TPU Film Liners: The Hidden Layer That Keeps Packs Dry

What a Liner Actually Is

A liner is the waterproof element of a pack, and it is worth stating that plainly because the shell fabric gets the credit on nearly every specification sheet. Woven cloth, however tightly constructed, is a grid of yarns with gaps between them, and a coating or a finish can shed water for a while but cannot hold it back under load. The barrier is a continuous thermoplastic film with no holes, and it appears in one of three forms: laminated to the back of the face fabric, extruded directly onto it as a coating, or built as a separate sealed inner body dropped inside a conventional pack. All three are legitimate, and they differ in repairability, weight and cost rather than in waterproofness.

The laminated route is the most common on a welded body, because the film is what the welding equipment fuses. The coating route is cheaper and appears on taped bodies, where the seam is covered rather than fused. A separate inner body is the heaviest and the most repairable, since the liner can be pulled out, turned inside and patched, and it is the construction used where the outer shell is expected to take severe abrasion, as on professional and rescue equipment. Buyers should ask which of the three a pack uses, because the answer predicts the repair options and the likely end of life far better than any rating printed on a hangtag.

The last row is included because it is what causes disappointment, and it is easy to spot. A pack with no film passes a spray test, since the surface sheds water for a minute or two, and fails the moment it is set down in a puddle or worn through sustained rain under a loaded strap. The giveaway is the absence of a laminate on the inside face: hold the cloth to the light and a lined fabric is opaque, while an unlined one shows the weave.

Film Chemistry: Polyester-Based and Polyether-Based

Thermoplastic polyurethane is a block copolymer built from hard segments that give strength and soft segments that give elasticity, and the soft segment is where the two commercial families differ. A polyester-based soft segment gives higher tensile strength, better abrasion resistance and better oil resistance at a lower price, and it is vulnerable to hydrolysis, which is the cleavage of the ester bond by water. A polyether-based soft segment resists hydrolysis and microbial attack, costs more and gives slightly lower mechanical properties. That is the entire trade, and it maps directly onto where the pack will be used and stored.

Hydrolysis deserves the closest attention, because it ends most liners and it is invisible until it is catastrophic. Water attacks the ester linkage continuously, faster in warm humid conditions and faster under stress, and the symptom is that the laminate turns tacky, then soft, then separates from the cloth in sheets. A pack stored dry at 20 degrees and 50 percent relative humidity shows little over five years, and the same pack in a tropical warehouse at 32 degrees and 85 percent shows it within three. Specify polyether-based film for any programme shipping to humid markets.

• Specify polyester-based film for abrasion-critical bodies sold into temperate markets, where hydrolysis is slow.
• Specify polyether-based film for tropical, marine and humid storage, where ester cleavage shortens life dramatically.
• Reject any laminate that feels tacky on the inside face, because tackiness is the first visible stage of hydrolysis.
• Store finished stock below 25 degrees and below 60 percent relative humidity, since hydrolysis rate roughly doubles per 10 degrees.

Thickness, Weight and the Numbers That Decide

Film thickness is the specification line with the widest practical effect, and it is quoted in millimetres or in microns, where 0.10 millimetres is 100 microns. Weight follows linearly from a density of about 1.20 grams per cubic centimetre, so 0.10 millimetres weighs about 120 grams per square metre and 0.20 millimetres about 240. Hydrostatic head does not rise linearly with thickness in any useful way, because a 0.08 millimetre film already exceeds the 10,000 millimetre column that any pack will ever see, so beyond that point thickness is bought for puncture resistance, weld tolerance and handling rather than for waterproofness.

The weld tolerance column matters in production and buyers never see it, and it explains why a thicker film can reduce total cost. A thicker film has a wider window between a weld that has not fused and one that has degraded, so it yields fewer rejects, and a 0.08 millimetre film welded on a line set up for 0.15 produces cold welds at a rate that outweighs the material saved. The rule is 0.10 millimetres as the floor for a welded body and 0.15 where panel geometry is complex. Coating weights are covered in coating thickness explained.

