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Internal Coating vs Lamination

Two routes to a waterproof shell

Every waterproof fabric is a sandwich: a structural face fabric that takes the abrasion and a polymer layer that stops water. The question is how the polymer got there. In internal coating, the polymer arrives as a liquid or paste, is spread onto the back of the shell at a controlled thickness, and is then cured in an oven so the solvents or water leave and the film sets. The polymer partly penetrates the weave, which is why coated fabric feels like one material rather than two.

In lamination, the polymer is made first, as a finished film with its own thickness and properties, and is then bonded to the shell with adhesive and heat, or by flame, or by hot-melt extrusion. The film keeps its identity: it sits on the surface of the weave as a distinct layer with a defined thickness. That is what gives lamination its advantage, because a film can be engineered independently of the fabric underneath it.

The practical consequence is that coating thickness is limited by how much liquid you can spread in one pass, typically 0.08 to 0.25 mm before the fabric goes boardy, while a laminated film can be 0.30 to 0.60 mm without ruining the hand. Thicker polymer means more puncture resistance and more weld material at the seam, which is why serious waterproof bags are laminated almost without exception. Our own welded range is built on laminated TPU, as used across the TPU waterproof backpack line.

How an internal coating is applied

Three methods dominate. Knife-over-roll coating drags the fabric under a fixed blade that meters the polymer to a set gap, giving repeatable thickness within about 0.02 mm. Rotary screen coating pushes polymer through a mesh, which suits patterned or breathable coatings. Dip coating saturates the fabric and is mostly used for cheap groundsheet-grade material, because it adds weight and blocks the weave completely.

Two or three passes are normal for a bag fabric: a thin tie coat that grips the fibres, then a thicker top coat that does the waterproofing. Each pass is cured separately, which is why coating lines run at 8 to 20 metres per minute rather than the 30 to 60 m/min of a simple finishing line. Energy and solvent handling dominate the cost, and the curing ovens are the reason coated fabric has a slightly higher embedded energy per metre than people assume.

The weakness is uniformity. A knife blade leaves streaks if the polymer viscosity drifts with temperature, and thin spots become the first place water finds a path. On a 0.10 mm coating, a 0.02 mm thin spot is a 20% reduction in barrier thickness at that point, and it is invisible from the outside. This is why coated bags often carry a higher hydrostatic head rating than their real-world performance suggests.

How lamination bonds film to fabric

Lamination starts with a finished film, usually extruded TPU, that already has its thickness, hardness and additive package fixed. The film and the shell are brought together with an adhesive layer, or with a hot-melt that becomes the adhesive, under heat and nip pressure in the range of 2 to 6 bar. The bond line is thin, often 0.02 to 0.05 mm, but it carries the entire peel load of the finished bag.

The critical variable is surface preparation. Shell fabric arrives with spinning oils and sizing that must be removed or the adhesive bonds to the contamination rather than the fibre. A corona or plasma treatment immediately before the nip raises surface energy and typically improves peel strength by 30 to 50%. Skipping that step is the most common cause of delamination returns, and it is invisible in a finished bag until the first hot car boot or the first cold morning.

Because the film is made separately, its properties can be tuned independently: a softer grade for a roll-top mouth that must fold 3,000 times, a harder grade for a base that drags over gravel. A coated fabric cannot do this, since the coating and the weave are cured together and share one chemical system. That independence is the real argument for lamination, more than the headline strength figures.

Weight, stiffness and the hand of the fabric

On paper the weight gap is modest: a coated 420D nylon at 320 g/m2 against a laminated equivalent at 380 g/m2, a difference of 60 g/m2. On a 20L bag with about 0.55 m2 of fabric that is 33 g, which nobody notices. What people do notice is stiffness. Coating penetrates the weave and locks the filaments together, so a coated fabric resists folding; laminated fabric folds along the film, which is more flexible at the same weight.

That stiffness shows up at the roll-top mouth. A coated bag needs a higher closing force, which encourages users to under-roll it, and under-rolling is a leading cause of leakage on the water. A 0.30 mm laminated TPU mouth folds with two fingers at 5 °C, where a 0.25 mm PVC-coated mouth at the same temperature needs a firm two-handed push. In cold conditions the gap widens further.

Stiffness also affects packing efficiency. A soft laminated bag conforms around irregular loads and recovers 5 to 10% more usable volume in a hull or pannier than a stiff coated bag of identical nominal size. If you have ever struggled to close a bag that measured large enough on paper, stiffness was probably the reason. See how this plays out in our comparison of fabric flexibility against durability.

