construction, welded rather than stitched assembly, and hardware that unclips. Our recyclability note and the PVC-free alternatives guide go through the options in order of practicality.

Which one you should actually buy

• Sea kayaking, sailing or paddling in cold water: TPU, 0.5 mm, polyether grade.
• Winter sports, ice fishing, high-altitude use: TPU, nothing else is reliable below minus 15 °C.
• Rental fleet or a club store used weekly: TPU, because the per-year cost is lower.
• Pool bag, beach tote, one summer of light use: PVC is fine and costs 40% less.
• Promotional handout with a fixed budget: PVC, unless the brand story needs a PVC-free claim.
• Food contact, childcare or a regulated retail channel: TPU, phthalate-free, with migration reports.
• Any bag you expect to repair rather than replace: TPU, because patches bond properly.

If you take one rule from this guide, make it the temperature rule. Below roughly 5 °C, TPU is not a preference, it is a requirement, because a closure that will not fold is not waterproof at any price. Above that and with light use, PVC remains a legitimate way to buy waterproof performance cheaply, and there is nothing wrong with choosing it deliberately for the right job.

Where you are unsure, ask for the numbers rather than the polymer name: film thickness in millimetres, face fabric in denier, weight in grams per square metre, weld strength in newtons per 50 mm, and a rated service life with the UV hours behind it. A production partner who can quote all five is worth more than a cheaper bag with a printed adjective. Our welded line has built TPU waterproof bags since 2014 in an SGS-verified 4,950 m2 facility, the formats are listed under products, and the fabric comparison gives the same arguments at specification level.