Waterproof Bag Coating and Laminate Technology Explained

Film, Coating and Laminate: Three Different Constructions

The three words are used interchangeably in supplier conversations and they describe three different things. A coating is a polymer applied as a liquid or paste onto a woven face, where it penetrates and cures. A film is a polymer extruded or calendered into a solid sheet first and joined to the face afterwards. A laminate is the resulting composite, and the term says nothing about which of the two routes produced it.

That distinction matters because the route determines the failure mode. A coating that has penetrated a weave fails by thinning and then by cracking along a fold. A film bonded to a face fails at the interface, by delamination, and it fails without warning because the surface still looks intact. Two bags can carry the same head figure and fail in completely different ways after a season.

The third construction is a homogeneous sheet with no face fabric at all, which is what most tarpaulin-style shells are. It welds beautifully because there is no interface to load, it punctures more easily than a woven composite because there is no yarn to arrest a tear, and it is the cheapest of the three to make.

• Coating: liquid polymer applied to cloth, penetrates, cures, fails by thinning and fold cracking.
• Film bonded to face: solid sheet joined afterwards, fails at the interface by delamination.
• Homogeneous sheet: no face at all, welds well, tears more easily than a woven composite.
• All three can be specified to the same head figure and still age very differently.
• The route is stated on a mill sheet, and it is worth reading before quoting head.

What Happens Inside a Knife-Over-Roll Coating Line

Knife-over-roll is the workhorse of coated fabric production and understanding it explains most of the variation buyers see between rolls. Cloth is drawn under a fixed blade set at a gap above a rotating roll, polymer is banked in front of the blade, and the gap between blade and cloth sets how much stays on the surface. The cloth then passes through a oven where the coating is gelled and fused.

Three process variables decide the result: the blade gap, the line speed and the oven temperature profile. Gap sets add-on mass, speed sets how much time the coating has to level, and the oven profile sets whether the coating fuses fully or skins over with solvent trapped beneath. A skin that forms too early is the origin of pinholes, and pinholes are the single largest cause of a head figure that measures well on one patch of a roll and poorly on the next.

The consequence for a buyer is that head variation within a lot is a process artefact rather than a design one. Asking for a head test at three points across a roll width, rather than one, is a cheap way to see whether the line was stable when the roll was made, and it is a request most mills will accommodate.

1. Blade gap sets the add-on mass, and it is the variable that moves coating weight in grams per square metre.
2. Line speed sets levelling time, and running faster than the coating can level leaves ridges.
3. Oven profile sets fusion, and skinning over traps solvent that later becomes pinholes.
4. Roll-to-roll tension sets how much the coating is stretched before it sets.
5. Test head at three points across the width to see whether the line was stable.

TPU Extrusion Versus PVC Calendering Compared

TPU film and PVC sheeting are made by different processes and the process shows in the product. TPU is usually extruded, melted and forced through a die into a sheet of controlled thickness, which gives a film with consistent gauge and a clean surface. PVC is usually calendered, passed between heated rolls that squeeze a compound into a sheet, which gives a sheet with a slightly different surface and a thickness controlled by roll gap rather than die.

Gauge consistency is the practical difference. An extruded film holds thickness tightly, which means the weld parameter window holds tightly too, because the energy needed to melt a joint is a function of thickness. A calendered sheet varies more, and that variation is one reason PVC weld windows tend to be narrower than TPU ones.

The second difference is what goes into the compound. TPU is a reacted polymer that arrives at the extruder essentially complete. PVC is a compound of resin plus plasticiser plus stabilizer plus filler, and the plasticiser is a mobile ingredient rather than a bound one. That mobility is the origin of both the soft hand buyers like and the migration problem that shows up later.

Polyurethane Coating Chemistry and the Hydrolysis Clock

Polyurethane coating is the most common route to a soft-hand waterproof cloth, and it carries a well-known liability: the ester-based soft segments in many polyurethane formulations are attacked by water in warm humid conditions, breaking chains and turning a coating first tacky and then crumbly. That is hydrolysis, and it is why a bag stored damp in a warm climate can fail without ever being used.