Peel strength: the number that predicts seam life

Peel adhesion is measured by pulling the polymer layer away from the shell at a fixed angle and recording the force. Coated fabrics typically record 8 to 12 N per 50 mm because the polymer is mechanically locked into the weave rather than chemically bonded to a surface. Laminated TPU records 25 to 45 N because the adhesive forms a continuous bond over the whole contact area.

Why it matters: a welded seam is loaded in peel, not in shear. When a 12 kg load is dropped onto a corner, or a bag is dragged across a beach, the seam tries to open by peeling the layers apart. A seam with 10 N of peel resistance will start to lift; the same seam at 35 N will not. Most warranty claims on waterproof bags are peel failures at a seam corner, not punctures through the middle of a panel.

The test method matters as much as the number. ISO 2411 defines the standard procedure for coated and laminated fabrics, and ASTM D751 covers the equivalent for coated textiles in the United States. A specification that quotes a number without naming the method is not comparable, which is exactly the sort of claim our guide to waterproofing claims tells you to challenge.

Why coatings weld badly and films weld well

Radio-frequency welding works by making polar molecules in the polymer oscillate until they heat and fuse. A laminated TPU film is a thick, uniform, polar layer, so it heats evenly and melts across the full joint. An internal coating is thin, uneven, and diluted by the weave it has soaked into, so there is simply less polymer at the joint, and the heat concentrates in the fibres rather than the coating.

The result is a seam that looks fine and performs inconsistently. Welded coated fabric often reaches 60 to 70% of the parent material strength, while welded laminated TPU reaches 85 to 95%, and the spread between individual joints is far tighter on laminated material. Tight spread is what a production line needs: a process that produces one weak seam in fifty is worse than a process that produces uniformly mediocre seams, because the weak one reaches the water.

There is a second-order effect on production yield. A weld that needs a narrow temperature window to succeed on coated fabric slows the line and raises the reject rate. On laminated TPU the window is wide enough that operators hold consistent quality across a shift, which is one reason a spec-built programme is quoted more confidently on laminated material. The welding process itself is described in our RF welding explainer.

Ageing: hydrolysis, plasticisers and UV

Coatings and films age by different chemical routes. Polyurethane coatings are vulnerable to hydrolysis: moisture attacks the ester soft segments, and a bag stored damp in a warm garage can soften and turn tacky in three to five years. Polyester-based PU is more susceptible than polyether-based PU, and the difference is worth two to three years of service life in a humid climate.

PVC coatings lose plasticiser instead. The plasticiser that makes the coating flexible migrates to the surface over time, leaving the coating brittle and slightly sticky. Warm storage accelerates it, which is why a PVC bag left in a car boot in summer stiffens visibly within two seasons. TPU films have no plasticiser to lose, so they age mainly by UV attack on the surface and mechanical fatigue at fold lines.

Cold behaviour is where the two constructions separate most clearly in the field. A PVC-coated bag taken from a -5 °C car and folded will crease permanently, and the crease becomes a crack within a season. A TPU laminated bag folds at -30 °C without marking. If your users ski tour, ice fish, or paddle in winter, the cold crack figure matters more than any waterproof rating on the hangtag.

Puncture, abrasion and the failure modes you will actually see

Puncture resistance scales with polymer thickness, not with fabric denier. A 0.30 mm laminated film resists a 3 mm steel pin at 45 to 70 N, while a 0.10 mm coating resists the same pin at 15 to 25 N. Doubling denier from 420D to 840D changes the figure by under 10%, which surprises people who assume a heavier fabric is automatically tougher. The polymer does the work against sharp objects; the weave does the work against rubbing.

Abrasion is the reverse: the face fabric takes the load and the polymer holds it together. A 500D shell with a 0.50 mm film survives 8,000 to 12,000 cycles on a standard abradant before the weave shows, against 3,000 to 5,000 for a 420D shell with a thin coating. For a bag that will be dragged over sand and rock, spend the grams on the face fabric; for a bag that carries sharp tools inside, spend them on the film.

• Corner peel at a welded joint: the most common warranty claim, caused by low peel strength and drag loading.
• Delamination blisters after hot storage, always traced to poor surface preparation before lamination.
• Crease cracks at the roll-top mouth, caused by folding the bag at the same line for years.
• Tacky surface on aged PU coating, a hydrolysis symptom that no cleaning product will fix.
• Pinhole punctures from grit trapped inside the bag, which abrade the film from the inside out.

One failure mode deserves special mention: abrasion from the inside. A single grain of sand left in a bag works against the polymer for an entire trip, and on a thin coating it can wear through in a few hundred kilometres of washboard road. Rinse the inside as often as the outside. Our puncture resistance testing guide sets out how these figures are produced.

