SANLI PIPE · Knowledge base · Materials & technology
PEX-AL-PEX Five-Layer Structure Explained: What Each Layer (Aluminium, Adhesive, Polyethylene) Actually Determines
A multilayer (aluminium-plastic) pipe is a five-layer composite structure: inner plastic + inner adhesive + aluminium strip + outer adhesive + outer plastic. The plastic layers decide what media can be carried and the temperature and pressure ratings; the aluminium strip decides dimensional stability, oxygen barrier and permeation resistance; the hot-melt adhesive layers decide whether the five layers are still bonded after years in service. Downgrade any of the three materials and the pipe's real service life falls far short of the nominal 50 years.
Content type: B pillar page (Products · Materials & technology) · illustrated long-read · approx. 12-minute read
What does the five-layer structure look like?
Cut a multilayer pipe open along its axis and magnify the cross-section: from the inside out there are five layers:
Common trade names are PEX-AL-PEX (cross-linked polyethylene on both sides), PE-RT-AL-PE (heat-resistant PE inside, ordinary PE outside) and PE-AL-PE (non-cross-linked PE on both sides). They are all the same five-layer skeleton — only the plastic grades differ. Our "PEX-AL-PEX vs PE-RT vs PP-R material comparison" goes further on material selection.
Remember it in one line: a multilayer pipe is not "a plastic pipe with aluminium in it" — it is a sandwich where the aluminium provides shape and the oxygen barrier, the plastics carry the water and the heat, and the adhesive keeps the two from parting.
What is each layer made of, and what performance does it own?
① Inner plastic — it decides what is carried, how hot, and how much pressure
The inner layer touches the medium directly and owns four things: food safety, temperature resistance, pressure resistance and creep resistance. The three grades most used in underfloor heating:
- PEX-a (peroxide cross-linked polyethylene) — degree of cross-linking ≥70%, long-term service up to 95 °C, the best creep resistance, the mainstream choice for European underfloor heating; SANLI's PEX-AL-PEX range uses silane/peroxide cross-linked compounds.
- PE-RT Type II (heat-resistant polyethylene) — non-cross-linked, fusion-weldable, 70 °C long-term, lower cost and recyclable; the workhorse of Russian and Central Asian district-heating secondary networks.
- MDPE (medium-density polyethylene) — non-cross-linked, cold water or low temperatures only; used in drinking-water and gas multilayer pipe.
Our experience: whether the inner layer is virgin compound of the declared grade is the one thing buyers cannot see — and the one that affects service life most.Regrind and filled compounds show up quickly in peel strength and in the 95 °C / 165 h hydrostatic test.
② Inner hot-melt adhesive — making PE and aluminium stick, and keep sticking
Polyethylene and aluminium chemically ignore each other; a grafted hot-melt adhesive has to play matchmaker. The industry mainstream:
- Maleic-anhydride grafted polyethylene (MAH-g-PE) — the standard for PE-based pipe.
- Maleic-anhydride grafted polypropylene (PP-g-MAH) — used with PP-R-type outer layers.
- EVA-g-MAH — a wider low-temperature processing window; used in some lap-weld lines.
Typical thickness is 0.05–0.15 mm: too thin and the bond fails, too thick and the film itself becomes the starting point of delamination. The grafting degree (MAH %) matters more than the adhesive brand: with insufficient grafting the pipe passes the incoming peel test, then loses peel strength after three years in service.
③ Aluminium strip — shape, oxygen barrier, tightness: three jobs in one layer
The aluminium strip is why a multilayer pipe "doesn't behave like plastic". The mainstream choice is 1xxx series pure aluminium (1060 / 1100), because pure aluminium offers the best corrosion resistance and formability; 3xxx/5xxx alloys are rarely used for household and underfloor-heating multilayer pipe.
| Parameter | Typical industry range | Effect on performance |
|---|---|---|
| Strip thickness | 0.25 – 0.40 mm | Thicker → less springback and lower oxygen permeation, but a larger minimum bend radius, so cracks are more likely when bending |
| Welding method | Lap-weld / butt-weld | Butt welds reach hoop strength close to the base metal and suit high-pressure/high-temperature duty; lap welds cost less and cover regular heating/water service |
| Temper | O (soft) / H (hard) | O temp forms well and survives bending; H temp is stiffer but cracks at small bend radii |
| Pinhole rate | typically ≤ 3 per m² | Pinholes are the weak point of the oxygen barrier — even a fully impermeable aluminium layer lets overall permeation rise if pinholes are numerous |
Many users believe "aluminium = 100% oxygen barrier". More precisely: the aluminium layer pushes oxygen permeation far below the DIN 4726 limit of 0.1 g/(m²·d), but pinhole rate, weld quality and any delamination between aluminium and plastic all degrade the barrier over the service years.The physics and the engineering consequences are covered in depth in "Why the oxygen barrier matters".
