Home › Field notes › TPU Film vs PU Coating for Pack Shells: Hydrolysis, Cold Flex and Cost

Modular Bag Pro · 2026-10-10 · 3,087 words · 14 min read

A TPU film is a pre-formed membrane laminated onto the shell; a PU coating is a liquid polyurethane cured in place, and the first costs more but resists hydrolysis, cold cracking and abrasion better. Three figures separate them in practice: coating weight in grams per square metre, the low-temperature bend figure the film or coating retains flexibility down to, and the number of days it survives a hot humid conditioning exposure before the surface turns tacky. Commercial mechanics are identical for both - 500 pieces per reference, a first piece 6-10 working days after a finished brief, and 35-50 days of production before release at AQL 2.5. Scope stays civilian: rain covers, travel shells, hiking bodies, worksite kits and camera protection for equipment carried by licensed operators, with no ballistic, weapon-related or military certification claim anywhere in the text.

What each one is: a laminated membrane against a cured liquid

The two are frequently quoted as alternatives to each other and the confusion starts with the name, since both are polyurethanes. The difference is how the layer arrives on the cloth.

A TPU film is made first and attached second. Thermoplastic polyurethane is extruded or cast into a membrane of controlled thickness, then bonded to the shell by heat, by an adhesive layer or by a flame lamination process. Because the film was formed separately, its thickness is uniform, its properties were set at the film mill, and the bond between film and cloth is a distinct interface that can be tested - and that can fail.

A PU coating is applied as a liquid and becomes a layer afterwards. Polyurethane in solution or dispersion is spread on the cloth by knife, roller or rotary screen, then dried and cured, leaving a film that has penetrated the surface yarns rather than sitting purely on top. Thickness is controlled by the gap and the solids content, so it varies more than a bought-in membrane, and there is no separate bond line because the coating is keyed into the cloth it sits on.

The practical consequences follow from that difference. A laminated film brings its own properties with it and can only fail at the bond or within the film; a coating takes some character from the substrate and the curing conditions, so two coating lines can produce different results from the same specification. Bond strength of coated fabrics is examined under ASTM D751, which also covers grab, tear and hydrostatic behaviour of the coated assembly.

A third route exists and is worth naming to avoid confusion: a PU coating can also be applied over a laminated film, giving a printable surface over a waterproof membrane. Where a programme needs artwork on the outside and a barrier underneath, that is usually the construction being offered, and the specification should say so rather than leaving it to be inferred from a price.

Spec rule: State the construction as film-laminated, liquid-coated, or coated over laminated, and give the layer mass in grams per square metre for each layer, because the failure modes and the test methods differ between the three.

Hydrolysis: the failure that decides most programmes

Hydrolysis is the chemical breakdown of the polymer by water, and it is the single most common reason a coated or laminated body comes back from a tropical market looking ruined. The mechanism is not mysterious: moisture attacks the soft segments of the polyurethane, chains shorten, and the layer loses cohesion. What the user sees is a surface that has gone tacky, then a film that cracks, then delamination spreading outward from a fold or a seam.

Chemistry decides the rate. Polyurethanes can be built on ester soft segments or on ether soft segments, and ester-based material hydrolyses markedly faster than ether-based material under the same conditions. Both a PU coating and a TPU film can be built either way, which is why a specification that names only "PU" or "TPU" has not actually specified hydrolysis performance - the soft-segment chemistry has to be named as well.

Heat and humidity together drive it, and the acceleration is steep. A body stored in a temperate warehouse may be unchanged after a year, while the same body in a sealed container in a tropical port, or in a non-climatised stockroom through a monsoon season, can degrade in a single season. This is why a screening exposure at elevated temperature and high relative humidity is worth running before launch: a week or two under those conditions compresses months of real storage into a decision.

Programmes selling into Southeast Asia, coastal Latin America, the Gulf or equatorial Africa should treat this as a qualification gate rather than a footnote. The cost of specifying the better chemistry is small next to the cost of a container of goods that arrives tacky, and the cost of discovering it after distribution is larger than both.

Two mitigations help whatever the chemistry. Keep the goods dry in transit - desiccant and a sealed inner bag are cheap - and avoid storing coated or laminated bodies under compression for long periods, because pressure plus humidity accelerates the same breakdown and concentrates it at folds.

Verdict: Name the soft-segment chemistry on the specification, require a conditioned hot-humid exposure before approval, and treat any tropical destination as a qualification gate rather than a shipping detail.

Low-temperature flexibility and the cold crack window

Flexibility at low temperature is the second great separator, and it is the one that produces warranty claims in winter markets. Every polymer stiffens as temperature falls; the question is where the useful floor sits, and what happens when a stiff layer is folded.

