TPU Air Retention Is Not Just a Seam Test: How to Separate Film Permeation, Pinholes, Valves, and Weld Leaks

An inflatable TPU product can lose pressure even when the main weld looks acceptable. Air may escape through a valve interface, a small pinhole, a cut edge, an accessory bond, or a discontinuity in a welded seam. It can also move gradually through intact film by permeation. A useful investigation separates these mechanisms instead of changing resin, film thickness, and welding conditions at the same time.

Quick answer

Diagnose air loss in this order:

1. Stabilize the product and the test environment. Temperature change alone can move the pressure reading.
2. Confirm the measurement system. Check the gauge, hoses, adapters, test fixture, and internal volume.
3. Isolate discrete leak paths. Examine valves, caps, weld starts and stops, corners, inserts, and accessory bonds.
4. Use localization methods. Bubble solution, immersion, sectional pressure tests, or an agreed instrument method can help locate a leak.
5. Evaluate intact-film permeation separately. Test representative film coupons at controlled thickness, temperature, humidity, gas, and pressure difference.
6. Recombine the system. Validate production-intent products after folding, flexing, inflation cycling, thermal aging, storage, and any required water or chemical exposure.

Do not call all pressure loss “film permeability,” and do not call a passed seam coupon an airtight product. The film, welds, valve, geometry, process, and test method form one air-retention system.

An inflatable boat illustrates the complete air-retention system: coated fabric or film, welded seams, valves, attachments, and production geometry. Contextual photo: Sebastos, Wikimedia Commons, CC0. The construction and material are not identified as GOAFLEX TPU.

Leakage and permeation are different failure mechanisms

A leak is gas flow through a discrete path. Examples include a pinhole, a poorly fused seam, a wrinkle trapped in a weld, an imperfect valve seat, a loose cap, a crack, or a channel along a reinforcement. Leak rate can change sharply with pressure, flexing, valve position, contamination, or opening of the defect.

Permeation is molecular transport through an apparently intact material. Gas dissolves into one side of the polymer, diffuses through it, and leaves the other side. It is distributed across area rather than concentrated at one visible defect. Film chemistry, morphology, thickness, temperature, humidity, gas type, and pressure difference can all affect the result.

This distinction changes the corrective action:

Finding	Leading variables	Typical next step
Repeatable bubbles at one point	Pinhole, weld channel, valve or accessory interface	Section, inspect, and correct the local design or process
Rapid pressure loss only when flexed	Intermittent crack, valve movement, edge peel, fold damage	Test under controlled motion and inspect the stressed zone
Similar gradual loss across defect-free samples	Film permeation, area/thickness effect, test-system background	Run controlled film transmission testing and mass-balance the whole product
Different results after room-temperature stabilization	Temperature or gauge artifact	Improve conditioning, logging, and absolute-pressure correction
Good film coupon data but poor finished product	Seams, valve, geometry, handling damage, or accessory bonding	Isolate product subsystems and map the failure location

A large inflatable has much more surface area than a laboratory coupon. Even a low transmission rate can become meaningful when multiplied by total area and service time. Conversely, one tiny defect can dominate total air loss and make improved barrier film irrelevant.

First remove test artifacts

Control temperature before interpreting pressure decay

Gas pressure changes with temperature. A product filled with warm compressed air can appear to leak as it cools. Sunlight, a cold floor, water immersion, and handling can also shift the reading.

Define and record:

• inflation medium and initial temperature;
• product and room temperature at inflation and every pressure reading;
• stabilization time before the official start;
• gauge type, range, resolution, calibration status, and connection method;
• initial gauge pressure and local atmospheric pressure when needed;
• test duration, orientation, support condition, and any applied load;
• whether product volume is assumed constant or changes as pressure falls.

For low-pressure inflatables, a high-range industrial gauge may be too coarse. Select an instrument whose resolution and repeatability can separate the allowed pressure loss from normal measurement noise.

A U-tube manometer illustrates differential-pressure measurement. It is a diagram, not a prescribed GOAFLEX test fixture. Image: Olivier Cleynen, Wikimedia Commons, CC0.