Field repair: what you can and cannot fix

Laminated TPU is the friendliest material to repair. A patch of the same film, applied with a hot iron through a silicone sheet at about 150 °C, fuses into the parent material and reaches 70 to 80% of original strength. Solvent-based TPU adhesives also work at room temperature, though the bond takes 24 hours to develop. A puncture anywhere on the panel area is a ten-minute fix with a kit weighing under 40 g.

Coated fabric is harder. The coating is thin and chemically interlocked with the weave, so a heat-fused patch tends to either fail to bond or scorch the fabric behind it. Adhesive patches work, but only on clean, dry, non-tacky coating; once hydrolysis has started, no adhesive will hold for long. In practice a punctured coated bag is a retired bag, which is worth factoring into the true cost per season.

Seam repairs follow the same pattern. A lifted weld on laminated material can be re-welded or taped successfully, and the repair holds because the base material has peel strength to spare. A lifted seam on coated material usually propagates, because the peel load that opened it in the first place is still there and the joint has no margin. Details in our guide to repairing a dry bag seam tear.

How to tell which construction you are holding

Three quick tests work without damaging anything. First, look at a cut edge or the inside of an open seam: laminated fabric shows a distinct film layer that you can lift with a fingernail, while coated fabric looks like a single material with the weave filled in. Second, flex the fabric sharply and listen: a laminated film makes a faint dry rustle, a coating is quieter. Third, press a warm thumb hard onto the surface for ten seconds and look for a shine: PVC coatings mark visibly, TPU does not.

Weight per square metre is the confirming measurement. Cut a 10 cm square, weigh it to 0.1 g, multiply by 100. A result near 320 g/m2 on a 420D shell means a thin coating; near 380 to 450 g/m2 means lamination. This is the cheapest quality check a buyer can run on a sample, and it catches a surprising number of specification substitutions before an order is placed.

Ask for the peel adhesion figure and the test method together. Any serious bag maker has this on a material data sheet, and the number should be quoted per 50 mm against a named standard. A bag maker that offers only a waterproof rating and a colour swatch has not characterised the material, and that gap will show up as seam failures rather than as marketing.

Choosing by activity

The pattern is consistent: lamination wins wherever the bag is welded, folded in the cold, or expected to survive more than two seasons. Coating wins where unit cost dominates and the product is genuinely disposable, such as a promotional beach bag used for one summer. Honesty about the intended life is the whole decision; there is no point paying for a ten-year material on a product that will be discarded in eight months.

What it costs and where the money goes

Using a thin PU-coated 420D nylon as the index at 1.0, a PVC-coated 500D polyester comes in around 0.9, a 0.30 mm TPU lamination at 1.6, and a 0.50 mm TPU lamination on a 500D face at 1.9. On a 20L bag with 0.55 m2 of fabric and roughly 0.9 m of welded seam, that is a material cost difference of about 1.10 to 1.80 US dollars per unit, before hardware and labour.

Lamination also affects yield and therefore labour. A wide welding window on laminated film means fewer rejects and faster cycle times, which recovers perhaps a third of the material premium. The rest is real cost, and it buys roughly two to three times the service life. Over a five-year ownership period the laminated bag is cheaper per season even at double the purchase price, which is the argument to make to a buyer who is comparing unit quotes only.

For a branded programme the decision also shapes what is possible later: laminated film takes a heat-transfer print and a welded handle cleanly, while coated fabric limits you to screen printing and stitched attachments. If the range will expand, the more capable material is usually the right platform even at a higher entry cost. Compare the options across the full product line before committing to one construction.

Specification checklist for a bag programme

• Name the construction: laminated film or internal coating, with polymer type and thickness in millimetres.
• Require peel adhesion in N per 50 mm, quoted against ISO 2411 or ASTM D751, with a minimum acceptance value.
• Specify face fabric denier separately from film thickness, because they resist different failure modes.
• Require a cold flex or cold crack temperature if the product will ship to a cold market.
• Set a hydrostatic head minimum after abrasion, not only on new material.
• Ask for weld strength retention as a percentage of parent material, with the spread, not just the average.
• Confirm repairability: can the end user patch this material with a domestic iron and a patch kit?

Those seven lines cost nothing to write and remove most of the ambiguity from a quotation. They also make two quotes genuinely comparable, which is otherwise impossible when one producer quotes a waterproof rating and another quotes a material. Our team has produced waterproof bags since 2014 in an SGS-verified 4,950 m2 facility running seven lines and 149 machines, and every programme starts from exactly this sheet; the process is documented with sampling in 6 to 10 working days and inspection to AQL 2.5.

If you want to go deeper on the material side, our comparison of TPU and PVC dry bag fabric covers the polymer choice, the laminated versus coated guide approaches the same question from the buyer angle, and our welded versus stitched seam guide explains what happens at the joint. For a broader material overview, start at the academy hub or read about dry bag material recyclability before you specify.