A typical polyether TPU membrane stays supple well below freezing, and grades are available that remain serviceable far lower again. Many liquid PU coatings stiffen noticeably sooner, and the practical symptom is the one users describe as the bag "going boardy" in the cold - panels that will not fold, a lid that resists closing, and coated surfaces that crack when they are forced.

Cracking concentrates at stress points: the fold line of a lid, the corner of a flap, the roll-top closure, the crease where a strap meets the body. Once a crack crosses the waterproof layer, the barrier is gone at that line even though the rest of the panel still looks intact. Cracks at folds are therefore a functional defect and not a cosmetic one, and they belong in the critical or major class at inspection rather than in the minor class.

Testing is straightforward and cheap. Condition the coated or laminated specimen at the lowest temperature in the service brief, bend it around a mandrel of an agreed diameter, and inspect the bend line under magnification. Repeat after abrasion or laundering, because a layer that survives cold bending when new can fail the same bend after its surface has been worked.

There is a related trap that catches travel programmes. A body that performs perfectly at sea level in temperate weather can behave differently in an unpressurised hold or on a winter mountain, and the combination of cold and compression - a packed bag strapped tight in a cold vehicle - is worse than either alone.

Takeaway: Set the low-temperature bend figure from the coldest market in the distribution plan, test it after abrasion and laundering rather than only on new cloth, and classify cracks that cross the barrier as functional defects.

Abrasion, puncture and how the layer is held on

Water resistance gets the attention and abrasion decides the life. A barrier layer only has to keep water out on the first day; it has to survive being dragged across a tailgate, a conveyor, a rock slab and a workshop floor for years afterwards.

Thermoplastic polyurethane is intrinsically tough and resilient, which is why a laminated membrane usually outlasts a liquid coating of similar mass against abrasion and puncture. The liquid coating answers with penetration: because it keys into the surface yarns, it cannot be peeled away as a sheet, and minor surface damage does not expose a clean interface in the way a scratched laminate can.

Mass matters for both. A thin coating gives a lighter, more supple body and a shorter abrasion life; adding grams per square metre buys life at the cost of hand feel, fold bulk and price. The useful exercise is to fix the end use first - will this panel meet concrete weekly or sit in an overhead locker - and then set the layer mass, rather than accepting whatever the mill's standard grade happens to be.

Bond strength is the laminate's specific risk and the reason adhesion belongs on the test schedule. A film can be perfectly intact and still fail because the adhesive let go, and the first sign is usually a bubble or a soft patch that spreads. Coated fabric behaviour, including adhesion, is examined under ASTM D751; welded or sewn joins on the same assembly are checked separately, because a seam can be the weakest part of an otherwise sound barrier.

Reading the table as a purchasing decision: the laminate buys life in difficult markets and the coating buys price and a simpler supply chain. Programmes that sell into one climate and one use case can often take the coating and spend the difference on a heavier cloth.

Bottom line: Choose a laminated membrane where the destination market is hot and humid or genuinely cold, and a liquid coating where the product lives in temperate conditions and price per body is the binding constraint.

Environmental position and what happens at end of life

The environmental comparison has three parts and only one of them favours the laminate. Thermoplastic polyurethane can in principle be re-melted and re-formed, so off-cuts and end-of-life bodies have a mechanical recovery route that a cured coating does not; a crosslinked coating is a thermoset layer sitting on a thermoplastic cloth, and it complicates both mechanical recycling and any chemical recovery of the substrate.

Processing chemistry is where the coating has improved fastest. Older solvent-based systems carried solvents of concern and required recovery plant; water-based dispersions avoid most of that and are now standard for many applications. Whatever route is used, the substances left in the finished article have to satisfy the market it enters - REACH across the European Union, Proposition 65 and related state rules in the United States - and plenty of buyers ask for OEKO-TEX Standard 100 on the finished article instead of relying on a supplier declaration alone.

Service life is the largest environmental lever and the one least often counted. A body that lasts twice as long halves the material, the freight and the disposal burden per year of use, so a laminate that costs more per unit and lasts materially longer is frequently the better environmental answer even where its recycling route is imperfect. Programmes making a public claim should be able to support it with a durability comparison rather than with a material name.

End-of-life reality deserves an honest sentence. Most carry goods are disposed of as mixed waste whatever they are made from, because separation infrastructure rarely reaches them. Designing for disassembly - fewer material types, separable hardware, a single polymer family where possible - does more in practice than optimising the recyclability of one layer.

Judgement: Weigh service life and disassembly ahead of theoretical recyclability, because a laminate that doubles useful life and a body built from fewer material types both beat a marginally more recyclable layer that fails in one season.