The clock is accelerated by heat and humidity together, not by either alone, which is why accelerated hydrolysis testing conditions specimens at elevated temperature and very high relative humidity and then looks for coating softening, loss of peel strength and surface tack. A coating that is fine at room conditions can be marginal at the accelerated condition, and the accelerated result is the one that predicts a container in a humid port.

Ether-based polyurethane resists hydrolysis far better than ester-based and costs more. Polycarbonate-based systems resist it better still. Specifying the soft-segment chemistry rather than just the head figure is the single most effective thing a buyer can do to remove this failure mode, and it is a specification line most briefs omit.

• Ester-based polyurethane: soft hand, lower cost, susceptible to hydrolysis in warm humidity.
• Ether-based polyurethane: better hydrolysis resistance, higher cost, slightly firmer hand.
• Polycarbonate-based: best resistance in this family, and priced accordingly.
• Hydrolysis needs heat and humidity together, so a cool damp store is far less risky than a hot one.
• A coating that has gone tacky has already lost peel strength, and the seam is next.

Adhesion Between Face Fabric and Film: The Peel Number

Peel adhesion is the force required to separate the film from the face, reported as force per unit width. It is the number that predicts delamination, and it is the number most often absent from a mill sheet because the mill is selling waterproofing rather than bond strength. For a welded build it is arguably more important than the head figure.

The reason is mechanical. A welded seam loads the interface in shear and peel, and a joint that separates at the interface rather than within the film is a joint where the interface was the weak link. The weld itself will look perfect and will pass a visual check, and it will fail under load or after ageing.

There is also a chemical dimension. Adhesion is achieved by a primer or by a tie layer, and both are affected by hydrolysis and by heat ageing in the same way the coating is. A bond that measures well on a fresh roll can measure substantially worse after an accelerated ageing sequence, which is why we peel-test before and after ageing rather than only at intake.

1. Ask for the peel figure at intake, in force per unit width, with the test method stated.
2. Peel-test again after an accelerated ageing sequence rather than only on fresh material.
3. Record the failure mode: adhesive failure at the interface is a different problem from cohesive failure in the film.
4. Re-check peel whenever the face supplier, the primer or the film gauge changes.
5. Treat a missing peel number as a reason to keep looking, not as a minor gap.

How Coating Weight in GSM Moves the Head Figure

Head rises with coating weight, but not linearly and not without limit. The first increments of coating close the interstices in the weave and produce a large jump in head. Later increments add thickness with diminishing effect on head and a steadily increasing penalty in weight, stiffness and cost.

There is also a floor effect. Below a certain add-on the coating cannot bridge the gaps between yarns consistently, and head results become scattered: some patches measure well and others leak immediately. That scatter is worse than a low average, because it means the roll contains weak spots that will not be found by a single test.

This is why we quote head as a band rather than as a point. A lightweight build sits at 10,000 mm and a heavy-duty one beyond 20,000 mm, reported to ISO 811, and the engineering question is not how to reach the highest number but how to reach the required number with the least weight and the widest process margin.

Plasticiser Migration in PVC and the Stiffening Drift

Plasticiser is what makes PVC soft, and it is not chemically bound into the polymer. It moves. Over time and under heat it migrates to the surface, it migrates into anything in contact with the sheet, and it leaves the sheet. When it leaves, the sheet stiffens, and a stiffened sheet cracks at the fold rather than flexing.

The visible symptoms are a slightly oily surface film, a change in hand from soft to boardy, and then cracking along the fold lines of a roll-top or a packed crease. Cracking at the fold is the end state, and by the time it appears the bag has already lost waterproofing at exactly the place where the closure depends on flexibility.

The second consequence is a blocking problem in storage. A sheet that has migrated plasticiser to its surface will stick to itself when stacked under pressure and warmth, and blocked rolls are difficult to separate without marking. That is a handling issue in the warehouse and a quality issue on the finished bag.

