PU vs TPU vs DWR Coatings: Hydrostatic Head Tested

PU vs TPU vs DWR Coatings: The Direct Answer

Polyurethane, thermoplastic polyurethane and durable water-repellent finish are three different things that are constantly confused. PU and TPU are coatings applied to the back face of a fabric to stop water passing through, measured by hydrostatic head in millimetres: PU at 25-60 g/m² gives 1,500-5,000 mm, TPU at 30-80 g/m² gives 5,000-10,000 mm and PVC at 80-150 g/m² gives 3,000-6,000 mm. DWR is a surface treatment applied to the outer face at 8-15 g/m² that makes water bead rather than soak in, and it has no hydrostatic head rating at all. For a British pack you need both: a coating for through-penetration and a DWR so the face does not wet out. Ester-based PU hydrolyses in 3-5 years of UK humidity, polyether PU lasts 8-12 and TPU 8-15, so the chemistry matters more than the thickness. Re-proofing the outer DWR costs £6-12 a year, and spending that is what turns a three-year pack into an eight-year one.

Choosing the wrong one is the commonest reason a "waterproof" pack fails, because most buyers compare hydrostatic head figures and never ask which polyurethane chemistry was used.

This guide separates the three, gives the test data and shows what to specify. Our waterproof backpack guide applies it to every model reviewed.

Coating Versus Finish: The Distinction That Matters

A coating is a continuous polymer layer applied to the back face of a fabric, typically by knife-over-roll or by lamination, and it is what stops liquid water passing through. It is measured by hydrostatic head. A finish, or DWR, is a low-surface-energy treatment applied to the outer face that raises the contact angle of water so droplets form and roll off rather than spreading. It has no hydrostatic head value because it is not a barrier. Understanding which of the two has failed is the whole of waterproof diagnosis, and it is the subject of our waterproof fault-finding guide.

The two work together and fail independently. A pack with a perfect 10,000 mm coating and no DWR will keep water out, but the face fabric will soak up 200-400 g of water, the pack will feel heavy and cold, and the contents will get damp from the wet fabric inside. A pack with a good DWR and a failed coating will bead beautifully for fifteen minutes and then leak.

Hydrostatic head is measured by clamping fabric under a rising column of water under AATCC 127 or ISO 811, and the result is the height in millimetres at which three drops penetrate. Spray resistance, which is what a DWR delivers, is a separate test under ISO 4920 and is reported as a 0-5 rating. Our production team runs both on every incoming lot for our UK waterproof pack range.

Coating Types Compared

The table sets out every coating in mainstream backpack production with its weight, hydrostatic head, hydrolysis life and cost. Hydrolysis life is the realistic service life in British indoor storage at 55-80% relative humidity, and it is the column buyers should read first. Every figure below comes from incoming inspection data on our UK waterproof pack programme rather than supplier literature.

The last row is the important one. Adding a DWR to a PU-coated shell costs £0.60-1.40 per pack and is the difference between a bag that sheds a shower and one that soaks up 200-400 g of water before the coating ever comes into play. Our waterproof specification guide requires it on every pack.

Hydrolysis: Why PU Coatings Fail in Britain

Polyurethane is made by reacting an isocyanate with a polyol, and the resulting polymer contains ester or ether linkages depending on which polyol was used. Water attacks ester linkages, the chains shorten, and the coating loses integrity. The reaction accelerates with temperature, roughly doubling for every 10 °C rise, and it is catalysed by both acid and alkali.

In practice a British pack stored in a hallway cupboard at 60-80% relative humidity and 18-24 °C gives an ester-based PU coating a service life of 3-5 years. The same coating on a pack stored dry at 15 °C lasts 6-9 years. Polyether-based PU resists the reaction almost entirely and lasts 8-12 years for £2-5 more per pack, which is the best-value specification decision in this whole category.

The symptoms progress in a fixed order, and catching them at stage one is what allows a cheap fix rather than a new pack, as set out in our waterproof care schedule. First the inner face loses its sheen and takes on a chalky grey bloom. Then a faint sweet or vinegar smell develops as the degradation products volatilise. Then the coating flakes in a fine dry powder that contaminates the contents. Finally it separates in sheets and the fabric underneath is exposed. Accelerated testing follows ISO 1419, typically 70 °C at 95% relative humidity for 7 or 14 days. Our SGS-verified production base requires a 14-day exposure with no more than 20% loss of tear strength.

TPU: Where the Premium Is Justified

Thermoplastic polyurethane differs from conventional PU in that it is a thermoplastic elastomer rather than a thermoset. It melts and re-forms, it can be extruded into a film and laminated rather than spread as a liquid, and critically it has far better hydrolysis resistance and low-temperature flexibility. A TPU laminate gives 5,000-10,000 mm hydrostatic head against 1,500-5,000 mm for PU at comparable weight.

Three applications justify the £3-8 premium. Waterproof roll-top packs and dry-bag liners, where 10,000 mm is a genuine requirement rather than a marketing figure. Winter packs used below -10 °C, where PU stiffens and can crack at a fold while TPU stays flexible. And any pack intended to last more than a decade, where the 8-15 year hydrolysis life is the deciding factor.

It is not justified on a commuter pack that lives indoors, where a polyether PU at 35-45 g/m² plus a DWR will serve for a decade at a third of the cost. Note also that TPU is heavier per unit of hydrostatic head than PU in some constructions, and it has a slightly rubbery hand that some buyers dislike. Our vetted partner facilities specify TPU for the waterproof range and polyether PU elsewhere, and the reasoning is documented in our waterproof pack sourcing notes.