Cost, processing and where yield is lost

A laminated membrane costs more per square metre than a liquid coating, and the gap has two parts: the film itself is bought in from a film mill with its own minimum quantities and lead times, and the lamination step adds a pass, an adhesive and a bond check. A coating is applied on the fabric line or at a coater, in one continuous operation, with no bought-in membrane and no separate bond interface.

Lead times follow the same logic. Coating capacity is widely available and a coating recipe can usually be adjusted inside an existing slot; a specific film grade may carry a mill lead time of weeks, and a non-standard thickness or colour may carry a minimum quantity far above what a single programme needs. Programmes that specify an unusual film should confirm availability before artwork is committed.

Yield is where the two converge. Coating defects - streaks, uneven mass, pinholes from trapped moisture - usually appear before cutting, and a defective roll is rejected at goods-in. Lamination defects are subtler: a bond that looks sound on the bench can release after laundering or after a hot humid exposure, so a percentage of the failure risk sits downstream of inspection. That argues for a bond check on finished bodies as well as on cloth.

That table is the diagnostic core of this topic. Most arguments about coating versus laminate are really arguments about one of those seven symptoms, and identifying which one is being discussed usually ends the argument quickly.

Selection rule: Fix the symptom you are trying to prevent, then buy the construction that prevents it - tackiness and cold cracking point to a laminate with ether soft segments, while stitch-line leakage is solved by seam construction rather than by changing the layer at all.

Qualification sequence, programme terms and who runs the work

Qualification runs in one sequence and each step feeds the next. Approve cloth and layer together; run bond assessment under ASTM D751; condition a second set through hot humidity and a third through the low-temperature bend; launder a finished body to ISO 6330 and re-check the bond; then weld or sew a production seam and test the join rather than the panel. Where an ingress rating is quoted for the finished article, the enclosure classification in IEC 60529 describes what the test rig demonstrated - not a promise about a specific pack in a specific storm - and the claim should be written that way.

On the lamination and coating side, our vetted partner facilities work 4,950 m² of SGS-verified floor space holding 149 machines laid out over 7 production lines and 137 people, at roughly 200,000 units a month; our production team schedules those lines by reference. The founder has been making bags since 2004 and the business began in 2014.

Volumes and dates do not depend on which layer is chosen. Entry is 500 units for a single reference, colourways combining to reach it provided a workable minimum run each; a first piece is ready 6-10 working days from a finished brief, or 12-15 when a laboratory submission or a new laminate stands in the way, and the order itself needs 35-50 days. Development pieces cost USD 50-150, credited once the order is placed, and jigs, screens or dies range from USD 300 to 2,500. Nothing leaves until a level II sample drawn to ISO 2859-1 has passed at AQL 2.5 with critical findings at nil, majors at 2.5 and minors at 4.0 counted as three classes; money is quoted FOB Xiamen on T/T 30/70 and an answer follows a finished brief within 24-48 hours. Shipping takes 25-35 days by ocean, 5-8 by air or 3-5 by express, and containers deserve attention from 20GP at about 28 CBM and 40HQ at about 68 CBM.

Two adjacent readings help place this decision. Our comparison of TPU against PVC for modular shells covers the third material many buyers are offered, and the note on ingress ratings for modular bodies explains what an IP figure does and does not promise. Where the shell decision feeds a platform choice, load carriage for wet and cold hiking routes treats both as primary variables, and buyers shortlisting suppliers can start from the enquiry route used for specification review.

Writing the barrier clause so it can be tested

The clause has to name what is applied, how much of it, and what happens if it fails. Six lines cover it, and each one blocks a specific ambiguity that otherwise turns into a dispute at goods-in.

• Construction named: film-laminated, liquid-coated, or coated over laminated
• Chemistry named: soft-segment type, because ester and ether behave differently against hydrolysis
• Layer mass in grams per square metre, with a tolerance band
• Bond requirement and the method used to assess it
• Low-temperature bend figure, and the conditioning that precedes the bend
• Hydrostatic performance of the finished assembly, and of the seam separately

Seam performance deserves its own line because a barrier is only as good as its weakest join. A sewn seam perforates the layer; that is why sewn-and-taped and welded constructions exist, and why a hydrostatic figure quoted for cloth tells a buyer nothing about the body. Test the join on a production sample, with the production thread and the production machine settings.

Re-qualification triggers close the clause. A change of film supplier, adhesive supplier, coating recipe, curing temperature or line speed can each alter bond behaviour, and every one of them should bring verification forward rather than wait for the next scheduled round. Record the approved construction against the contract number, keep a retained swatch, and the next season's order can be matched to what actually passed.