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TPU vs PVC for Dry Bag Fabric

What TPU and PVC actually are at the film level

TPU is a thermoplastic polyurethane built from alternating hard and soft segments. The hard segments supply tensile strength and abrasion resistance; the soft segments supply stretch. It arrives as a solid extruded or calendered film that is laminated to a woven face fabric under heat and pressure, so the barrier and the textile remain two distinct layers. There is no liquid plasticiser in the film, and that single fact explains most of what follows.

PVC bag fabric is a polyvinyl chloride resin blended with plasticiser, heat stabiliser and filler, then coated or calendered onto a woven base. The plasticiser is what turns a rigid polymer into a flexible sheet, and it is not chemically bonded to the chain: it sits between the molecules and slowly leaves. A typical bag coating carries 25 to 35 percent plasticiser by weight, and that fraction is why a PVC bag feels supple on day one and board-like after four summers.

Weight and packability per square metre

The weight gap sounds small until you multiply it. A 20L bag uses roughly 0.46 m² of shell, so a 150 g/m² difference becomes about 70 g on the finished bag, which is 22 to 28 percent of its empty weight. On a 90L expedition bag using 1.6 m² the same arithmetic removes 240 g, and on a backpack harness the saving shows up directly in shoulder load over a six-hour carry.

Packability matters more than absolute weight for most people. TPU has less mechanical memory, so a 10L bag folds to roughly a 12 cm by 8 cm bundle at 20 °C where the PVC equivalent takes 15 cm by 10 cm, and the gap widens as the temperature drops. A bag that stuffs into a kayak hatch or a jacket pocket gets used; one that fights you gets left in the car, and that is the real failure mode.

Cold flexibility and the cracking threshold

Flexibility at low temperature is measured by bending a strip around a mandrel at a set temperature and looking for cracks. A well-formulated 0.5 mm TPU passes at about -30 °C; a general-purpose PVC coating typically passes at -10 °C and cracks below that. For ice fishing, snowmobiling, winter paddling or any trip that starts before dawn in a cold climate, this one number decides the fabric.

The risk is not only in the field. A PVC bag shipped and stored in an unheated container at -20 °C can arrive with hairline cracks along the fold line before a user ever touches it, and those cracks do not heal. TPU also stiffens when cold, but it stiffens reversibly: twenty minutes at 15 °C and it folds again. Storage and freight temperature is therefore part of the material specification, not an afterthought.

Abrasion, puncture and tear in numbers

Laboratory abrasion tests translate well to field behaviour. On a Taber abrader with CS-17 wheels and a 1 kg load, 0.5 mm TPU loses about 5 mg per 1,000 cycles while PVC loses about 21 mg, and 0.3 mm TPU still loses only 8 mg. Puncture resistance with a 2 mm probe follows the same order: 320 N for 0.5 mm TPU, 210 N for 0.3 mm TPU and 145 N for 0.5 mm PVC. In practice the TPU shell survives roughly three times the dragging distance over wet sand.

Test methods matter when you compare two spec sheets. Coated-fabric tensile and tear behaviour is defined in the ASTM standards for coated fabrics, and a sheet that names the method is worth more than one that names only a number. Tear propagation is the more useful figure for a dry bag, because almost every real failure starts as a small nick that then runs, rather than as a clean puncture.

How each film joins: RF, hot wedge and solvent

Polyvinyl chloride is strongly polar, which makes it almost ideal for radio-frequency welding: the molecules heat under an alternating field and fuse in two to four seconds. TPU also welds under RF but needs tighter control of power, pressure and dwell, and the welding window is narrower. Both films accept hot-wedge and hot-air welding, which is what most bag lines use for long straight seams.

Solvent welding is the exception, and it belongs to PVC. Tetrahydrofuran or methyl ethyl ketone softens a PVC surface so two pieces fuse into one, producing a joint that can exceed the strength of the surrounding film. TPU resists those solvents, so its repairs rely on heat or on polyurethane adhesives. The practical consequence: an aged PVC bag with depleted plasticiser welds and glues badly, because its surface chemistry has already changed.

Plasticiser migration and why PVC stiffens with age

Plasticiser loss is a diffusion process, and its rate roughly doubles for every 10 °C rise in temperature. A bag stored in a garage that reaches 35 °C loses plasticiser several times faster than the same bag in a 15 °C hallway, which is why users in warm climates report stiff bags after only two seasons while users in temperate climates get four. Contact with sunscreen, insect repellent and body oil accelerates it further.

The first visible sign is bloom: a slightly tacky surface film that attracts dust and feels faintly greasy. After that comes shrinkage of one to two percent, which puts the welded seams under permanent tension, and then fold-line cracking. TPU has no plasticiser to lose, but it has its own ageing path: polyester-based grades can hydrolyse in hot, humid storage, which is why marine-grade bags specify polyether TPU for long service in the tropics.