The engineering responses are to select a higher molecular weight plasticiser that migrates more slowly, to reduce the plasticiser loading and accept a firmer hand, or to move to a polymer that does not need plasticising at all. All three are specification decisions and all three are cheaper to make before sampling than after a customer complaint.

Hydrostatic Head Results Across Five Build-Ups

The table below is the reference used when a buyer asks what a given construction actually delivers. It is expressed as bands rather than points because a single number implies a precision that neither the process nor the test supports, and because the useful decision is between bands rather than within one.

The last row is worth a note because it is the one buyers mis-specify most often. A liner film inside a cloth shell does not need a high head figure, because the outer cloth is not the waterproof element and the liner is not the abrasion element. Specifying the outer cloth to a heavy-duty head figure on a lined bag buys nothing and adds weight.

Where the two are combined, the meaningful test is on the composite as assembled rather than on either ply alone, because the seam and any quilting stitch through the composite create paths the plies did not have individually.

Cold Crack and Low-Temperature Flex by Chemistry

Cold crack testing conditions a specimen at a low temperature and then bends it over a mandrel of defined diameter, and the result is reported as the lowest temperature at which the specimen survives without cracking. It is a different measurement from head and it eliminates materials that head figures pass without comment.

The ranking by chemistry is stable and worth memorising. TPU holds flexibility to low temperatures and is the default for winter programmes. Polyurethane coating behaves well if the formulation was chosen for it. PVC stiffens as temperature falls, and the amount it stiffens depends on how much plasticiser was loaded and which plasticiser was used.

The bag-level consequence is concentrated at the closure. A roll-top is folded three or four times and then compressed by a buckle, and that fold is the coldest, most loaded and most flexed feature on the product. A shell that passes a cold bend test on a flat specimen can still crack at a four-turn fold, which is why the fold itself should be conditioned and flexed rather than only the panel.

• TPU: holds flexibility well below freezing, the default for winter markets.
• Polyurethane coating: depends on formulation, and should be specified with a low-temperature requirement.
• PVC: stiffens with falling temperature, and the degree depends on plasticiser loading.
• Test the fold, not only the panel, because the closure fold is the most loaded feature.
• Condition and flex, rather than condition and inspect, since cracking appears on movement.

Abrasion Cycles to Coating Breakdown by Film Type

Abrasion resistance in a coated or laminated shell is a property of the surface layer, not of the composite, and it is the property that decides service life on any bag that touches the ground. The test cycles a weighted abradant across the face until the waterproof layer is breached, and the result is reported as cycles to breakdown.

The pattern by chemistry is that polyurethane coatings abrade well and TPU films abrade very well, while PVC sheeting abrades adequately but tends to mark and scuff visibly before it actually breaches. Visible scuffing generates returns well before it generates leaks, so the cosmetic threshold matters commercially even where the functional threshold is not reached.

The dominant variable is not the chemistry but the add-on. A thicker surface layer takes more cycles to breach, which is why an abrasion requirement usually resolves into a coating weight decision. That is also why the two requirements should be written together: a buyer who specifies abrasion cycles and then cost-reduces the add-on has specified the first and then removed the means of achieving it.

Ultraviolet and Hydrolysis Ageing Run as One Sequence

Ultraviolet and hydrolysis are usually tested separately and they usually occur together. A bag left on a boat deck is exposed to ultraviolet at the same time as it is warm and damp, and testing one without the other understates the damage. Running them as a single sequence, ultraviolet exposure followed by warm humid conditioning followed by a functional test, is closer to service and catches combinations the single tests miss.

The ultraviolet stage is run in a cabinet with fluorescent lamps or a xenon arc source, cycling exposure with condensation, and the result is read as colour change, gloss loss, chalking and then as retained tensile and peel strength. The hydrolysis stage is run warm and very humid, and the result is read as coating softening and peel loss.

The functional test at the end is what makes the sequence useful. Ageing followed by a head test, a peel test and a water ingress test on the finished body tells a buyer whether an aged product still performs, which is a different and more useful statement than a colour change number on its own.