④ Outer hot-melt adhesive — same spec as the inner one, but a harsher job
It performs the same function as the inner adhesive, but it faces the screed backfill, alkaline cement, abrasion and UV, so it is more sensitive to weathering and shear. To cut cost, some plants run non-grafted PE masterbatch as the outer adhesive — peel strength scrapes through the acceptance test, then bubbles and delamination appear after a year of sun exposure.
⑤ Outer plastic — it never touches the water, but it touches the job site
The outer layer is MDPE / HDPE, providing scratch resistance, UV resistance and impact resistance. Exposed installations (above-ceiling runs, on-site storage) need a UV masterbatch, otherwise chalking and surface cracking appear within two or three years. For buried indoor runs, the outer layer's main contribution is protection during transport and installation.
Which layer decides what — in one table
| Layer | Typical material | Typical thickness | Performance owned | Failure mode | SANLI factory test |
|---|---|---|---|---|---|
| ① Inner PE | PEX-a / PE-RT II / MDPE | 0.5 – 2.0 mm | Temperature, pressure, creep resistance, food contact | Premature failure in long-term 95 °C hydrostatic duty; scaling or odour on the bore | Hydrostatic tests (20 °C/1 h, 95 °C/165 h, 80 °C/1000 h), melt flow index, ash content |
| ② Inner adhesive | MAH-g-PE / EVA-g-MAH | 0.05 – 0.15 mm | PE↔aluminium interface bond | Bubbling, delamination, peel-strength decay | 90°/180° peel strength, microscopic interface inspection |
| ③ Aluminium strip | 1060 / 1100 pure aluminium | 0.25 – 0.40 mm | Dimensional stability, oxygen barrier, water-vapour tightness | Cracking/springback when bending, rising oxygen permeation, seam leakage | 100% spark / eddy-current seam testing, tensile strength, pinhole rate |
| ④ Outer adhesive | MAH-g-PE / PP-g-MAH | 0.05 – 0.15 mm | Aluminium↔outer PE bond | As inner adhesive, plus weathering decay | Peel strength after accelerated weathering (xenon/UV) |
| ⑤ Outer PE | MDPE / HDPE (UV option) | 0.4 – 1.2 mm | Scratch, UV and impact resistance; site protection | Surface chalking, cracking, fading | Carbon-black content, UV ageing, slow crack growth resistance |
The table shows typical industry ranges; specific products are governed by their technical data sheets. The "Global standards comparison for multilayer pipe" lists the mandatory per-layer requirements of EN ISO 21003, ASTM F1281/F1282 and GB/T 18997.
Multilayer pipe vs PEX-a vs PE-RT vs PP-R: seven dimensions scored
All four products serve the same "underfloor heating circuit" job, each with its own strengths. The scores below are SANLI's product team's judgement for common residential/commercial heating duty (long-term ≤70 °C, system pressure ≤6 bar, buried or in-screed installation), not absolute metrics; re-weight them for other duties.
| Dimension | Multilayer pipe (PEX-AL-PEX) | PEX-a (single layer) | PE-RT II | PP-R |
|---|---|---|---|---|
| Oxygen barrier | ★★★★★ intrinsic aluminium barrier | ★★☆ needs EVOH | ★★☆ needs EVOH | ★☆ none |
| Dimensional stability (no springback) | ★★★★★ holds its shape once bent | ★★★ shape memory | ★★★ | ★★ rigid, hard to bend |
| Flexibility / ease of laying | ★★★★ coilable, no clips needed | ★★★★★ the softest | ★★★★★ | ★★☆ |
| Long-term heat resistance | ★★★★ 95 °C continuous capable | ★★★★ | ★★★ 70 °C long-term | ★★★ 70 °C long-term |
| Joint reliability | ★★★★★ press or threaded both work | ★★★ needs expansion fittings | ★★★ fusion welding | ★★★ fusion welding |
| Water tightness (permeation) | ★★★★★ | ★★★ | ★★★ | ★★★ |
| Overall value for money | ★★★★ | ★★★ | ★★★★ | ★★★★ |
Scores reflect relative suitability under common heating duty only; combine system temperature, pressure, installation method and budget when selecting. Pricing is out of scope for this article — quotes are calculated per specification, volume and terms; send an enquiry via the RFQ channel and we respond within 24 hours.
Field experience: the five ways the five-layer structure goes wrong
① "If there's aluminium, oxygen is blocked" — not entirely
The aluminium itself is a near-perfect oxygen barrier, but long-term protection is decided by pinhole rate + weld quality + whether the interface delaminates. In project reclamations we have cut open cheap multilayer pipe that ran 3–5 years while the boiler corroded badly: the aluminium had no pinholes, but the aluminium–plastic interface had visibly debonded, creating a motorway for oxygen diffusion along the interface.
② 0.3 mm vs 0.4 mm aluminium: thicker is not better
0.4 mm does give better springback and oxygen numbers — and a larger minimum bend radius at the same time. A 16×2.0 pipe with 0.4 mm strip can micro-crack in the aluminium when cold-bent to 5D (80 mm), while 0.25–0.3 mm handles a 6D bend comfortably. Choose thickness to match the bending method, not because more is more.