UV, ozone and salt: outdoor ageing behaviour

Neither polymer survives strong sunlight without help. Both are formulated with hindered amine light stabilisers and UV absorbers, and the difference shows up in retention rather than in whether damage occurs at all. After 3,000 hours of accelerated UV exposure, a stabilised TPU retains about 88 percent of its tensile strength against 52 percent for a stabilised PVC, and the colour shift is 2.1 Delta E against 6.8, which is the gap between barely noticeable and clearly faded.

Saltwater itself does not attack either polymer; sodium chloride is chemically inert to both. What salt does is mechanical: crystals left in the fold act as an abrasive paste, and chloride drives corrosion in buckles, D-rings and rivets. The most common sun failure is not in the panel at all but along the thin flash at the edge of a weld, because that is where the material is thinnest and most stretched.

Hydrostatic head and what the number hides

Both films clear a hydrostatic head of 10,000 mm comfortably, and a good laminate reaches 20,000 mm or more. The trouble is that the test described in the ISO fabric penetration standards measures a flat clamped sample under a rising water column. It says nothing about the seam, the closure, or the fold, which is where every real leak starts. A 20,000 mm bag with a badly rolled top still fills with water.

Where the polymer does matter is creep. Under a sustained load a film slowly deforms, and the two materials behave differently afterwards. TPU has elastic recovery: a fold compressed for twelve hours inside a full hatch springs back when the bag is opened. PVC deforms plastically and takes a set, so the crease stays and the next closure has to work against a permanent kink in the fabric.

Odour, skin contact and restricted substances

New-bag odour comes from volatiles leaving the film. TPU emits a faint residual monomer smell that most people stop noticing within a day or two. PVC emits plasticiser and stabiliser volatiles that are obvious for two to six weeks, and noticeable again every time the bag has been sealed in a hot car. For a bag that lives inside a tent, a campervan or a small cabin, this is a genuine comfort difference.

Compliance is the heavier issue. Several orthophthalate plasticisers are restricted in the European Union and in children’s products in the United States, and California requires warnings for listed substances. Guidance on phthalates is published by the EPA. A well-managed PVC formulation can comply with all of it, but the dossier is larger and the risk of a reformulation sitting with the brand owner is real.

Field repairability of each film

A punctured TPU bag can be repaired with a pre-cut TPU patch and heat: 140 °C through a silicone release sheet for about twenty seconds produces a bond of roughly 60 percent of the original strength, and a polyurethane adhesive works for larger damage. Carry patches cut to a 6 cm disc so a 5 mm hole still gets a 25 mm margin all round, and keep the repair away from the fold line where it will flex.

PVC repairs rely on solvent cement or a vinyl patch with vinyl adhesive, and both work well while the plasticiser is still in the film. On a bag that has already stiffened, the surface keeps changing under the patch and the bond fails within weeks. One rule applies to both materials: never use cyanoacrylate. It cures brittle, cracks at the first fold, and leaves a hard edge that abrades everything packed next to it.

Cost per litre and cost per season

On unit cost, PVC wins clearly. The film costs roughly 30 to 40 percent less per square metre, and a 20L bag built in PVC at the same thickness lands at about 68 percent of the TPU price. Across a 500-piece first order that difference is a meaningful line in a budget, and for a bag that will be filled once and left in the bottom of a boat, it is money well saved.

Total cost inverts the comparison once folding enters the picture. Using observed service lives of eight to ten years for TPU and three to five years for PVC in weekend recreational use, the cost per season of a TPU bag is roughly half that of the PVC bag at any given size. The break-even point is about three years of use, which is why rental fleets and clubs almost always specify TPU despite the higher ticket.

Choosing by use case: where each film wins

Start with temperature, then with how often the bag will be folded. If the bag is used below freezing, or folded and squeezed on every trip, TPU is the only sensible answer and the premium is small against the service life. If the bag is filled once a season and sits still, PVC gives you stiffness, print quality and a lower unit cost with no meaningful penalty.

The second filter is compliance and market. Retail channels in the European Union and school or youth programmes in the United States ask for phthalate declarations, REACH statements and sometimes OEKO-TEX documentation on the fabric. Those are far easier to assemble for a phthalate-free TPU laminate than for a plasticised vinyl, and the paperwork cost is real money on a small range.

Our own line has produced waterproof bags since 2014 in an SGS-verified 4,950 m2 facility, running seven lines and 149 machines with a monthly output of 200,000 pieces, and the film choice is the first question in every brief we receive. Sampling takes 6 to 10 working days and bulk runs 35 to 50 days from FOB Xiamen, with T/T 30/70 terms and AQL 2.5 inspection.

Compare the two films against the rest of our waterproof product line and the TPU waterproof backpack range, then read the deeper cuts: 0.3 mm versus 0.5 mm film, phthalate-free TPU, PVC-free alternatives, abrasion data for TPU and 420D against 500D face fabric.