PET vs HDPE Liners in Type IV COPVs: A Permeation and Mass Trade-off
Both HDPE and modified PET are used as polymer liners in Type IV composite pressure vessels (COPVs). On paper, they sound equivalent. In service, they perform nothing alike. The choice between them comes down to one engineering trade-off: liner wall thickness versus gas permeation. Get it right and you save mass; get it wrong and your tank loses gas.
The two polymer liners
A Type IV COPV consists of three layers, working from inside to outside:
- Polymer liner — the gas-tight barrier (does not carry pressure load)
- Composite overwrap — carbon fibre reinforced polymer (CFRP) carrying all the pressure load
- End fittings — metal bosses bonded into the dome ends for thread/valve attachment
The liner has two jobs: seal the gas in, and act as a mandrel on which the composite is wound. It does not share the structural load — the CFRP overwrap does that work. So the liner can, in principle, be very thin. How thin depends on the polymer.
HDPE — the proven choice
High-density polyethylene (HDPE) is the canonical Type IV liner material. It has been the industry default for CNG vehicle tanks for over two decades. HDPE liners are typically ~4 mm thick, manufactured by blow-moulding or extrusion. The thickness comes from two factors: HDPE has lower mechanical strength per millimetre than other polymers, and it’s relatively permeable to small gas molecules, so designers compensate by adding wall thickness.
HDPE strengths:
- Mature supply chain — many vendors, well-understood manufacturing
- Cost-effective in volume
- Robust handling tolerance during composite winding
- Acceptable permeation for most CNG and industrial gas applications
HDPE limits:
- 4 mm wall consumes internal volume — significant penalty in compact applications
- Brittle below approximately −20 °C without specific cold-weather modifications
- Permeation is meaningful at high pressures (around 15% / month for hydrogen at 300 bar based on industry-typical formulations)
Modified PET — the lightweight choice
Polyethylene terephthalate (PET), modified for high-pressure cylinder service, is a more recent option. The defining property is wall thickness: a modified PET liner is typically ~0.3 mm — more than ten times thinner than HDPE. This is what makes Meyer’s HDRX cylinders the lightest Type IV format in production.
Modified PET strengths:
- Minimum mass and maximum internal volume per outer-diameter envelope
- Wide operating temperature range (−60 °C to +80 °C in production formulations)
- Long cycle life — no metal fatigue, no plasticiser leaching at typical service conditions
- NLL (No Limited Lifespan) qualification possible
Modified PET trade-offs:
- Higher unit cost than HDPE
- Demanding processing window — controlled production environment required
- Permeation is higher than HDPE because the wall is thirteen times thinner (around 30% / month for hydrogen at 300 bar in production formulations)
The permeation comparison, in numbers
Indicative hydrogen permeation at 300 bar, 20 °C, for both polymer liners:
| Liner | Wall thickness | H₂ permeation | Mass implication |
|---|---|---|---|
| HDPE | ~4 mm | ~15% / month | Heavier, less internal volume |
| Modified PET | ~0.3 mm | ~30% / month | Lighter, more internal volume |
The numbers look counter-intuitive. PET is intrinsically a better gas barrier than HDPE per millimetre of thickness. But because the PET wall is thirteen times thinner, the net leak rate is higher. This is the central trade-off: you can have low mass, or you can have low permeation, but with polymer-lined Type IV you can’t have both.
Does the permeation actually matter?
This is the question. Permeation rate per month sounds alarming. In practice it depends entirely on how often the cylinder is filled and what gas is stored.
Daily-fill applications: permeation is irrelevant
A hydrogen-powered UAV refills its tank between every flight, often multiple times a day. Even at 30% / month permeation, the tank loses negligible gas in the few hours between fill and use. For UAV operators, fleet vehicles, daily-use SCBA, and any application where the cylinder cycles fast, the permeation difference between HDPE and PET is academic.
For these uses, mass wins. Modified PET is the right answer.
Long-duration storage: permeation dominates
A CubeSat sitting in low-earth orbit on stored cold-gas propellant has months of mission life. A helium pressurant tank on a rocket upper stage may sit fuelled for weeks before launch. A strategic-reserve cylinder may stay unopened for years.
For these applications, polymer-lined Type IV is rarely the right answer. Metal-lined formats (Type III aluminium, or thin-metal-lined Type IV-M) maintain gas integrity over long durations because metal is a near-perfect barrier (< 0.05% / month). The mass penalty is real but acceptable when the alternative is losing your propellant before you use it.
Gas type matters too
The numbers above are for hydrogen — the smallest, most permeable gas. Other gases permeate polymer liners much more slowly:
- Helium — permeates polymer liners faster than H₂ (small atomic radius, nonpolar). Polymer liners generally unsuitable for long-duration helium storage.
- Nitrogen, Argon — permeate roughly 5–10× slower than H₂. PET works fine for daily-use N₂ systems, less ideal for years-long storage.
- Xenon, Krypton — heavy noble gases, large molecules, permeate very slowly. PET viable for most CubeSat propulsion timelines.
- CNG / methane — well within HDPE’s design envelope; PET works equally fine.
The selection rule
A useful starting heuristic:
- Mass-critical AND filled often (UAV, drone, SCBA, vehicle) — choose modified PET. Mass savings translate directly to flight time, payload, range, or wearer comfort.
- Cost-critical AND moderate pressure (CNG, industrial gas, stationary mobile) — choose HDPE. Mature, cheap, sufficient permeation for the duty cycle.
- Permeation-critical OR long-duration storage (helium pressurant, satellite storage, long mission) — choose a metal-lined format (Type III aluminium or Type IV-M).
For a more nuanced answer that includes cycle life, working pressure, and cost sensitivity, run your application through the MEYER COPV Selector — the tool maps your requirements onto all five COPV formats and shows you the trade-offs.
What MEYER manufactures
MEYER specialises in modified PET-lined Type IV COPVs — the HDRX cylinder family. Capacities from 0.5 L to 350 L, working pressures up to 700 bar, qualified for hydrogen UAVs, CubeSat propulsion, and orbital launch programmes. Browse the full HDRX catalog.
If your application calls for HDPE Type IV, Type III aluminium, or any other format we don’t manufacture, send us the spec anyway — we’ll give you an honest assessment and refer you to the right partner if needed.