Weight is the counter-argument and it is real. A 35 litre body uses roughly 1.6 to 2.1 square metres of laminate, so moving from 0.10 to 0.20 millimetres adds 190 to 250 grams to the finished pack. On a travel or expedition body above 1.2 kilograms that is a fair price for puncture resistance; on a 20 litre daypack sold on lightness it is not, and the right answer there is a thinner film with a reinforced base panel. The comparison against the alternative film family is set out in TPU versus PVC.

How the Film Is Bonded, and How It Comes Off

Bonding is where a laminated pack succeeds or fails over time, and the adhesive line is the least discussed and most consequential part of the specification. A film that is not bonded well delaminates, and delamination is not repairable in any practical sense because the bond fails over the whole area rather than at one point. Three bonding routes exist, and they differ in cost, in the equipment needed and in how the bond ages. Thermal bonding uses heat and pressure to fuse the film directly to a compatible substrate, adhesive lamination uses a reactive hot melt applied at 15 to 40 grams per square metre, and flame lamination melts a foam or film surface and presses it on.

Two controls decide whether a bonded laminate ages well, and both are cheap. Surface preparation comes first: recycled and some synthetic yarns carry spin finish and processing residue, and bonding without a corona or plasma treatment produces a patchy bond that looks fine and fails unevenly, so a dyne level of 38 or above is the standard check. Cure time comes second: reactive hot melt reaches handling strength in minutes and full strength in 24 to 72 hours, so a laminate welded on the day it was bonded is welded before the bond has developed. The bodies these laminates go into are shown in the waterproof backpack collection.

Why Liners Fail

Four mechanisms account for essentially every liner failure, and they present differently enough to be diagnosed on the bench. Hydrolysis attacks the polymer and produces tackiness then separation. Delamination attacks the bond and produces bubbles or sheets lifting from the cloth. Puncture is mechanical and sudden. Ultraviolet embrittles the surface and precedes cracking. The order matters, because two of the four are delayable through specification and storage.

The storage point is worth emphasising to any distributor, because liner life is largely determined before the pack is sold. Hydrolysis rate roughly doubles for every 10 degrees of temperature rise, so a container held at 40 degrees for a season does more damage than three years in a temperate warehouse. Store below 25 degrees and below 60 percent relative humidity, keep stock out of sunlight, and rotate so nothing sits for more than two years.

Cold, Heat and Ultraviolet

Temperature behaviour is where film choice shows most clearly to a user, because a liner that stiffens makes a pack unpleasant to open and a liner that cracks ends it. Thermoplastic polyurethane stays flexible well below any temperature a hiker will meet, with a brittle point around minus 30 to minus 40 degrees for polyester-based film and lower still for polyether-based. Plasticised polyvinyl chloride stiffens noticeably below freezing and cracks around minus 10 to minus 20, which is why it has largely been displaced on cold-weather goods. Heat behaves differently: both soften, but only the plasticised material loses something, since the plasticiser migrates out over time and takes the flexibility with it.

Ultraviolet is the last environmental factor and the easiest to manage, because the liner is on the inside of the pack and sees the sun only when the pack is open or stored inside out. It causes chain scission at the film surface, which shows as yellowing, then a chalky surface, then cracking along fold lines. Pigment changes this substantially: a carbon-black or opaque film absorbs ultraviolet in the first few microns and protects the bulk, while a clear film transmits it, so specifying an opaque liner on a pack used open in strong sun is a free improvement. Cold-weather behaviour across the whole pack is treated in the cold weather guide.

Condensation Inside a Sealed Liner

A sealed liner keeps water out and keeps water in, which is the one genuine disadvantage of a continuous film. Moisture enters with the contents, as a damp jacket, and as vapour in the air trapped at closing. The vapour arithmetic is small: a 40 litre body holding air at 30 degrees and 90 percent relative humidity carries about 1.1 grams of water, and cooling that air to 10 degrees drops roughly 0.7 grams onto the inner face. A 500 gram wet jacket carries far more.