1. Condition the specimens, then run ultraviolet exposure with condensation cycling.
2. Read colour change and gloss, then retained tensile and peel on the exposed face.
3. Move the same specimens into warm humid conditioning for the hydrolysis stage.
4. Re-read peel, and look for surface tack as the early hydrolysis signal.
5. Finish with a head test and a water ingress test on assembled bodies from the same lot.
6. Compare against unaged controls from the same roll, never against a specification value alone.

Which Constructions Accept a Welded Seam

Welding is the cheapest and most reliable way to make a waterproof seam, and not every construction accepts it. Radio-frequency welding works by heating a polar polymer in an alternating field, which means the polymer has to be polar enough to respond. Hot-air and hot-wedge welding work by melting the surface, which means the construction has to have a meltable surface on both sides of the joint.

TPU and PVC both weld well by radio-frequency, and both weld by hot air. Polyethylene and polypropylene do not respond to radio-frequency at all and must be hot-wedge welded, which is a slower process with a narrower window. A coated woven can be welded where the coating is thick enough to melt across the joint, and stitched and taped where it is not.

The practical test is a parameter ladder followed by a peel test on every rung, and the test should be run on the actual composite rather than on the film alone. A film that welds beautifully on its own can weld badly once it is bonded to a face, because the face changes how heat moves through the stack.

• TPU: welds by radio-frequency and hot air, with a wide window.
• PVC: welds by both methods, with a somewhat narrower window.
• Polyethylene and polypropylene: no radio-frequency response, hot-wedge only.
• Coated woven: weldable where the coating is thick enough to melt across the joint.
• Always run the ladder on the composite, never on the film alone.

Specifying Coating Technology in a Tech Pack

A tech pack that names a head figure has specified an outcome and left the technology open, which means two suppliers can meet it with constructions that age completely differently. Naming the construction closes that gap. The minimum useful set is the face construction, the coating or film chemistry, the add-on or gauge, the adhesion requirement and the ageing requirements.

The ageing requirement is the line most often skipped and the one that prevents the most complaints. Stating a retained peel percentage after a defined ultraviolet and warm-humid sequence costs nothing to write and removes the most common source of a second-season failure. Without it, a supplier is free to deliver a construction that passes every intake test and fails in a warm warehouse.

QUANZHOU JUNYUAN BAGS was established in 2014, with our founder in the bag trade since 2004, and coating technology is qualified by process behaviour rather than by a single number. Head is reported to ISO 811 across a band from 10,000 mm for lightweight builds to beyond 20,000 mm for heavy-duty ones, weld windows are established by parameter ladder and peel testing on the composite, and ageing is run as an ultraviolet and warm-humid sequence ending in a functional test. Capacity at our validated production partner facility is verified by SGS at 4,950 square metres, 7 production lines, 149 pieces of equipment, 137 staff and 200,000 pieces per month, with market documentation able to cover CE, CPC, RoHS and REACH, with QC run to ISO 9001-aligned procedures. Sampling runs 6 to 10 working days, or 12 to 15 for a complex build, and bulk runs 35 to 50 days, quoted FOB Xiamen on T/T 30% deposit and 70% before shipment.

To have a construction reviewed before sampling, write to service@junyuanbags.com or message the team on WhatsApp at +86 177 5002 0688. The rating language used across constructions is set out in the waterproof ratings guide, the pressure method itself is described in the ISO 811 hydrostatic pressure guide, seam options are compared in the waterproof standards guide, and a construction can be specified from the custom programme page.

Frequently Asked Questions

Q1: What is the difference between a coating and a laminate on waterproof fabric?

A: A coating is a polymer applied as a liquid or paste onto a woven face where it penetrates and cures, while a laminate is a solid film joined to the face afterwards. The distinction matters because a coating fails by thinning and fold cracking and a film laminate fails by delamination at the interface, so two shells can share a head figure and age completely differently.

Q2: Why do head results vary across a single roll of fabric?

A: Because head variation within a lot is usually a process artefact rather than a design issue. Blade gap, line speed and oven profile on a knife-over-roll line decide add-on and fusion, and a coating that skins over too early traps solvent and produces pinholes, which appear as weak spots scattered across the roll.