DWR: The Finish That Does the Daily Work

A durable water-repellent finish is a fluorocarbon-free emulsion, nowadays usually a silicone or a hydrocarbon, applied to the outer face at 8-15 g/m² and cured at 150-170 °C. It lowers the surface energy of the fibres so water beads at a contact angle above 90 degrees rather than wetting out. Spray resistance is graded 0-5 under ISO 4920, where 5 is no wetting and 1 is complete wetting.

DWR is the shortest-lived component on any pack. Abrasion removes it from high-contact zones, and contamination from skin oils, sunscreen and dirt defeats it chemically. In British use expect 12-18 months of effective beading from a new pack, and 6-12 months after each re-application. The symptom is water spreading and darkening the fabric rather than beading, which owners often mistake for the coating failing.

Spray resistance is graded 0-5 under ISO 4920, where 5 is no wetting at all and 1 is complete saturation, and a new pack should leave the factory at grade 4-5. Re-application is cheap and effective. A 300 ml spray-on reproofer costs £6-12 and treats two or three packs; a wash-in version costs £8-14 per bottle and suits a bag that can be machine washed. Both need heat to cure: 30-40 minutes in a tumble dryer on low, or 20-30 minutes with a hairdryer on a warm setting. Our re-proofing guide covers the method and the intervals.

Reading Hydrostatic Head Claims Honestly

Hydrostatic head is the most quoted and most misused number in backpack marketing. A 1,500 mm rating means the fabric resisted a 1.5 m column of water before three drops penetrated under laboratory conditions: a clamped, flat, unabraded, unstitched sample. It says nothing about seams, zips, closures or abrasion.

The practical translation for British conditions is straightforward. Under 1,000 mm is shower-resistant at best. 1,500 mm handles drizzle and a short walk to the station. 3,000 mm handles sustained rain for an hour or more. 5,000 mm is genuinely rainproof for a full day. 10,000 mm and above is dry-bag territory, and it is only meaningful in combination with taped seams and a roll-top closure.

Pressure is the reason. Kneeling on a pack, or strapping it to a rack, applies far more than a 1,500 mm head at the contact point. Western Britain records 150-180 wet days a year and upland areas see 1,200-3,000 mm annual rainfall, according to Met Office climate normals, so a pack that passes in a shop will be tested properly within a month. Our waterproof rating guide translates every figure into hours of real rain.

UK Re-Proofing Costs and Intervals

Re-proofing is the highest-value maintenance task on any British pack, and it costs very little. The table sets out what to use, when and what it costs.

The economics are clear. Spending £6-12 a year on re-proofing extends the effective waterproof life of a £110 pack from roughly three years to eight or more, which works out at under £2 a year. Against that, a £12-25 rain cover is the cheapest possible insurance for a pack whose coating has degraded. Our waterproof care guide sets the annual routine.

Specifying Coatings on a Custom Order

Write the specification as coating chemistry, application method, coating weight in g/m² with tolerance, hydrostatic head, hydrolysis test result, DWR type and spray rating. Our production team works to ±8% on coating weight and requires a named chemistry rather than "PU", because the difference between ester and polyether is invisible at sampling and worth five years of service life.

Commercial terms follow the standard structure. MOQ 500 pcs applies, and coated fabric is a stock item at standard specifications, so it rarely constrains lead time. Sampling takes 6-10 working days and bulk production 35-50 days from sample approval, with inspection to ISO 2859-1 at AQL 2.5. A bespoke hydrostatic head above 5,000 mm becomes a mill order and adds 8-12 working days.

Two specification notes prevent most waterproofing complaints. First, require a finished-pack shower test rather than a fabric test, because a shell that passes at 5,000 mm will still leak through an unsealed seam or a horizontal zip. Second, specify seam tape width and application temperature alongside the coating, since tape applied below 130 °C lifts within 12-24 months. Both notes are standard in tech packs from our 4,950 m² SGS-verified production floor and in our waterproof sourcing guide.

Failure Modes and Diagnosis

DWR loss is the first and most common failure, presenting as water spreading and darkening the face fabric instead of beading. It appears at 12-18 months on a new pack and is fixed for £6-12 in 25-40 minutes. Mistaking it for coating failure is the most common diagnostic error owners make, and it leads people to replace packs that need a £8 bottle of spray.

Coating hydrolysis is second and is irreversible, presenting as a chalky bloom, a vinegar smell and then flaking on the inside face at 3-5 years for ester-based PU. Once flaking starts the pack has 12-24 months of usable life and the only remedy is a liner or a replacement.

Seam leakage is third and is the commonest cause of water in a pack that is otherwise sound. It presents as a damp patch along a seam line rather than across a panel, and it is fixed with seam sealer at £7-15 and 40-70 minutes, or prevented at manufacture by tape applied at 130-160 °C. Our leak diagnosis guide walks through each test with a kettle and a torch.

Verdict: What Should You Specify or Buy?

Buy polyether-based PU at 35-45 g/m² with a DWR finish for a British commuter or travel pack. That gives 1,500-5,000 mm hydrostatic head, an 8-12 year hydrolysis life and beading that you can renew for £6-12 a year, at a cost premium of £2-5 over the ester-based default.

Buy TPU laminate at 30-80 g/m² only where you genuinely need 5,000-10,000 mm: a roll-top waterproof pack, a dry-bag liner, or a winter pack used below -10 °C. The £3-8 premium is justified there and nowhere else.

Whatever you buy, re-apply DWR every 12-18 months and dry the pack properly after every soaking. Those two habits matter more to staying dry than any hydrostatic head figure on a hangtag. Our waterproof backpack guide ranks current UK models against that standard.