• Close the pack with dry contents where it can be arranged; the contents are a far larger moisture source than the trapped air.
• Use a wet compartment or a separate dry body for anything damp rather than relying on the liner to manage it.
• Open the pack overnight at the end of a trip so the interior can exchange air and dry.

Breathability is the obvious question and it has an honest answer: a solid thermoplastic polyurethane film passes essentially no water vapour, with a moisture vapour transmission rate near zero, so a fully sealed liner will not dry out through the film. Hydrophilic films exist that transmit vapour by a solution-diffusion mechanism and reach 5,000 to 10,000 grams per square metre per day, but they do so by absorbing water into the polymer, and the same mechanism lowers the hydrostatic head and makes the film swell when wet. That trade is acceptable on a garment and it is wrong on a pack, where the whole point of the liner is to be a barrier. The ventilation-versus-barrier question is treated in the breathability trade-off guide.

For a buyer the consequence is short: a liner body keeps contents dry from outside and holds whatever moisture was inside, so packs in this category should be packed dry and opened at the end of the day. That is a behaviour to communicate in the product copy rather than a defect to hide, because the customer who understands it will be satisfied and the one who does not will report a leak that is not a leak. Hydrostatic resistance is measured by ISO 811, coated fabric adhesion by ASTM methods for coated fabrics, and finished-body ingress codes by IEC 60529.

Specifying a Liner: Cost, Sampling and Orders

Specifying a liner is four decisions: chemistry, thickness, bonding route and colour, each with a number attached. Chemistry sets service life. Thickness sets puncture and weld tolerance at about 60 grams of pack weight per 0.05 millimetres on a 35 litre body. Bonding sets whether the laminate delaminates in year three or year eight. Colour sets ultraviolet resistance, since an opaque pigment protects the film in a way a clear one does not.

• Laminates are peel-tested after full cure, with acceptance above 90 percent of the base cloth strength.
• Materials are screened against REACH and California Prop 65, and monthly output of 200,000 units is verified by SGS.
• Programmes run under BSCI and ISO 9001 systems, with AQL 2.5 inspection before release.
• A five-line brief starts a quotation: film chemistry, thickness, bonding route, pack volume and quantity.

Write the liner into the specification before the fabric, because the fabric is what a buyer sees and the liner is what does the work. Specify the chemistry for the market the pack will be stored in rather than sold in, since hydrolysis happens in a warehouse and not on a trail. Never go below 0.10 millimetres on a welded body, and ask for the peel figure after full cure rather than after handling strength. Liners, laminates, closures and the rigs used to verify them are shown. Enquiries go to service@junyuanbags.com.

People Also Ask

What is a TPU film liner in a backpack?

A continuous thermoplastic polyurethane membrane, 0.08 to 0.25 millimetres thick, bonded to the inside face of the shell or built as a separate sealed inner body. It is the layer that stops water, since woven cloth is a grid of yarns with gaps and cannot hold back water under load.

How thick should a waterproof backpack liner be?

0.10 millimetres is the floor for any welded body, weighing about 120 grams per square metre and exceeding 10,000 millimetres of hydrostatic head. 0.15 to 0.20 millimetres suits travel, expedition and professional bodies, and buying thickness beyond that is buying puncture resistance rather than waterproofness.

What is the difference between polyester-based and polyether-based TPU film?

Polyester-based film is stronger at 40 to 55 megapascals and cheaper, but retains only 45 to 60 percent of its strength after five years in humid storage. Polyether-based film costs 20 to 35 percent more and retains 80 to 90 percent, which is why it is specified for tropical and marine markets.

Why does my waterproof backpack liner feel sticky?