Q3: What is hydrolysis and why does it affect waterproof bags?

A: Hydrolysis is the breaking of polymer chains by water under warm humid conditions, and it attacks the ester-based soft segments in many polyurethane coatings. The coating becomes tacky, loses peel strength and then crumbles, which is why a bag stored damp in a warm climate can fail without ever being used.

Q4: Does TPU resist hydrolysis better than PU coating?

A: Yes, substantially. TPU is a reacted polymer and holds up well, while ester-based polyurethane coatings are the susceptible family. Ether-based and polycarbonate-based polyurethane formulations resist hydrolysis far better than ester-based ones, and naming the soft-segment chemistry is the cheapest way to remove this failure mode.

Q5: Why is peel adhesion more important than head for a welded bag?

A: Because a welded seam loads the interface between face and film in shear and peel. A joint that separates at the interface rather than within the film looks perfect and passes a visual check, then fails under load or after ageing, so the interface strength governs whether the weld survives.

Q6: Does a higher coating weight always give a higher head figure?

A: It gives diminishing returns. The first increments close the gaps in the weave and produce a large jump, later increments add thickness with little head gain and rising penalties in weight, stiffness and cost. Below a floor the results scatter badly, which is worse than a low average because the roll contains undetected weak spots.

Q7: What causes PVC to stiffen and crack over time?

A: Plasticiser migration. The plasticiser that makes PVC soft is not chemically bound, so it moves to the surface and away from the sheet, and the sheet stiffens and then cracks at the fold. The visible sequence is an oily surface film, a boardy hand, and then cracking along roll-top folds and packed creases.

Q8: Which polymers can be radio-frequency welded?

A: TPU and PVC both weld well by radio-frequency because both respond to an alternating field, and both also weld by hot air. Polyethylene and polypropylene have no radio-frequency response and must be hot-wedge welded, which is slower and has a narrower parameter window.

Q9: Should ultraviolet and hydrolysis be tested together?

A: Yes, because they occur together in service. A bag on a boat deck is exposed to ultraviolet while being warm and damp, and running ultraviolet exposure followed by warm humid conditioning and then a functional ingress test is much closer to service than either test run alone.

Q10: What head figure should be specified for a lightweight day bag?

A: Ten thousand millimetres to ISO 811 is the figure quoted for lightweight builds, and it is correct for a hand-carried product that never generates meaningful pressure against its shell. Heavy-duty builds go beyond 20,000 mm where a pack is kneeled on, strapped down or used in ground contact.

Q11: Is a high head figure needed on a liner film inside a cloth shell?

A: No, and specifying one is a common waste. On a lined bag the outer cloth is not the waterproof element and the liner is not the abrasion element, so the meaningful test is on the assembled composite including the seam, rather than on either ply measured alone.

Q12: What should a tech pack state about coating technology?

A: The face construction, the coating or film chemistry, the add-on or gauge, the adhesion requirement and an ageing requirement. The ageing line is the one most often skipped and the one that prevents the most complaints, because it stops a construction passing every intake test and failing in a warm warehouse.

Q13: How does coating technology affect the sampling window?

A: A library construction with an established weld window keeps sampling inside 6 to 10 working days, while one requiring a parameter study moves toward 12 to 15. Bulk then runs 35 to 50 days, and a construction that must be booked from the mill shifts the start of that window.

Q14: What capacity and documentation support a coating programme?

A: Capacity is verified by SGS at 4,950 square metres, 7 production lines, 149 pieces of equipment, 137 staff and 200,000 pieces per month. Documentation can cover CE, CPC, RoHS and REACH, with QC run to ISO 9001-aligned procedures, and lot acceptance is recorded to AQL 2.5 under ISO 2859-1.

People Also Ask

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• What is hydrolysis in PU coating?
• Is TPU or PVC better for welded seams?
• How does coating weight affect waterproof rating?
• What head figure does a day bag need?
• Why does PVC get stiff and crack?