③ Lap-weld vs butt-weld: not a cosmetic difference, a life difference
Lap welding is simple and cheap: the strip edges overlap, and the seam reaches only 60–70% of the base metal's hoop strength. Butt welding forms the tube first and welds it square, reaching hoop strength close to the base metal. For ordinary domestic heating and water, lap-weld is fully adequate; for hot radiator circuits and medium-pressure secondary networks we always recommend butt-weld. Measured parameter differences between the two processes (hoop strength, burst pressure, hydrostatic life extrapolation) are being compiled by our product team into a dedicated article for this knowledge base.
④ "Same-grade virgin material" in the inner layer is an invisible cost item
In a five-layer pipe you can measure the aluminium thickness with your eyes; you cannot see whether the inner layer is virgin, same-grade compound. Two industry tricks — selling PE80 as PEX-a and blending regrind — rarely show up in the incoming peel test, but crack en masse in the 95 °C / 165 h hydrostatic test. Every SANLI PEX-AL-PEX batch undergoes hydrostatic sampling for the 8760 h (1-year) equivalent life extrapolation, precisely to close this invisible gap.
⑤ Whether the outer layer needs UV stabiliser is decided by the job site, not the standard
EN ISO 21003 does not mandate UV resistance for the outer layer, but "pipe standing outdoors on site for three months before burial" is normal. For Central Asian, Middle East and North African projects we add 2–2.5% carbon black or UV masterbatch to the outer layer by default; for European indoor buried work it can be omitted per customer practice. Standards are the pass mark; service conditions decide the material.
How SANLI builds the five layers — and proves them
As a multilayer-pipe manufacturer founded in 2002, with 20 composite-pipe lines, 100,000 metres per day and exports to 50+ countries, SANLI does three things about the structural layers:
- Layer-separated incoming control: plastic compounds, aluminium strip and adhesive arrive on three independent incoming lines; every batch is checked for melt flow index, ash content, tensile and peel strength first thing. Aluminium strip is 100% spark-tested for pinholes before it enters the warehouse.
- 26 in-line checks: lamination temperature, the thickness ratio across all five layers, weld status of the strip, out-of-roundness, coil length, in-line eddy-current/spark seam testing and finished-pipe hydrostatics — 100% coil-by-coil, every batch, not sampling.
- Annual full type testing: 50-year life extrapolation (8760 h hydrostatic), 95 °C/165 h, xenon ageing, UV resistance and slow crack growth performed by SKZ / the National Chemical Building Materials Quality Inspection Centre; reports travel with the goods.
In multilayer pipe, SANLI PIPE holds EAC certification across the full product line and works to TR CU and GOST requirements — one of the compliant manufacturers for the Eurasian Economic Union market.
Frequently asked questions
Q1: Is more aluminium always better in a multilayer pipe?
No. 0.25–0.40 mm are all normal. Thicker strip improves the oxygen barrier and reduces springback, but enlarges the minimum cold-bend radius. Select against the installation method (straight lengths + fittings vs coils) and the system temperature — not by adding thickness blindly.
Q2: Which layer differs between PEX-AL-PEX and PE-RT-AL-PE?
The ① inner plastic: the former uses cross-linked polyethylene rated up to 95 °C long-term; the latter uses non-cross-linked heat-resistant polyethylene for 70 °C long-term service. The aluminium strip and adhesive structure are identical — choose by the system's maximum temperature.
Q3: Which of the five layers fails first?
Neither the plastic nor the aluminium — it is the ②/④ hot-melt adhesive layers. Once the adhesive degrades or the interface debonds, the pipe degenerates into "a plastic sleeve around a loose aluminium tube", and oxygen barrier and pressure capability collapse together. In industry, interface delamination accounts for a far larger share of reclamations than aluminium pinholes or PE embrittlement.
Q4: Does a multilayer pipe still need extra oxygen-barrier treatment?
No. The aluminium layer already holds oxygen permeation far below the DIN 4726 limit, so multilayer pipe does not need an EVOH coating the way single-layer PEX does. EVOH is the substitute for the "no-aluminium" solution, not an add-on.
Q5: How can you tell lap-weld from butt-weld by eye?
Look at the seam on a longitudinal cut: a lap weld is a diagonal overlap seam in the strip (roughly a 25–30° angle); a butt weld is an almost straight seam perpendicular to the pipe axis with slight upsetting. SANLI offers both; select by system pressure and temperature ceiling.
Q6: What is the relationship between the five-layer structure and the 50-year life claim?
50 years is the theoretical ceiling extrapolated from 8760 hours (1 year) of hydrostatic testing per ISO 22391 / EN ISO 21003, conditional on every layer meeting specification, every interface bonding correctly and the system staying within duty limits. Cut any corner in any layer and the 50-year claim discounts accordingly — which is why the structure deserves an article of its own.
Next steps
If you are evaluating multilayer-pipe suppliers, send us the system temperature, working pressure, target market (certification requirements) and expected volumes — within 24 hours you will get a layer-by-layer brief of "what compound in each layer, what we test". WhatsApp video factory tours are available, with live access to the lines and the lab.