Prove the fixture does not leak

Before judging the product, pressure-test the hose, fitting, adapter, gauge, and closure used by the test. A temporary adapter inserted into the valve can create the very leak being investigated. Run a blank or known-tight reference and document the system background loss.

Repeatability matters more than a single “best” result. Use multiple products, independent inflations, fixed timing, and the same stabilization sequence. If operators obtain different readings, fix the method before comparing materials.

Diagnose the product in stages

Stage 1: inspect high-risk locations

Map the construction before testing. Mark every:

• valve body, valve seat, cap, gasket, insert, and inflation adapter;
• weld start, stop, corner, overlap, branch, and multi-layer intersection;
• handle, D-ring, reinforcement, patch, print, coating transition, and repair;
• fold line, abrasion zone, cut edge, and area exposed to heat or solvent;
• film roll position, web direction, lot, and conversion date.

Use good lighting and magnification. Look for contamination, wrinkles, insufficient melt flow, edge channels, burn-through, thinning, foreign particles, coating voids, and damage caused after welding.

The valve is a separate leak path from the film and main seam. Contextual photo: Gidev, Wikimedia Commons, CC BY 2.0. The photographed product is not identified as TPU or GOAFLEX material.

Stage 2: localize discrete leaks

A bubble solution can be useful around valves, seam edges, and attachments when the liquid is compatible with the product. Apply it systematically, allow enough observation time, and distinguish growing bubbles from foam created during application. Clean and dry the product afterward using an approved procedure.

Immersion can reveal bubbles from small leaks, but buoyancy, water temperature, hydrostatic pressure, trapped surface air, product size, and material compatibility must be controlled. Never immerse an assembly when water could create an electrical, corrosion, hygiene, or product-safety hazard.

ASTM F2096 describes bubble-emission testing for gross leaks in packaging. It can provide useful principles for pressure, observation, and sensitivity, but it is not automatically the governing test for an inflatable product. Use the current customer or product standard where one exists, and document any adapted method rather than claiming unqualified compliance.

Stage 3: isolate subsystems

If the source remains uncertain, divide the system:

• seal or replace the valve with a verified reference component;
• test individual chambers or sections;
• compare flat welded coupons with the finished three-dimensional joint;
• compare film from suspect and approved roll locations;
• cover or clamp one accessory interface at a time using a validated temporary method;
• compare initial results with results after flexing or inflation cycling.

Change one variable at a time. A new film combined with a new valve and a new weld recipe may improve the product, but it will not identify the root cause.

Stage 4: investigate process-linked defects

For welded TPU constructions, record the actual process window rather than only the nominal machine setting:

• film or coated-fabric identity, thickness, lot, storage, and conditioning;
• weld technology, tool geometry, energy or temperature, pressure, dwell, and cooling;
• overlap width, stack thickness, reinforcement transitions, and surface cleanliness;
• line speed, start/stop position, operator, machine, and shift;
• destructive seam strength, failure mode, and cross-sections where appropriate.

A seam can have acceptable peel or tensile strength yet contain a narrow gas channel. Mechanical strength and airtightness should therefore be measured as separate outputs.

Evaluate film permeation with controlled coupons

Only move to a film-barrier conclusion after discrete leaks and test artifacts have been addressed. Gas-transmission testing should control:

• gas species or gas mixture;
• specimen thickness and area;
• temperature and relative humidity;
• pressure difference and flow conditions;
• conditioning time and test direction;
• replicate count, blank correction, and reporting units.

ASTM D1434 and ISO 15105 provide standardized frameworks for gas-transmission testing of plastic film and sheeting. Select the method and apparatus appropriate to the expected transmission range and purchase the current standard when contractual compliance is required.

A coupon result is not the finished product’s hourly pressure loss. To estimate system impact, the engineering team must consider total exposed area, local thickness, seams, coated textiles, valves, pressure history, product volume, temperature, and the gas used. Confirm any model against a finished-product pressure or volume-retention test.

Published research on TPU gas permeability also shows that composition and morphology matter; a generic “TPU” label is not enough to predict barrier performance. Compare specific production grades and processed films.