Because the film is hydrolysing. Water cleaves the ester bonds in a polyester-based film, and the first visible stage is a tacky inner face, followed by softening and then separation from the cloth in sheets. It is the end of the liner life, not a surface problem.

Does a waterproof liner make a pack sweat?

Yes, in the sense that a solid film passes essentially no water vapour. Trapped air contributes little, about 0.7 grams of condensation when a 40 litre body cools from 30 to 10 degrees, but damp contents carry far more, so packs should be packed dry and opened overnight.

Can a delaminated liner be repaired?

Not practically. Delamination fails across the whole bonded area rather than at one point, so there is no edge to weld and no patch large enough. It is prevented at specification by surface treatment to a dyne level of 38 or above and by allowing the adhesive a full 24 to 72 hour cure before welding.

Frequently Asked Questions

Q1. What does a TPU film liner do in a waterproof backpack?

It is the waterproof barrier. Woven cloth cannot hold back water under load, so a continuous thermoplastic film of 0.08 to 0.25 millimetres is bonded to the inside face of the shell or built as a separate sealed inner body, and that film carries the hydrostatic head the specification quotes.

Q2. How thick is a typical backpack liner?

0.10 millimetres on general packs, weighing about 120 grams per square metre, rising to 0.15 or 0.20 millimetres on travel, expedition and professional bodies. Below 0.10 the weld window narrows enough to raise rejection rates, and above 0.20 the weight penalty stops buying anything useful.

Q3. Which TPU chemistry lasts longer?

Polyether-based. It retains 80 to 90 percent of its strength after five years in humid storage against 45 to 60 percent for polyester-based film, and it stays flexible to minus 40 or minus 50 degrees. It costs 20 to 35 percent more on the film line, which is under four percent on a finished pack.

Q4. What is hydrolysis and how do you slow it down?

Hydrolysis is the cleavage of ester bonds in the polymer by water, and it shows as a tacky inner face, then softening, then separation. Its rate roughly doubles per 10 degrees of temperature, so storing below 25 degrees and below 60 percent relative humidity, and specifying polyether film for humid markets, are the two effective measures.

Q5. Why does a liner delaminate?

Two causes dominate: bonding to a surface carrying spin finish or processing residue, which needs a corona or plasma treatment to a dyne level of 38 or above, and welding before a reactive adhesive has cured, which needs 24 to 72 hours. Both produce a bond that looks sound and fails across the whole area.

Q6. Is a thicker liner more waterproof?

Not meaningfully. A 0.08 millimetre film already exceeds 8,000 millimetres and a 0.10 millimetre film exceeds 10,000, which is beyond any load a pack sees. Thickness above that buys puncture resistance and a wider weld tolerance rather than a higher head figure.

Q7. Does a waterproof liner make the pack heavier?

Yes, by about 60 grams per 0.05 millimetres on a 35 litre body, since a film has a density near 1.20 grams per cubic centimetre. Moving from 0.10 to 0.20 millimetres adds 190 to 250 grams, which suits an expedition pack and does not suit an ultralight daypack.

Q8. Will a liner crack in cold weather?

Not with thermoplastic polyurethane, which stays flexible to minus 30 or minus 40 degrees for polyester-based film and lower for polyether-based. Plasticised PVC is the material that stiffens below freezing and cracks around minus 10 to minus 20, which is why it has largely been displaced.

Q9. Why is there water inside my sealed pack?

Because a sealed liner keeps water in as well as out. Trapped air contributes about 0.7 grams of condensation when a 40 litre body cools from 30 to 10 degrees, but damp contents carry far more. Pack dry, use a wet compartment for damp items and open the pack overnight.

Q10. What is the MOQ and lead time for a lined waterproof backpack?

500 pcs per design and colourway, with bulk in 35 to 50 days and samples in 6 to 10 working days, extended to 8 to 12 where peel and hydrolysis data are requested. Shipments are FOB Xiamen or DDP to more than 100 countries, inspected at AQL 2.5.