Validate service damage, not only new-product airtightness

A product that passes immediately after manufacture may develop leakage after use. Build an approval sequence around the actual risk:

1. initial stabilized pressure-retention test;
2. valve operation and cap/seat cycling;
3. repeated inflation and deflation;
4. folded storage and reopening;
5. flex, roll, drop, abrasion, or load cycling relevant to the application;
6. hot/cold and thermal-cycle exposure;
7. humidity, water, salt, cleaner, sweat, oil, or UV exposure where applicable;
8. repeat leak localization and pressure retention;
9. destructive inspection of failed and retained controls.

Pressure-system validation requires controlled instrumentation, interfaces, and procedures. Contextual NASA image: NASA Johnson Space Center / Radislav Sinyak, Wikimedia Commons, public domain. The test shown is not a TPU inflatable-product method.

For safety-critical or load-bearing inflatables, pressure vessels, flotation devices, medical products, protective equipment, and regulated products, use the applicable product standards and qualified laboratories. This article is a diagnostic framework, not a safety certification protocol.

Build a clear air-retention specification

Avoid a requirement such as “must not leak.” Define:

• product configuration, nominal volume, and number of chambers;
• inflation medium and target pressure range;
• conditioning and stabilization procedure;
• temperature, humidity, orientation, support, and applied load;
• test duration and reading intervals;
• allowed pressure or volume change after correcting for agreed environmental effects;
• permitted test-system background and gauge uncertainty;
• sample size, lot selection, retest rule, and failure classification;
• separate acceptance for visible leaks, film transmission, seam strength, valve function, and aged performance.

The specification should reflect what the customer actually needs. A compact bladder, rescue product, marine inflatable, sports product, and temporary packaging cushion do not share the same acceptable loss rate or safety margin.

Common mistakes

Blaming the weld first

Welds are visible and easy to suspect, but valves, adapters, pinholes, test hoses, and temperature shifts often produce similar symptoms.

Using room pressure without temperature logging

A small thermal change can look like leakage, especially in low-pressure products.

Treating bubble testing as a quantitative permeation test

Bubbles localize certain discrete leaks. They do not measure distributed molecular transport through intact film.

Treating gas-transmission data as a finished-product guarantee

Coupon data must be scaled and then verified in the assembled product.

Passing a flat seam coupon and skipping geometry

Corners, branches, reinforcements, valve patches, and start/stop zones create process conditions that a straight coupon does not reproduce.

Changing several variables together

The product may improve, but the factory loses the ability to control the real cause in future lots.

What to send GOAFLEX for a useful review

Send the following to hello@goaflex.com:

• product drawing, chamber volume, surface area, and critical geometry;
• film or coated-fabric grade, thickness, construction, lot, and process history;
• valve, gasket, insert, reinforcement, and accessory materials;
• welding method, tool design, overlap, settings, and actual process records;
• target pressure, acceptable loss, test duration, temperature, and service environment;
• raw pressure and temperature logs, instrument details, and blank/reference results;
• leak-location map, photos, failure surfaces, and whether bubbles were repeatable;
• results before and after flexing, folding, cycling, aging, or chemical exposure;
• retained approved and failed samples from known lots.

GOAFLEX can help compare TPU film families, thicknesses, processing behavior, and application-level validation plans. Related guides include Weldable TPU Film and Coated Fabric, Aliphatic TPU for Marine Inflatable Materials, and TPU Film Extrusion Troubleshooting.

References

• ASTM International, ASTM D1434, Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting.
• ISO, ISO 15105-1, Plastics — Film and sheeting — Determination of gas-transmission rate — Part 1: Differential-pressure methods.
• ISO, ISO 15105-2, Plastics — Film and sheeting — Determination of gas-transmission rate — Part 2: Equal-pressure method.
• ASTM International, ASTM F2096, Standard Test Method for Detecting Gross Leaks in Packaging by Internal Pressurization (Bubble Test).
• M. Heuchel et al., Gas Permeability of Thermoplastic Polyurethane Elastomers, Polymer Journal.

Contextual images do not prove GOAFLEX ownership, endorsement, exact TPU chemistry, test compliance, or product performance.